diff --git a/bar.py b/bar.py
new file mode 100644
index 0000000000..064dad8773
--- /dev/null
+++ b/bar.py
@@ -0,0 +1,2486 @@
+import copy
+import gc
+import pickle
+import sys
+import doctest
+import unittest
+import weakref
+import inspect
+
+from test import support
+
+try:
+    import _testcapi
+except ImportError:
+    _testcapi = None
+
+
+# This tests to make sure that if a SIGINT arrives just before we send into a
+# yield from chain, the KeyboardInterrupt is raised in the innermost
+# generator (see bpo-30039).
+@unittest.skipUnless(_testcapi is not None and
+                     hasattr(_testcapi, "raise_SIGINT_then_send_None"),
+                     "needs _testcapi.raise_SIGINT_then_send_None")
+class SignalAndYieldFromTest(unittest.TestCase):
+
+    def generator1(self):
+        return (yield from self.generator2())
+
+    def generator2(self):
+        try:
+            yield
+        except KeyboardInterrupt:
+            return "PASSED"
+        else:
+            return "FAILED"
+
+    def test_raise_and_yield_from(self):
+        gen = self.generator1()
+        gen.send(None)
+        try:
+            _testcapi.raise_SIGINT_then_send_None(gen)
+        except BaseException as _exc:
+            exc = _exc
+        self.assertIs(type(exc), StopIteration)
+        self.assertEqual(exc.value, "PASSED")
+
+
+class FinalizationTest(unittest.TestCase):
+
+    def test_frame_resurrect(self):
+        # A generator frame can be resurrected by a generator's finalization.
+        def gen():
+            nonlocal frame
+            try:
+                yield
+            finally:
+                frame = sys._getframe()
+
+        g = gen()
+        wr = weakref.ref(g)
+        next(g)
+        del g
+        support.gc_collect()
+        self.assertIs(wr(), None)
+        self.assertTrue(frame)
+        del frame
+        support.gc_collect()
+
+    def test_refcycle(self):
+        # A generator caught in a refcycle gets finalized anyway.
+        old_garbage = gc.garbage[:]
+        finalized = False
+        def gen():
+            nonlocal finalized
+            try:
+                yield
+                yield 1
+            finally:
+                finalized = True
+
+        g = gen()
+        next(g)
+        g.send(g)
+        self.assertGreater(sys.getrefcount(g), 2)
+        self.assertFalse(finalized)
+        del g
+        support.gc_collect()
+        self.assertTrue(finalized)
+        self.assertEqual(gc.garbage, old_garbage)
+
+    def test_lambda_generator(self):
+        # Issue #23192: Test that a lambda returning a generator behaves
+        # like the equivalent function
+        f = lambda: (yield 1)
+        def g(): return (yield 1)
+
+        # test 'yield from'
+        f2 = lambda: (yield from g())
+        def g2(): return (yield from g())
+
+        f3 = lambda: (yield from f())
+        def g3(): return (yield from f())
+
+        for gen_fun in (f, g, f2, g2, f3, g3):
+            gen = gen_fun()
+            self.assertEqual(next(gen), 1)
+            with self.assertRaises(StopIteration) as cm:
+                gen.send(2)
+            self.assertEqual(cm.exception.value, 2)
+
+
+class GeneratorTest(unittest.TestCase):
+
+    def test_name(self):
+        def func():
+            yield 1
+
+        # check generator names
+        gen = func()
+        self.assertEqual(gen.__name__, "func")
+        self.assertEqual(gen.__qualname__,
+                         "GeneratorTest.test_name.<locals>.func")
+
+        # modify generator names
+        gen.__name__ = "name"
+        gen.__qualname__ = "qualname"
+        self.assertEqual(gen.__name__, "name")
+        self.assertEqual(gen.__qualname__, "qualname")
+
+        # generator names must be a string and cannot be deleted
+        self.assertRaises(TypeError, setattr, gen, '__name__', 123)
+        self.assertRaises(TypeError, setattr, gen, '__qualname__', 123)
+        self.assertRaises(TypeError, delattr, gen, '__name__')
+        self.assertRaises(TypeError, delattr, gen, '__qualname__')
+
+        # modify names of the function creating the generator
+        func.__qualname__ = "func_qualname"
+        func.__name__ = "func_name"
+        gen = func()
+        self.assertEqual(gen.__name__, "func_name")
+        self.assertEqual(gen.__qualname__, "func_qualname")
+
+        # unnamed generator
+        gen = (x for x in range(10))
+        self.assertEqual(gen.__name__,
+                         "<genexpr>")
+        self.assertEqual(gen.__qualname__,
+                         "GeneratorTest.test_name.<locals>.<genexpr>")
+
+    def test_copy(self):
+        def f():
+            yield 1
+        g = f()
+        with self.assertRaises(TypeError):
+            copy.copy(g)
+
+    def test_pickle(self):
+        def f():
+            yield 1
+        g = f()
+        for proto in range(pickle.HIGHEST_PROTOCOL + 1):
+            with self.assertRaises((TypeError, pickle.PicklingError)):
+                pickle.dumps(g, proto)
+
+    def test_send_non_none_to_new_gen(self):
+        def f():
+            yield 1
+        g = f()
+        with self.assertRaises(TypeError):
+            g.send(0)
+        self.assertEqual(next(g), 1)
+
+    def test_handle_frame_object_in_creation(self):
+
+        #Attempt to expose partially constructed frames
+        #See https://github.com/python/cpython/issues/94262
+
+        def cb(*args):
+            inspect.stack()
+
+        def gen():
+            yield 1
+
+        thresholds = gc.get_threshold()
+
+        gc.callbacks.append(cb)
+        gc.set_threshold(1, 0, 0)
+        try:
+            gen()
+        finally:
+            gc.set_threshold(*thresholds)
+            gc.callbacks.pop()
+
+        class Sneaky:
+            def __del__(self):
+                inspect.stack()
+
+        sneaky = Sneaky()
+        sneaky._s = Sneaky()
+        sneaky._s._s = sneaky
+
+        gc.set_threshold(1, 0, 0)
+        try:
+            del sneaky
+            gen()
+        finally:
+            gc.set_threshold(*thresholds)
+
+    def test_ag_frame_f_back(self):
+        async def f():
+            yield
+        ag = f()
+        self.assertIsNone(ag.ag_frame.f_back)
+
+    def test_cr_frame_f_back(self):
+        async def f():
+            pass
+        cr = f()
+        self.assertIsNone(cr.cr_frame.f_back)
+        cr.close()  # Suppress RuntimeWarning.
+
+    def test_gi_frame_f_back(self):
+        def f():
+            yield
+        gi = f()
+        self.assertIsNone(gi.gi_frame.f_back)
+
+
+
+class ExceptionTest(unittest.TestCase):
+    # Tests for the issue #23353: check that the currently handled exception
+    # is correctly saved/restored in PyEval_EvalFrameEx().
+
+    def test_except_throw(self):
+        def store_raise_exc_generator():
+            try:
+                self.assertEqual(sys.exc_info()[0], None)
+                yield
+            except Exception as exc:
+                # exception raised by gen.throw(exc)
+                self.assertEqual(sys.exc_info()[0], ValueError)
+                self.assertIsNone(exc.__context__)
+                yield
+
+                # ensure that the exception is not lost
+                self.assertEqual(sys.exc_info()[0], ValueError)
+                yield
+
+                # we should be able to raise back the ValueError
+                raise
+
+        make = store_raise_exc_generator()
+        next(make)
+
+        try:
+            raise ValueError()
+        except Exception as exc:
+            try:
+                make.throw(exc)
+            except Exception:
+                pass
+
+        next(make)
+        with self.assertRaises(ValueError) as cm:
+            next(make)
+        self.assertIsNone(cm.exception.__context__)
+
+        self.assertEqual(sys.exc_info(), (None, None, None))
+
+    def test_except_next(self):
+        def gen():
+            self.assertEqual(sys.exc_info()[0], ValueError)
+            yield "done"
+
+        g = gen()
+        try:
+            raise ValueError
+        except Exception:
+            self.assertEqual(next(g), "done")
+        self.assertEqual(sys.exc_info(), (None, None, None))
+
+    def test_except_gen_except(self):
+        def gen():
+            try:
+                self.assertEqual(sys.exc_info()[0], None)
+                yield
+                # we are called from "except ValueError:", TypeError must
+                # inherit ValueError in its context
+                raise TypeError()
+            except TypeError as exc:
+                self.assertEqual(sys.exc_info()[0], TypeError)
+                self.assertEqual(type(exc.__context__), ValueError)
+            # here we are still called from the "except ValueError:"
+            self.assertEqual(sys.exc_info()[0], ValueError)
+            yield
+            self.assertIsNone(sys.exc_info()[0])
+            yield "done"
+
+        g = gen()
+        next(g)
+        try:
+            raise ValueError
+        except Exception:
+            next(g)
+
+        self.assertEqual(next(g), "done")
+        self.assertEqual(sys.exc_info(), (None, None, None))
+
+    def test_nested_gen_except_loop(self):
+        def gen():
+            for i in range(100):
+                self.assertIsInstance(sys.exception(), TypeError)
+                yield "doing"
+
+        def outer():
+            try:
+                raise TypeError
+            except:
+                for x in gen():
+                    yield x
+
+        try:
+            raise ValueError
+        except Exception:
+            for x in outer():
+                self.assertEqual(x, "doing")
+        self.assertEqual(sys.exception(), None)
+
+    def test_except_throw_exception_context(self):
+        def gen():
+            try:
+                try:
+                    self.assertEqual(sys.exc_info()[0], None)
+                    yield
+                except ValueError:
+                    # we are called from "except ValueError:"
+                    self.assertEqual(sys.exc_info()[0], ValueError)
+                    raise TypeError()
+            except Exception as exc:
+                self.assertEqual(sys.exc_info()[0], TypeError)
+                self.assertEqual(type(exc.__context__), ValueError)
+            # we are still called from "except ValueError:"
+            self.assertEqual(sys.exc_info()[0], ValueError)
+            yield
+            self.assertIsNone(sys.exc_info()[0])
+            yield "done"
+
+        g = gen()
+        next(g)
+        try:
+            raise ValueError
+        except Exception as exc:
+            g.throw(exc)
+
+        self.assertEqual(next(g), "done")
+        self.assertEqual(sys.exc_info(), (None, None, None))
+
+    def test_except_throw_bad_exception(self):
+        class E(Exception):
+            def __new__(cls, *args, **kwargs):
+                return cls
+
+        def boring_generator():
+            yield
+
+        gen = boring_generator()
+
+        err_msg = 'should have returned an instance of BaseException'
+
+        with self.assertRaisesRegex(TypeError, err_msg):
+            gen.throw(E)
+
+        self.assertRaises(StopIteration, next, gen)
+
+        def generator():
+            with self.assertRaisesRegex(TypeError, err_msg):
+                yield
+
+        gen = generator()
+        next(gen)
+        with self.assertRaises(StopIteration):
+            gen.throw(E)
+
+    def test_gen_3_arg_deprecation_warning(self):
+        def g():
+            yield 42
+
+        gen = g()
+        with self.assertWarns(DeprecationWarning):
+            with self.assertRaises(TypeError):
+                gen.throw(TypeError, TypeError(24), None)
+
+    def test_stopiteration_error(self):
+        # See also PEP 479.
+
+        def gen():
+            raise StopIteration
+            yield
+
+        with self.assertRaisesRegex(RuntimeError, 'raised StopIteration'):
+            next(gen())
+
+    def test_tutorial_stopiteration(self):
+        # Raise StopIteration" stops the generator too:
+
+        def f():
+            yield 1
+            raise StopIteration
+            yield 2 # never reached
+
+        g = f()
+        self.assertEqual(next(g), 1)
+
+        with self.assertRaisesRegex(RuntimeError, 'raised StopIteration'):
+            next(g)
+
+    def test_return_tuple(self):
+        def g():
+            return (yield 1)
+
+        gen = g()
+        self.assertEqual(next(gen), 1)
+        with self.assertRaises(StopIteration) as cm:
+            gen.send((2,))
+        self.assertEqual(cm.exception.value, (2,))
+
+    def test_return_stopiteration(self):
+        def g():
+            return (yield 1)
+
+        gen = g()
+        self.assertEqual(next(gen), 1)
+        with self.assertRaises(StopIteration) as cm:
+            gen.send(StopIteration(2))
+        self.assertIsInstance(cm.exception.value, StopIteration)
+        self.assertEqual(cm.exception.value.value, 2)
+
+
+class GeneratorThrowTest(unittest.TestCase):
+
+    def test_exception_context_with_yield(self):
+        def f():
+            try:
+                raise KeyError('a')
+            except Exception:
+                yield
+
+        gen = f()
+        gen.send(None)
+        with self.assertRaises(ValueError) as cm:
+            gen.throw(ValueError)
+        context = cm.exception.__context__
+        self.assertEqual((type(context), context.args), (KeyError, ('a',)))
+
+    def test_exception_context_with_yield_inside_generator(self):
+        # Check that the context is also available from inside the generator
+        # with yield, as opposed to outside.
+        def f():
+            try:
+                raise KeyError('a')
+            except Exception:
+                try:
+                    yield
+                except Exception as exc:
+                    self.assertEqual(type(exc), ValueError)
+                    context = exc.__context__
+                    self.assertEqual((type(context), context.args),
+                        (KeyError, ('a',)))
+                    yield 'b'
+
+        gen = f()
+        gen.send(None)
+        actual = gen.throw(ValueError)
+        # This ensures that the assertions inside were executed.
+        self.assertEqual(actual, 'b')
+
+    def test_exception_context_with_yield_from(self):
+        def f():
+            yield
+
+        def g():
+            try:
+                raise KeyError('a')
+            except Exception:
+                yield from f()
+
+        gen = g()
+        gen.send(None)
+        with self.assertRaises(ValueError) as cm:
+            gen.throw(ValueError)
+        context = cm.exception.__context__
+        self.assertEqual((type(context), context.args), (KeyError, ('a',)))
+
+    def test_exception_context_with_yield_from_with_context_cycle(self):
+        # Check trying to create an exception context cycle:
+        # https://bugs.python.org/issue40696
+        has_cycle = None
+
+        def f():
+            yield
+
+        def g(exc):
+            nonlocal has_cycle
+            try:
+                raise exc
+            except Exception:
+                try:
+                    yield from f()
+                except Exception as exc:
+                    has_cycle = (exc is exc.__context__)
+            yield
+
+        exc = KeyError('a')
+        gen = g(exc)
+        gen.send(None)
+        gen.throw(exc)
+        # This also distinguishes from the initial has_cycle=None.
+        self.assertEqual(has_cycle, False)
+
+    def test_throw_after_none_exc_type(self):
+        def g():
+            try:
+                raise KeyError
+            except KeyError:
+                pass
+
+            try:
+                yield
+            except Exception:
+                raise RuntimeError
+
+        gen = g()
+        gen.send(None)
+        with self.assertRaises(RuntimeError):
+            gen.throw(ValueError)
+
+
+class GeneratorStackTraceTest(unittest.TestCase):
+
+    def check_stack_names(self, frame, expected):
+        names = []
+        while frame:
+            name = frame.f_code.co_name
+            # Stop checking frames when we get to our test helper.
+            if name.startswith('check_') or name.startswith('call_'):
+                break
+
+            names.append(name)
+            frame = frame.f_back
+
+        self.assertEqual(names, expected)
+
+    def check_yield_from_example(self, call_method):
+        def f():
+            self.check_stack_names(sys._getframe(), ['f', 'g'])
+            try:
+                yield
+            except Exception:
+                pass
+            self.check_stack_names(sys._getframe(), ['f', 'g'])
+
+        def g():
+            self.check_stack_names(sys._getframe(), ['g'])
+            yield from f()
+            self.check_stack_names(sys._getframe(), ['g'])
+
+        gen = g()
+        gen.send(None)
+        try:
+            call_method(gen)
+        except StopIteration:
+            pass
+
+    def test_send_with_yield_from(self):
+        def call_send(gen):
+            gen.send(None)
+
+        self.check_yield_from_example(call_send)
+
+    def test_throw_with_yield_from(self):
+        def call_throw(gen):
+            gen.throw(RuntimeError)
+
+        self.check_yield_from_example(call_throw)
+
+
+class YieldFromTests(unittest.TestCase):
+    def test_generator_gi_yieldfrom(self):
+        def a():
+            self.assertEqual(inspect.getgeneratorstate(gen_b), inspect.GEN_RUNNING)
+            self.assertIsNone(gen_b.gi_yieldfrom)
+            yield
+            self.assertEqual(inspect.getgeneratorstate(gen_b), inspect.GEN_RUNNING)
+            self.assertIsNone(gen_b.gi_yieldfrom)
+
+        def b():
+            self.assertIsNone(gen_b.gi_yieldfrom)
+            yield from a()
+            self.assertIsNone(gen_b.gi_yieldfrom)
+            yield
+            self.assertIsNone(gen_b.gi_yieldfrom)
+
+        gen_b = b()
+        self.assertEqual(inspect.getgeneratorstate(gen_b), inspect.GEN_CREATED)
+        self.assertIsNone(gen_b.gi_yieldfrom)
+
+        gen_b.send(None)
+        self.assertEqual(inspect.getgeneratorstate(gen_b), inspect.GEN_SUSPENDED)
+        self.assertEqual(gen_b.gi_yieldfrom.gi_code.co_name, 'a')
+
+        gen_b.send(None)
+        self.assertEqual(inspect.getgeneratorstate(gen_b), inspect.GEN_SUSPENDED)
+        self.assertIsNone(gen_b.gi_yieldfrom)
+
+        [] = gen_b  # Exhaust generator
+        self.assertEqual(inspect.getgeneratorstate(gen_b), inspect.GEN_CLOSED)
+        self.assertIsNone(gen_b.gi_yieldfrom)
+
+
+tutorial_tests = """
+Let's try a simple generator:
+
+    >>> def f():
+    ...    yield 1
+    ...    yield 2
+
+    >>> for i in f():
+    ...     print(i)
+    1
+    2
+    >>> g = f()
+    >>> next(g)
+    1
+    >>> next(g)
+    2
+
+"Falling off the end" stops the generator:
+
+    >>> next(g)
+    Traceback (most recent call last):
+      File "<stdin>", line 1, in ?
+      File "<stdin>", line 2, in g
+    StopIteration
+
+"return" also stops the generator:
+
+    >>> def f():
+    ...     yield 1
+    ...     return
+    ...     yield 2 # never reached
+    ...
+    >>> g = f()
+    >>> next(g)
+    1
+    >>> next(g)
+    Traceback (most recent call last):
+      File "<stdin>", line 1, in ?
+      File "<stdin>", line 3, in f
+    StopIteration
+    >>> next(g) # once stopped, can't be resumed
+    Traceback (most recent call last):
+      File "<stdin>", line 1, in ?
+    StopIteration
+
+However, "return" and StopIteration are not exactly equivalent:
+
+    >>> def g1():
+    ...     try:
+    ...         return
+    ...     except:
+    ...         yield 1
+    ...
+    >>> list(g1())
+    []
+
+    >>> def g2():
+    ...     try:
+    ...         raise StopIteration
+    ...     except:
+    ...         yield 42
+    >>> print(list(g2()))
+    [42]
+
+This may be surprising at first:
+
+    >>> def g3():
+    ...     try:
+    ...         return
+    ...     finally:
+    ...         yield 1
+    ...
+    >>> list(g3())
+    [1]
+
+Let's create an alternate range() function implemented as a generator:
+
+    >>> def yrange(n):
+    ...     for i in range(n):
+    ...         yield i
+    ...
+    >>> list(yrange(5))
+    [0, 1, 2, 3, 4]
+
+Generators always return to the most recent caller:
+
+    >>> def creator():
+    ...     r = yrange(5)
+    ...     print("creator", next(r))
+    ...     return r
+    ...
+    >>> def caller():
+    ...     r = creator()
+    ...     for i in r:
+    ...             print("caller", i)
+    ...
+    >>> caller()
+    creator 0
+    caller 1
+    caller 2
+    caller 3
+    caller 4
+
+Generators can call other generators:
+
+    >>> def zrange(n):
+    ...     for i in yrange(n):
+    ...         yield i
+    ...
+    >>> list(zrange(5))
+    [0, 1, 2, 3, 4]
+
+"""
+
+# The examples from PEP 255.
+
+pep_tests = """
+
+Specification:  Yield
+
+    Restriction:  A generator cannot be resumed while it is actively
+    running:
+
+    >>> def g():
+    ...     i = next(me)
+    ...     yield i
+    >>> me = g()
+    >>> next(me)
+    Traceback (most recent call last):
+     ...
+      File "<string>", line 2, in g
+    ValueError: generator already executing
+
+Specification: Return
+
+    Note that return isn't always equivalent to raising StopIteration:  the
+    difference lies in how enclosing try/except constructs are treated.
+    For example,
+
+        >>> def f1():
+        ...     try:
+        ...         return
+        ...     except:
+        ...        yield 1
+        >>> print(list(f1()))
+        []
+
+    because, as in any function, return simply exits, but
+
+        >>> def f2():
+        ...     try:
+        ...         raise StopIteration
+        ...     except:
+        ...         yield 42
+        >>> print(list(f2()))
+        [42]
+
+    because StopIteration is captured by a bare "except", as is any
+    exception.
+
+Specification: Generators and Exception Propagation
+
+    >>> def f():
+    ...     return 1//0
+    >>> def g():
+    ...     yield f()  # the zero division exception propagates
+    ...     yield 42   # and we'll never get here
+    >>> k = g()
+    >>> next(k)
+    Traceback (most recent call last):
+      File "<stdin>", line 1, in ?
+      File "<stdin>", line 2, in g
+      File "<stdin>", line 2, in f
+    ZeroDivisionError: integer division or modulo by zero
+    >>> next(k)  # and the generator cannot be resumed
+    Traceback (most recent call last):
+      File "<stdin>", line 1, in ?
+    StopIteration
+    >>>
+
+Specification: Try/Except/Finally
+
+    >>> def f():
+    ...     try:
+    ...         yield 1
+    ...         try:
+    ...             yield 2
+    ...             1//0
+    ...             yield 3  # never get here
+    ...         except ZeroDivisionError:
+    ...             yield 4
+    ...             yield 5
+    ...             raise
+    ...         except:
+    ...             yield 6
+    ...         yield 7     # the "raise" above stops this
+    ...     except:
+    ...         yield 8
+    ...     yield 9
+    ...     try:
+    ...         x = 12
+    ...     finally:
+    ...         yield 10
+    ...     yield 11
+    >>> print(list(f()))
+    [1, 2, 4, 5, 8, 9, 10, 11]
+    >>>
+
+Guido's binary tree example.
+
+    >>> # A binary tree class.
+    >>> class Tree:
+    ...
+    ...     def __init__(self, label, left=None, right=None):
+    ...         self.label = label
+    ...         self.left = left
+    ...         self.right = right
+    ...
+    ...     def __repr__(self, level=0, indent="    "):
+    ...         s = level*indent + repr(self.label)
+    ...         if self.left:
+    ...             s = s + "\\n" + self.left.__repr__(level+1, indent)
+    ...         if self.right:
+    ...             s = s + "\\n" + self.right.__repr__(level+1, indent)
+    ...         return s
+    ...
+    ...     def __iter__(self):
+    ...         return inorder(self)
+
+    >>> # Create a Tree from a list.
+    >>> def tree(list):
+    ...     n = len(list)
+    ...     if n == 0:
+    ...         return []
+    ...     i = n // 2
+    ...     return Tree(list[i], tree(list[:i]), tree(list[i+1:]))
+
+    >>> # Show it off: create a tree.
+    >>> t = tree("ABCDEFGHIJKLMNOPQRSTUVWXYZ")
+
+    >>> # A recursive generator that generates Tree labels in in-order.
+    >>> def inorder(t):
+    ...     if t:
+    ...         for x in inorder(t.left):
+    ...             yield x
+    ...         yield t.label
+    ...         for x in inorder(t.right):
+    ...             yield x
+
+    >>> # Show it off: create a tree.
+    >>> t = tree("ABCDEFGHIJKLMNOPQRSTUVWXYZ")
+    >>> # Print the nodes of the tree in in-order.
+    >>> for x in t:
+    ...     print(' '+x, end='')
+     A B C D E F G H I J K L M N O P Q R S T U V W X Y Z
+
+    >>> # A non-recursive generator.
+    >>> def inorder(node):
+    ...     stack = []
+    ...     while node:
+    ...         while node.left:
+    ...             stack.append(node)
+    ...             node = node.left
+    ...         yield node.label
+    ...         while not node.right:
+    ...             try:
+    ...                 node = stack.pop()
+    ...             except IndexError:
+    ...                 return
+    ...             yield node.label
+    ...         node = node.right
+
+    >>> # Exercise the non-recursive generator.
+    >>> for x in t:
+    ...     print(' '+x, end='')
+     A B C D E F G H I J K L M N O P Q R S T U V W X Y Z
+
+"""
+
+# Examples from Iterator-List and Python-Dev and c.l.py.
+
+email_tests = """
+
+The difference between yielding None and returning it.
+
+>>> def g():
+...     for i in range(3):
+...         yield None
+...     yield None
+...     return
+>>> list(g())
+[None, None, None, None]
+
+Ensure that explicitly raising StopIteration acts like any other exception
+in try/except, not like a return.
+
+>>> def g():
+...     yield 1
+...     try:
+...         raise StopIteration
+...     except:
+...         yield 2
+...     yield 3
+>>> list(g())
+[1, 2, 3]
+
+Next one was posted to c.l.py.
+
+>>> def gcomb(x, k):
+...     "Generate all combinations of k elements from list x."
+...
+...     if k > len(x):
+...         return
+...     if k == 0:
+...         yield []
+...     else:
+...         first, rest = x[0], x[1:]
+...         # A combination does or doesn't contain first.
+...         # If it does, the remainder is a k-1 comb of rest.
+...         for c in gcomb(rest, k-1):
+...             c.insert(0, first)
+...             yield c
+...         # If it doesn't contain first, it's a k comb of rest.
+...         for c in gcomb(rest, k):
+...             yield c
+
+>>> seq = list(range(1, 5))
+>>> for k in range(len(seq) + 2):
+...     print("%d-combs of %s:" % (k, seq))
+...     for c in gcomb(seq, k):
+...         print("   ", c)
+0-combs of [1, 2, 3, 4]:
+    []
+1-combs of [1, 2, 3, 4]:
+    [1]
+    [2]
+    [3]
+    [4]
+2-combs of [1, 2, 3, 4]:
+    [1, 2]
+    [1, 3]
+    [1, 4]
+    [2, 3]
+    [2, 4]
+    [3, 4]
+3-combs of [1, 2, 3, 4]:
+    [1, 2, 3]
+    [1, 2, 4]
+    [1, 3, 4]
+    [2, 3, 4]
+4-combs of [1, 2, 3, 4]:
+    [1, 2, 3, 4]
+5-combs of [1, 2, 3, 4]:
+
+From the Iterators list, about the types of these things.
+
+>>> def g():
+...     yield 1
+...
+>>> type(g)
+<class 'function'>
+>>> i = g()
+>>> type(i)
+<class 'generator'>
+>>> [s for s in dir(i) if not s.startswith('_')]
+['close', 'gi_code', 'gi_frame', 'gi_running', 'gi_suspended', 'gi_yieldfrom', 'send', 'throw']
+>>> from test.support import HAVE_DOCSTRINGS
+>>> print(i.__next__.__doc__ if HAVE_DOCSTRINGS else 'Implement next(self).')
+Implement next(self).
+>>> iter(i) is i
+True
+>>> import types
+>>> isinstance(i, types.GeneratorType)
+True
+
+And more, added later.
+
+>>> i.gi_running
+0
+>>> type(i.gi_frame)
+<class 'frame'>
+>>> i.gi_running = 42
+Traceback (most recent call last):
+  ...
+AttributeError: attribute 'gi_running' of 'generator' objects is not writable
+>>> def g():
+...     yield me.gi_running
+>>> me = g()
+>>> me.gi_running
+0
+>>> next(me)
+1
+>>> me.gi_running
+0
+
+A clever union-find implementation from c.l.py, due to David Eppstein.
+Sent: Friday, June 29, 2001 12:16 PM
+To: python-list@python.org
+Subject: Re: PEP 255: Simple Generators
+
+>>> class disjointSet:
+...     def __init__(self, name):
+...         self.name = name
+...         self.parent = None
+...         self.generator = self.generate()
+...
+...     def generate(self):
+...         while not self.parent:
+...             yield self
+...         for x in self.parent.generator:
+...             yield x
+...
+...     def find(self):
+...         return next(self.generator)
+...
+...     def union(self, parent):
+...         if self.parent:
+...             raise ValueError("Sorry, I'm not a root!")
+...         self.parent = parent
+...
+...     def __str__(self):
+...         return self.name
+
+>>> names = "ABCDEFGHIJKLM"
+>>> sets = [disjointSet(name) for name in names]
+>>> roots = sets[:]
+
+>>> import random
+>>> gen = random.Random(42)
+>>> while 1:
+...     for s in sets:
+...         print(" %s->%s" % (s, s.find()), end='')
+...     print()
+...     if len(roots) > 1:
+...         s1 = gen.choice(roots)
+...         roots.remove(s1)
+...         s2 = gen.choice(roots)
+...         s1.union(s2)
+...         print("merged", s1, "into", s2)
+...     else:
+...         break
+ A->A B->B C->C D->D E->E F->F G->G H->H I->I J->J K->K L->L M->M
+merged K into B
+ A->A B->B C->C D->D E->E F->F G->G H->H I->I J->J K->B L->L M->M
+merged A into F
+ A->F B->B C->C D->D E->E F->F G->G H->H I->I J->J K->B L->L M->M
+merged E into F
+ A->F B->B C->C D->D E->F F->F G->G H->H I->I J->J K->B L->L M->M
+merged D into C
+ A->F B->B C->C D->C E->F F->F G->G H->H I->I J->J K->B L->L M->M
+merged M into C
+ A->F B->B C->C D->C E->F F->F G->G H->H I->I J->J K->B L->L M->C
+merged J into B
+ A->F B->B C->C D->C E->F F->F G->G H->H I->I J->B K->B L->L M->C
+merged B into C
+ A->F B->C C->C D->C E->F F->F G->G H->H I->I J->C K->C L->L M->C
+merged F into G
+ A->G B->C C->C D->C E->G F->G G->G H->H I->I J->C K->C L->L M->C
+merged L into C
+ A->G B->C C->C D->C E->G F->G G->G H->H I->I J->C K->C L->C M->C
+merged G into I
+ A->I B->C C->C D->C E->I F->I G->I H->H I->I J->C K->C L->C M->C
+merged I into H
+ A->H B->C C->C D->C E->H F->H G->H H->H I->H J->C K->C L->C M->C
+merged C into H
+ A->H B->H C->H D->H E->H F->H G->H H->H I->H J->H K->H L->H M->H
+
+"""
+# Emacs turd '
+
+# Fun tests (for sufficiently warped notions of "fun").
+
+fun_tests = """
+
+Build up to a recursive Sieve of Eratosthenes generator.
+
+>>> def firstn(g, n):
+...     return [next(g) for i in range(n)]
+
+>>> def intsfrom(i):
+...     while 1:
+...         yield i
+...         i += 1
+
+>>> firstn(intsfrom(5), 7)
+[5, 6, 7, 8, 9, 10, 11]
+
+>>> def exclude_multiples(n, ints):
+...     for i in ints:
+...         if i % n:
+...             yield i
+
+>>> firstn(exclude_multiples(3, intsfrom(1)), 6)
+[1, 2, 4, 5, 7, 8]
+
+>>> def sieve(ints):
+...     prime = next(ints)
+...     yield prime
+...     not_divisible_by_prime = exclude_multiples(prime, ints)
+...     for p in sieve(not_divisible_by_prime):
+...         yield p
+
+>>> primes = sieve(intsfrom(2))
+>>> firstn(primes, 20)
+[2, 3, 5, 7, 11, 13, 17, 19, 23, 29, 31, 37, 41, 43, 47, 53, 59, 61, 67, 71]
+
+
+Another famous problem:  generate all integers of the form
+    2**i * 3**j  * 5**k
+in increasing order, where i,j,k >= 0.  Trickier than it may look at first!
+Try writing it without generators, and correctly, and without generating
+3 internal results for each result output.
+
+>>> def times(n, g):
+...     for i in g:
+...         yield n * i
+>>> firstn(times(10, intsfrom(1)), 10)
+[10, 20, 30, 40, 50, 60, 70, 80, 90, 100]
+
+>>> def merge(g, h):
+...     ng = next(g)
+...     nh = next(h)
+...     while 1:
+...         if ng < nh:
+...             yield ng
+...             ng = next(g)
+...         elif ng > nh:
+...             yield nh
+...             nh = next(h)
+...         else:
+...             yield ng
+...             ng = next(g)
+...             nh = next(h)
+
+The following works, but is doing a whale of a lot of redundant work --
+it's not clear how to get the internal uses of m235 to share a single
+generator.  Note that me_times2 (etc) each need to see every element in the
+result sequence.  So this is an example where lazy lists are more natural
+(you can look at the head of a lazy list any number of times).
+
+>>> def m235():
+...     yield 1
+...     me_times2 = times(2, m235())
+...     me_times3 = times(3, m235())
+...     me_times5 = times(5, m235())
+...     for i in merge(merge(me_times2,
+...                          me_times3),
+...                    me_times5):
+...         yield i
+
+Don't print "too many" of these -- the implementation above is extremely
+inefficient:  each call of m235() leads to 3 recursive calls, and in
+turn each of those 3 more, and so on, and so on, until we've descended
+enough levels to satisfy the print stmts.  Very odd:  when I printed 5
+lines of results below, this managed to screw up Win98's malloc in "the
+usual" way, i.e. the heap grew over 4Mb so Win98 started fragmenting
+address space, and it *looked* like a very slow leak.
+
+>>> result = m235()
+>>> for i in range(3):
+...     print(firstn(result, 15))
+[1, 2, 3, 4, 5, 6, 8, 9, 10, 12, 15, 16, 18, 20, 24]
+[25, 27, 30, 32, 36, 40, 45, 48, 50, 54, 60, 64, 72, 75, 80]
+[81, 90, 96, 100, 108, 120, 125, 128, 135, 144, 150, 160, 162, 180, 192]
+
+Heh.  Here's one way to get a shared list, complete with an excruciating
+namespace renaming trick.  The *pretty* part is that the times() and merge()
+functions can be reused as-is, because they only assume their stream
+arguments are iterable -- a LazyList is the same as a generator to times().
+
+>>> class LazyList:
+...     def __init__(self, g):
+...         self.sofar = []
+...         self.fetch = g.__next__
+...
+...     def __getitem__(self, i):
+...         sofar, fetch = self.sofar, self.fetch
+...         while i >= len(sofar):
+...             sofar.append(fetch())
+...         return sofar[i]
+
+>>> def m235():
+...     yield 1
+...     # Gack:  m235 below actually refers to a LazyList.
+...     me_times2 = times(2, m235)
+...     me_times3 = times(3, m235)
+...     me_times5 = times(5, m235)
+...     for i in merge(merge(me_times2,
+...                          me_times3),
+...                    me_times5):
+...         yield i
+
+Print as many of these as you like -- *this* implementation is memory-
+efficient.
+
+>>> m235 = LazyList(m235())
+>>> for i in range(5):
+...     print([m235[j] for j in range(15*i, 15*(i+1))])
+[1, 2, 3, 4, 5, 6, 8, 9, 10, 12, 15, 16, 18, 20, 24]
+[25, 27, 30, 32, 36, 40, 45, 48, 50, 54, 60, 64, 72, 75, 80]
+[81, 90, 96, 100, 108, 120, 125, 128, 135, 144, 150, 160, 162, 180, 192]
+[200, 216, 225, 240, 243, 250, 256, 270, 288, 300, 320, 324, 360, 375, 384]
+[400, 405, 432, 450, 480, 486, 500, 512, 540, 576, 600, 625, 640, 648, 675]
+
+Ye olde Fibonacci generator, LazyList style.
+
+>>> def fibgen(a, b):
+...
+...     def sum(g, h):
+...         while 1:
+...             yield next(g) + next(h)
+...
+...     def tail(g):
+...         next(g)    # throw first away
+...         for x in g:
+...             yield x
+...
+...     yield a
+...     yield b
+...     for s in sum(iter(fib),
+...                  tail(iter(fib))):
+...         yield s
+
+>>> fib = LazyList(fibgen(1, 2))
+>>> firstn(iter(fib), 17)
+[1, 2, 3, 5, 8, 13, 21, 34, 55, 89, 144, 233, 377, 610, 987, 1597, 2584]
+
+
+Running after your tail with itertools.tee (new in version 2.4)
+
+The algorithms "m235" (Hamming) and Fibonacci presented above are both
+examples of a whole family of FP (functional programming) algorithms
+where a function produces and returns a list while the production algorithm
+suppose the list as already produced by recursively calling itself.
+For these algorithms to work, they must:
+
+- produce at least a first element without presupposing the existence of
+  the rest of the list
+- produce their elements in a lazy manner
+
+To work efficiently, the beginning of the list must not be recomputed over
+and over again. This is ensured in most FP languages as a built-in feature.
+In python, we have to explicitly maintain a list of already computed results
+and abandon genuine recursivity.
+
+This is what had been attempted above with the LazyList class. One problem
+with that class is that it keeps a list of all of the generated results and
+therefore continually grows. This partially defeats the goal of the generator
+concept, viz. produce the results only as needed instead of producing them
+all and thereby wasting memory.
+
+Thanks to itertools.tee, it is now clear "how to get the internal uses of
+m235 to share a single generator".
+
+>>> from itertools import tee
+>>> def m235():
+...     def _m235():
+...         yield 1
+...         for n in merge(times(2, m2),
+...                        merge(times(3, m3),
+...                              times(5, m5))):
+...             yield n
+...     m1 = _m235()
+...     m2, m3, m5, mRes = tee(m1, 4)
+...     return mRes
+
+>>> it = m235()
+>>> for i in range(5):
+...     print(firstn(it, 15))
+[1, 2, 3, 4, 5, 6, 8, 9, 10, 12, 15, 16, 18, 20, 24]
+[25, 27, 30, 32, 36, 40, 45, 48, 50, 54, 60, 64, 72, 75, 80]
+[81, 90, 96, 100, 108, 120, 125, 128, 135, 144, 150, 160, 162, 180, 192]
+[200, 216, 225, 240, 243, 250, 256, 270, 288, 300, 320, 324, 360, 375, 384]
+[400, 405, 432, 450, 480, 486, 500, 512, 540, 576, 600, 625, 640, 648, 675]
+
+The "tee" function does just what we want. It internally keeps a generated
+result for as long as it has not been "consumed" from all of the duplicated
+iterators, whereupon it is deleted. You can therefore print the hamming
+sequence during hours without increasing memory usage, or very little.
+
+The beauty of it is that recursive running-after-their-tail FP algorithms
+are quite straightforwardly expressed with this Python idiom.
+
+Ye olde Fibonacci generator, tee style.
+
+>>> def fib():
+...
+...     def _isum(g, h):
+...         while 1:
+...             yield next(g) + next(h)
+...
+...     def _fib():
+...         yield 1
+...         yield 2
+...         next(fibTail) # throw first away
+...         for res in _isum(fibHead, fibTail):
+...             yield res
+...
+...     realfib = _fib()
+...     fibHead, fibTail, fibRes = tee(realfib, 3)
+...     return fibRes
+
+>>> firstn(fib(), 17)
+[1, 2, 3, 5, 8, 13, 21, 34, 55, 89, 144, 233, 377, 610, 987, 1597, 2584]
+
+"""
+
+# syntax_tests mostly provokes SyntaxErrors.  Also fiddling with #if 0
+# hackery.
+
+syntax_tests = """
+
+These are fine:
+
+>>> def f():
+...     yield 1
+...     return
+
+>>> def f():
+...     try:
+...         yield 1
+...     finally:
+...         pass
+
+>>> def f():
+...     try:
+...         try:
+...             1//0
+...         except ZeroDivisionError:
+...             yield 666
+...         except:
+...             pass
+...     finally:
+...         pass
+
+>>> def f():
+...     try:
+...         try:
+...             yield 12
+...             1//0
+...         except ZeroDivisionError:
+...             yield 666
+...         except:
+...             try:
+...                 x = 12
+...             finally:
+...                 yield 12
+...     except:
+...         return
+>>> list(f())
+[12, 666]
+
+>>> def f():
+...    yield
+>>> type(f())
+<class 'generator'>
+
+
+>>> def f():
+...    if 0:
+...        yield
+>>> type(f())
+<class 'generator'>
+
+
+>>> def f():
+...     if 0:
+...         yield 1
+>>> type(f())
+<class 'generator'>
+
+>>> def f():
+...    if "":
+...        yield None
+>>> type(f())
+<class 'generator'>
+
+>>> def f():
+...     return
+...     try:
+...         if x==4:
+...             pass
+...         elif 0:
+...             try:
+...                 1//0
+...             except SyntaxError:
+...                 pass
+...             else:
+...                 if 0:
+...                     while 12:
+...                         x += 1
+...                         yield 2 # don't blink
+...                         f(a, b, c, d, e)
+...         else:
+...             pass
+...     except:
+...         x = 1
+...     return
+>>> type(f())
+<class 'generator'>
+
+>>> def f():
+...     if 0:
+...         def g():
+...             yield 1
+...
+>>> type(f())
+<class 'NoneType'>
+
+>>> def f():
+...     if 0:
+...         class C:
+...             def __init__(self):
+...                 yield 1
+...             def f(self):
+...                 yield 2
+>>> type(f())
+<class 'NoneType'>
+
+>>> def f():
+...     if 0:
+...         return
+...     if 0:
+...         yield 2
+>>> type(f())
+<class 'generator'>
+
+This one caused a crash (see SF bug 567538):
+
+>>> def f():
+...     for i in range(3):
+...         try:
+...             continue
+...         finally:
+...             yield i
+...
+>>> g = f()
+>>> print(next(g))
+0
+>>> print(next(g))
+1
+>>> print(next(g))
+2
+>>> print(next(g))
+Traceback (most recent call last):
+StopIteration
+
+
+Test the gi_code attribute
+
+>>> def f():
+...     yield 5
+...
+>>> g = f()
+>>> g.gi_code is f.__code__
+True
+>>> next(g)
+5
+>>> next(g)
+Traceback (most recent call last):
+StopIteration
+>>> g.gi_code is f.__code__
+True
+
+
+Test the __name__ attribute and the repr()
+
+>>> def f():
+...    yield 5
+...
+>>> g = f()
+>>> g.__name__
+'f'
+>>> repr(g)  # doctest: +ELLIPSIS
+'<generator object f at ...>'
+
+Lambdas shouldn't have their usual return behavior.
+
+>>> x = lambda: (yield 1)
+>>> list(x())
+[1]
+
+>>> x = lambda: ((yield 1), (yield 2))
+>>> list(x())
+[1, 2]
+"""
+
+# conjoin is a simple backtracking generator, named in honor of Icon's
+# "conjunction" control structure.  Pass a list of no-argument functions
+# that return iterable objects.  Easiest to explain by example:  assume the
+# function list [x, y, z] is passed.  Then conjoin acts like:
+#
+# def g():
+#     values = [None] * 3
+#     for values[0] in x():
+#         for values[1] in y():
+#             for values[2] in z():
+#                 yield values
+#
+# So some 3-lists of values *may* be generated, each time we successfully
+# get into the innermost loop.  If an iterator fails (is exhausted) before
+# then, it "backtracks" to get the next value from the nearest enclosing
+# iterator (the one "to the left"), and starts all over again at the next
+# slot (pumps a fresh iterator).  Of course this is most useful when the
+# iterators have side-effects, so that which values *can* be generated at
+# each slot depend on the values iterated at previous slots.
+
+def simple_conjoin(gs):
+
+    values = [None] * len(gs)
+
+    def gen(i):
+        if i >= len(gs):
+            yield values
+        else:
+            for values[i] in gs[i]():
+                for x in gen(i+1):
+                    yield x
+
+    for x in gen(0):
+        yield x
+
+# That works fine, but recursing a level and checking i against len(gs) for
+# each item produced is inefficient.  By doing manual loop unrolling across
+# generator boundaries, it's possible to eliminate most of that overhead.
+# This isn't worth the bother *in general* for generators, but conjoin() is
+# a core building block for some CPU-intensive generator applications.
+
+def conjoin(gs):
+
+    n = len(gs)
+    values = [None] * n
+
+    # Do one loop nest at time recursively, until the # of loop nests
+    # remaining is divisible by 3.
+
+    def gen(i):
+        if i >= n:
+            yield values
+
+        elif (n-i) % 3:
+            ip1 = i+1
+            for values[i] in gs[i]():
+                for x in gen(ip1):
+                    yield x
+
+        else:
+            for x in _gen3(i):
+                yield x
+
+    # Do three loop nests at a time, recursing only if at least three more
+    # remain.  Don't call directly:  this is an internal optimization for
+    # gen's use.
+
+    def _gen3(i):
+        assert i < n and (n-i) % 3 == 0
+        ip1, ip2, ip3 = i+1, i+2, i+3
+        g, g1, g2 = gs[i : ip3]
+
+        if ip3 >= n:
+            # These are the last three, so we can yield values directly.
+            for values[i] in g():
+                for values[ip1] in g1():
+                    for values[ip2] in g2():
+                        yield values
+
+        else:
+            # At least 6 loop nests remain; peel off 3 and recurse for the
+            # rest.
+            for values[i] in g():
+                for values[ip1] in g1():
+                    for values[ip2] in g2():
+                        for x in _gen3(ip3):
+                            yield x
+
+    for x in gen(0):
+        yield x
+
+# And one more approach:  For backtracking apps like the Knight's Tour
+# solver below, the number of backtracking levels can be enormous (one
+# level per square, for the Knight's Tour, so that e.g. a 100x100 board
+# needs 10,000 levels).  In such cases Python is likely to run out of
+# stack space due to recursion.  So here's a recursion-free version of
+# conjoin too.
+# NOTE WELL:  This allows large problems to be solved with only trivial
+# demands on stack space.  Without explicitly resumable generators, this is
+# much harder to achieve.  OTOH, this is much slower (up to a factor of 2)
+# than the fancy unrolled recursive conjoin.
+
+def flat_conjoin(gs):  # rename to conjoin to run tests with this instead
+    n = len(gs)
+    values = [None] * n
+    iters  = [None] * n
+    _StopIteration = StopIteration  # make local because caught a *lot*
+    i = 0
+    while 1:
+        # Descend.
+        try:
+            while i < n:
+                it = iters[i] = gs[i]().__next__
+                values[i] = it()
+                i += 1
+        except _StopIteration:
+            pass
+        else:
+            assert i == n
+            yield values
+
+        # Backtrack until an older iterator can be resumed.
+        i -= 1
+        while i >= 0:
+            try:
+                values[i] = iters[i]()
+                # Success!  Start fresh at next level.
+                i += 1
+                break
+            except _StopIteration:
+                # Continue backtracking.
+                i -= 1
+        else:
+            assert i < 0
+            break
+
+# A conjoin-based N-Queens solver.
+
+class Queens:
+    def __init__(self, n):
+        self.n = n
+        rangen = range(n)
+
+        # Assign a unique int to each column and diagonal.
+        # columns:  n of those, range(n).
+        # NW-SE diagonals: 2n-1 of these, i-j unique and invariant along
+        # each, smallest i-j is 0-(n-1) = 1-n, so add n-1 to shift to 0-
+        # based.
+        # NE-SW diagonals: 2n-1 of these, i+j unique and invariant along
+        # each, smallest i+j is 0, largest is 2n-2.
+
+        # For each square, compute a bit vector of the columns and
+        # diagonals it covers, and for each row compute a function that
+        # generates the possibilities for the columns in that row.
+        self.rowgenerators = []
+        for i in rangen:
+            rowuses = [(1 << j) |                  # column ordinal
+                       (1 << (n + i-j + n-1)) |    # NW-SE ordinal
+                       (1 << (n + 2*n-1 + i+j))    # NE-SW ordinal
+                            for j in rangen]
+
+            def rowgen(rowuses=rowuses):
+                for j in rangen:
+                    uses = rowuses[j]
+                    if uses & self.used == 0:
+                        self.used |= uses
+                        yield j
+                        self.used &= ~uses
+
+            self.rowgenerators.append(rowgen)
+
+    # Generate solutions.
+    def solve(self):
+        self.used = 0
+        for row2col in conjoin(self.rowgenerators):
+            yield row2col
+
+    def printsolution(self, row2col):
+        n = self.n
+        assert n == len(row2col)
+        sep = "+" + "-+" * n
+        print(sep)
+        for i in range(n):
+            squares = [" " for j in range(n)]
+            squares[row2col[i]] = "Q"
+            print("|" + "|".join(squares) + "|")
+            print(sep)
+
+# A conjoin-based Knight's Tour solver.  This is pretty sophisticated
+# (e.g., when used with flat_conjoin above, and passing hard=1 to the
+# constructor, a 200x200 Knight's Tour was found quickly -- note that we're
+# creating 10s of thousands of generators then!), and is lengthy.
+
+class Knights:
+    def __init__(self, m, n, hard=0):
+        self.m, self.n = m, n
+
+        # solve() will set up succs[i] to be a list of square #i's
+        # successors.
+        succs = self.succs = []
+
+        # Remove i0 from each of its successor's successor lists, i.e.
+        # successors can't go back to i0 again.  Return 0 if we can
+        # detect this makes a solution impossible, else return 1.
+
+        def remove_from_successors(i0, len=len):
+            # If we remove all exits from a free square, we're dead:
+            # even if we move to it next, we can't leave it again.
+            # If we create a square with one exit, we must visit it next;
+            # else somebody else will have to visit it, and since there's
+            # only one adjacent, there won't be a way to leave it again.
+            # Finally, if we create more than one free square with a
+            # single exit, we can only move to one of them next, leaving
+            # the other one a dead end.
+            ne0 = ne1 = 0
+            for i in succs[i0]:
+                s = succs[i]
+                s.remove(i0)
+                e = len(s)
+                if e == 0:
+                    ne0 += 1
+                elif e == 1:
+                    ne1 += 1
+            return ne0 == 0 and ne1 < 2
+
+        # Put i0 back in each of its successor's successor lists.
+
+        def add_to_successors(i0):
+            for i in succs[i0]:
+                succs[i].append(i0)
+
+        # Generate the first move.
+        def first():
+            if m < 1 or n < 1:
+                return
+
+            # Since we're looking for a cycle, it doesn't matter where we
+            # start.  Starting in a corner makes the 2nd move easy.
+            corner = self.coords2index(0, 0)
+            remove_from_successors(corner)
+            self.lastij = corner
+            yield corner
+            add_to_successors(corner)
+
+        # Generate the second moves.
+        def second():
+            corner = self.coords2index(0, 0)
+            assert self.lastij == corner  # i.e., we started in the corner
+            if m < 3 or n < 3:
+                return
+            assert len(succs[corner]) == 2
+            assert self.coords2index(1, 2) in succs[corner]
+            assert self.coords2index(2, 1) in succs[corner]
+            # Only two choices.  Whichever we pick, the other must be the
+            # square picked on move m*n, as it's the only way to get back
+            # to (0, 0).  Save its index in self.final so that moves before
+            # the last know it must be kept free.
+            for i, j in (1, 2), (2, 1):
+                this  = self.coords2index(i, j)
+                final = self.coords2index(3-i, 3-j)
+                self.final = final
+
+                remove_from_successors(this)
+                succs[final].append(corner)
+                self.lastij = this
+                yield this
+                succs[final].remove(corner)
+                add_to_successors(this)
+
+        # Generate moves 3 through m*n-1.
+        def advance(len=len):
+            # If some successor has only one exit, must take it.
+            # Else favor successors with fewer exits.
+            candidates = []
+            for i in succs[self.lastij]:
+                e = len(succs[i])
+                assert e > 0, "else remove_from_successors() pruning flawed"
+                if e == 1:
+                    candidates = [(e, i)]
+                    break
+                candidates.append((e, i))
+            else:
+                candidates.sort()
+
+            for e, i in candidates:
+                if i != self.final:
+                    if remove_from_successors(i):
+                        self.lastij = i
+                        yield i
+                    add_to_successors(i)
+
+        # Generate moves 3 through m*n-1.  Alternative version using a
+        # stronger (but more expensive) heuristic to order successors.
+        # Since the # of backtracking levels is m*n, a poor move early on
+        # can take eons to undo.  Smallest square board for which this
+        # matters a lot is 52x52.
+        def advance_hard(vmid=(m-1)/2.0, hmid=(n-1)/2.0, len=len):
+            # If some successor has only one exit, must take it.
+            # Else favor successors with fewer exits.
+            # Break ties via max distance from board centerpoint (favor
+            # corners and edges whenever possible).
+            candidates = []
+            for i in succs[self.lastij]:
+                e = len(succs[i])
+                assert e > 0, "else remove_from_successors() pruning flawed"
+                if e == 1:
+                    candidates = [(e, 0, i)]
+                    break
+                i1, j1 = self.index2coords(i)
+                d = (i1 - vmid)**2 + (j1 - hmid)**2
+                candidates.append((e, -d, i))
+            else:
+                candidates.sort()
+
+            for e, d, i in candidates:
+                if i != self.final:
+                    if remove_from_successors(i):
+                        self.lastij = i
+                        yield i
+                    add_to_successors(i)
+
+        # Generate the last move.
+        def last():
+            assert self.final in succs[self.lastij]
+            yield self.final
+
+        if m*n < 4:
+            self.squaregenerators = [first]
+        else:
+            self.squaregenerators = [first, second] + \
+                [hard and advance_hard or advance] * (m*n - 3) + \
+                [last]
+
+    def coords2index(self, i, j):
+        assert 0 <= i < self.m
+        assert 0 <= j < self.n
+        return i * self.n + j
+
+    def index2coords(self, index):
+        assert 0 <= index < self.m * self.n
+        return divmod(index, self.n)
+
+    def _init_board(self):
+        succs = self.succs
+        del succs[:]
+        m, n = self.m, self.n
+        c2i = self.coords2index
+
+        offsets = [( 1,  2), ( 2,  1), ( 2, -1), ( 1, -2),
+                   (-1, -2), (-2, -1), (-2,  1), (-1,  2)]
+        rangen = range(n)
+        for i in range(m):
+            for j in rangen:
+                s = [c2i(i+io, j+jo) for io, jo in offsets
+                                     if 0 <= i+io < m and
+                                        0 <= j+jo < n]
+                succs.append(s)
+
+    # Generate solutions.
+    def solve(self):
+        self._init_board()
+        for x in conjoin(self.squaregenerators):
+            yield x
+
+    def printsolution(self, x):
+        m, n = self.m, self.n
+        assert len(x) == m*n
+        w = len(str(m*n))
+        format = "%" + str(w) + "d"
+
+        squares = [[None] * n for i in range(m)]
+        k = 1
+        for i in x:
+            i1, j1 = self.index2coords(i)
+            squares[i1][j1] = format % k
+            k += 1
+
+        sep = "+" + ("-" * w + "+") * n
+        print(sep)
+        for i in range(m):
+            row = squares[i]
+            print("|" + "|".join(row) + "|")
+            print(sep)
+
+conjoin_tests = """
+
+Generate the 3-bit binary numbers in order.  This illustrates dumbest-
+possible use of conjoin, just to generate the full cross-product.
+
+>>> for c in conjoin([lambda: iter((0, 1))] * 3):
+...     print(c)
+[0, 0, 0]
+[0, 0, 1]
+[0, 1, 0]
+[0, 1, 1]
+[1, 0, 0]
+[1, 0, 1]
+[1, 1, 0]
+[1, 1, 1]
+
+For efficiency in typical backtracking apps, conjoin() yields the same list
+object each time.  So if you want to save away a full account of its
+generated sequence, you need to copy its results.
+
+>>> def gencopy(iterator):
+...     for x in iterator:
+...         yield x[:]
+
+>>> for n in range(10):
+...     all = list(gencopy(conjoin([lambda: iter((0, 1))] * n)))
+...     print(n, len(all), all[0] == [0] * n, all[-1] == [1] * n)
+0 1 True True
+1 2 True True
+2 4 True True
+3 8 True True
+4 16 True True
+5 32 True True
+6 64 True True
+7 128 True True
+8 256 True True
+9 512 True True
+
+And run an 8-queens solver.
+
+>>> q = Queens(8)
+>>> LIMIT = 2
+>>> count = 0
+>>> for row2col in q.solve():
+...     count += 1
+...     if count <= LIMIT:
+...         print("Solution", count)
+...         q.printsolution(row2col)
+Solution 1
++-+-+-+-+-+-+-+-+
+|Q| | | | | | | |
++-+-+-+-+-+-+-+-+
+| | | | |Q| | | |
++-+-+-+-+-+-+-+-+
+| | | | | | | |Q|
++-+-+-+-+-+-+-+-+
+| | | | | |Q| | |
++-+-+-+-+-+-+-+-+
+| | |Q| | | | | |
++-+-+-+-+-+-+-+-+
+| | | | | | |Q| |
++-+-+-+-+-+-+-+-+
+| |Q| | | | | | |
++-+-+-+-+-+-+-+-+
+| | | |Q| | | | |
++-+-+-+-+-+-+-+-+
+Solution 2
++-+-+-+-+-+-+-+-+
+|Q| | | | | | | |
++-+-+-+-+-+-+-+-+
+| | | | | |Q| | |
++-+-+-+-+-+-+-+-+
+| | | | | | | |Q|
++-+-+-+-+-+-+-+-+
+| | |Q| | | | | |
++-+-+-+-+-+-+-+-+
+| | | | | | |Q| |
++-+-+-+-+-+-+-+-+
+| | | |Q| | | | |
++-+-+-+-+-+-+-+-+
+| |Q| | | | | | |
++-+-+-+-+-+-+-+-+
+| | | | |Q| | | |
++-+-+-+-+-+-+-+-+
+
+>>> print(count, "solutions in all.")
+92 solutions in all.
+
+And run a Knight's Tour on a 10x10 board.  Note that there are about
+20,000 solutions even on a 6x6 board, so don't dare run this to exhaustion.
+
+>>> k = Knights(10, 10)
+>>> LIMIT = 2
+>>> count = 0
+>>> for x in k.solve():
+...     count += 1
+...     if count <= LIMIT:
+...         print("Solution", count)
+...         k.printsolution(x)
+...     else:
+...         break
+Solution 1
++---+---+---+---+---+---+---+---+---+---+
+|  1| 58| 27| 34|  3| 40| 29| 10|  5|  8|
++---+---+---+---+---+---+---+---+---+---+
+| 26| 35|  2| 57| 28| 33|  4|  7| 30| 11|
++---+---+---+---+---+---+---+---+---+---+
+| 59|100| 73| 36| 41| 56| 39| 32|  9|  6|
++---+---+---+---+---+---+---+---+---+---+
+| 74| 25| 60| 55| 72| 37| 42| 49| 12| 31|
++---+---+---+---+---+---+---+---+---+---+
+| 61| 86| 99| 76| 63| 52| 47| 38| 43| 50|
++---+---+---+---+---+---+---+---+---+---+
+| 24| 75| 62| 85| 54| 71| 64| 51| 48| 13|
++---+---+---+---+---+---+---+---+---+---+
+| 87| 98| 91| 80| 77| 84| 53| 46| 65| 44|
++---+---+---+---+---+---+---+---+---+---+
+| 90| 23| 88| 95| 70| 79| 68| 83| 14| 17|
++---+---+---+---+---+---+---+---+---+---+
+| 97| 92| 21| 78| 81| 94| 19| 16| 45| 66|
++---+---+---+---+---+---+---+---+---+---+
+| 22| 89| 96| 93| 20| 69| 82| 67| 18| 15|
++---+---+---+---+---+---+---+---+---+---+
+Solution 2
++---+---+---+---+---+---+---+---+---+---+
+|  1| 58| 27| 34|  3| 40| 29| 10|  5|  8|
++---+---+---+---+---+---+---+---+---+---+
+| 26| 35|  2| 57| 28| 33|  4|  7| 30| 11|
++---+---+---+---+---+---+---+---+---+---+
+| 59|100| 73| 36| 41| 56| 39| 32|  9|  6|
++---+---+---+---+---+---+---+---+---+---+
+| 74| 25| 60| 55| 72| 37| 42| 49| 12| 31|
++---+---+---+---+---+---+---+---+---+---+
+| 61| 86| 99| 76| 63| 52| 47| 38| 43| 50|
++---+---+---+---+---+---+---+---+---+---+
+| 24| 75| 62| 85| 54| 71| 64| 51| 48| 13|
++---+---+---+---+---+---+---+---+---+---+
+| 87| 98| 89| 80| 77| 84| 53| 46| 65| 44|
++---+---+---+---+---+---+---+---+---+---+
+| 90| 23| 92| 95| 70| 79| 68| 83| 14| 17|
++---+---+---+---+---+---+---+---+---+---+
+| 97| 88| 21| 78| 81| 94| 19| 16| 45| 66|
++---+---+---+---+---+---+---+---+---+---+
+| 22| 91| 96| 93| 20| 69| 82| 67| 18| 15|
++---+---+---+---+---+---+---+---+---+---+
+"""
+
+weakref_tests = """\
+Generators are weakly referencable:
+
+>>> import weakref
+>>> def gen():
+...     yield 'foo!'
+...
+>>> wr = weakref.ref(gen)
+>>> wr() is gen
+True
+>>> p = weakref.proxy(gen)
+
+Generator-iterators are weakly referencable as well:
+
+>>> gi = gen()
+>>> wr = weakref.ref(gi)
+>>> wr() is gi
+True
+>>> p = weakref.proxy(gi)
+>>> list(p)
+['foo!']
+
+"""
+
+coroutine_tests = """\
+>>> from test.support import gc_collect
+
+Sending a value into a started generator:
+
+>>> def f():
+...     print((yield 1))
+...     yield 2
+>>> g = f()
+>>> next(g)
+1
+>>> g.send(42)
+42
+2
+
+Sending a value into a new generator produces a TypeError:
+
+>>> f().send("foo")
+Traceback (most recent call last):
+...
+TypeError: can't send non-None value to a just-started generator
+
+
+Yield by itself yields None:
+
+>>> def f(): yield
+>>> list(f())
+[None]
+
+
+Yield is allowed only in the outermost iterable in generator expression:
+
+>>> def f(): list(i for i in [(yield 26)])
+>>> type(f())
+<class 'generator'>
+
+
+A yield expression with augmented assignment.
+
+>>> def coroutine(seq):
+...     count = 0
+...     while count < 200:
+...         count += yield
+...         seq.append(count)
+>>> seq = []
+>>> c = coroutine(seq)
+>>> next(c)
+>>> print(seq)
+[]
+>>> c.send(10)
+>>> print(seq)
+[10]
+>>> c.send(10)
+>>> print(seq)
+[10, 20]
+>>> c.send(10)
+>>> print(seq)
+[10, 20, 30]
+
+
+Check some syntax errors for yield expressions:
+
+>>> f=lambda: (yield 1),(yield 2)
+Traceback (most recent call last):
+  ...
+SyntaxError: 'yield' outside function
+
+# Pegen does not produce this error message yet
+# >>> def f(): x = yield = y
+# Traceback (most recent call last):
+#   ...
+# SyntaxError: assignment to yield expression not possible
+
+>>> def f(): (yield bar) = y
+Traceback (most recent call last):
+  ...
+SyntaxError: cannot assign to yield expression here. Maybe you meant '==' instead of '='?
+
+>>> def f(): (yield bar) += y
+Traceback (most recent call last):
+  ...
+SyntaxError: 'yield expression' is an illegal expression for augmented assignment
+
+
+Now check some throw() conditions:
+
+>>> def f():
+...     while True:
+...         try:
+...             print((yield))
+...         except ValueError as v:
+...             print("caught ValueError (%s)" % (v))
+>>> import sys
+>>> g = f()
+>>> next(g)
+
+>>> g.throw(ValueError) # type only
+caught ValueError ()
+
+>>> g.throw(ValueError("xyz"))  # value only
+caught ValueError (xyz)
+
+>>> import warnings
+>>> warnings.filterwarnings("ignore", category=DeprecationWarning)
+
+# Filter DeprecationWarning: regarding the (type, val, tb) signature of throw().
+# Deprecation warnings are re-enabled below.
+
+>>> g.throw(ValueError, ValueError(1))   # value+matching type
+caught ValueError (1)
+
+>>> g.throw(ValueError, TypeError(1))  # mismatched type, rewrapped
+caught ValueError (1)
+
+>>> g.throw(ValueError, ValueError(1), None)   # explicit None traceback
+caught ValueError (1)
+
+>>> g.throw(ValueError(1), "foo")       # bad args
+Traceback (most recent call last):
+  ...
+TypeError: instance exception may not have a separate value
+
+>>> g.throw(ValueError, "foo", 23)      # bad args
+Traceback (most recent call last):
+  ...
+TypeError: throw() third argument must be a traceback object
+
+>>> g.throw("abc")
+Traceback (most recent call last):
+  ...
+TypeError: exceptions must be classes or instances deriving from BaseException, not str
+
+>>> g.throw(0)
+Traceback (most recent call last):
+  ...
+TypeError: exceptions must be classes or instances deriving from BaseException, not int
+
+>>> g.throw(list)
+Traceback (most recent call last):
+  ...
+TypeError: exceptions must be classes or instances deriving from BaseException, not type
+
+>>> def throw(g,exc):
+...     try:
+...         raise exc
+...     except:
+...         g.throw(*sys.exc_info())
+>>> throw(g,ValueError) # do it with traceback included
+caught ValueError ()
+
+>>> g.send(1)
+1
+
+>>> throw(g,TypeError)  # terminate the generator
+Traceback (most recent call last):
+  ...
+TypeError
+
+>>> print(g.gi_frame)
+None
+
+>>> g.send(2)
+Traceback (most recent call last):
+  ...
+StopIteration
+
+>>> g.throw(ValueError,6)       # throw on closed generator
+Traceback (most recent call last):
+  ...
+ValueError: 6
+
+>>> f().throw(ValueError,7)     # throw on just-opened generator
+Traceback (most recent call last):
+  ...
+ValueError: 7
+
+>>> warnings.filters.pop(0)
+('ignore', None, <class 'DeprecationWarning'>, None, 0)
+
+# Re-enable DeprecationWarning: the (type, val, tb) exception representation is deprecated,
+#                               and may be removed in a future version of Python.
+
+Plain "raise" inside a generator should preserve the traceback (#13188).
+The traceback should have 3 levels:
+- g.throw()
+- f()
+- 1/0
+
+>>> def f():
+...     try:
+...         yield
+...     except:
+...         raise
+>>> g = f()
+>>> try:
+...     1/0
+... except ZeroDivisionError as v:
+...     try:
+...         g.throw(v)
+...     except Exception as w:
+...         tb = w.__traceback__
+>>> levels = 0
+>>> while tb:
+...     levels += 1
+...     tb = tb.tb_next
+>>> levels
+3
+
+Now let's try closing a generator:
+
+>>> def f():
+...     try: yield
+...     except GeneratorExit:
+...         print("exiting")
+
+>>> g = f()
+>>> next(g)
+>>> g.close()
+exiting
+>>> g.close()  # should be no-op now
+
+>>> f().close()  # close on just-opened generator should be fine
+
+>>> def f(): yield      # an even simpler generator
+>>> f().close()         # close before opening
+>>> g = f()
+>>> next(g)
+>>> g.close()           # close normally
+
+And finalization:
+
+>>> def f():
+...     try: yield
+...     finally:
+...         print("exiting")
+
+>>> g = f()
+>>> next(g)
+>>> del g; gc_collect()  # For PyPy or other GCs.
+exiting
+
+
+GeneratorExit is not caught by except Exception:
+
+>>> def f():
+...     try: yield
+...     except Exception:
+...         print('except')
+...     finally:
+...         print('finally')
+
+>>> g = f()
+>>> next(g)
+>>> del g; gc_collect()  # For PyPy or other GCs.
+finally
+
+
+Now let's try some ill-behaved generators:
+
+>>> def f():
+...     try: yield
+...     except GeneratorExit:
+...         yield "foo!"
+>>> g = f()
+>>> next(g)
+>>> g.close()
+Traceback (most recent call last):
+  ...
+RuntimeError: generator ignored GeneratorExit
+>>> g.close()
+
+
+Our ill-behaved code should be invoked during GC:
+
+>>> with support.catch_unraisable_exception() as cm:
+...     g = f()
+...     next(g)
+...     del g
+...
+...     cm.unraisable.exc_type == RuntimeError
+...     "generator ignored GeneratorExit" in str(cm.unraisable.exc_value)
+...     cm.unraisable.exc_traceback is not None
+True
+True
+True
+
+And errors thrown during closing should propagate:
+
+>>> def f():
+...     try: yield
+...     except GeneratorExit:
+...         raise TypeError("fie!")
+>>> g = f()
+>>> next(g)
+>>> g.close()
+Traceback (most recent call last):
+  ...
+TypeError: fie!
+
+
+Ensure that various yield expression constructs make their
+enclosing function a generator:
+
+>>> def f(): x += yield
+>>> type(f())
+<class 'generator'>
+
+>>> def f(): x = yield
+>>> type(f())
+<class 'generator'>
+
+>>> def f(): lambda x=(yield): 1
+>>> type(f())
+<class 'generator'>
+
+>>> def f(d): d[(yield "a")] = d[(yield "b")] = 27
+>>> data = [1,2]
+>>> g = f(data)
+>>> type(g)
+<class 'generator'>
+>>> g.send(None)
+'a'
+>>> data
+[1, 2]
+>>> g.send(0)
+'b'
+>>> data
+[27, 2]
+>>> try: g.send(1)
+... except StopIteration: pass
+>>> data
+[27, 27]
+
+"""
+
+refleaks_tests = """
+Prior to adding cycle-GC support to itertools.tee, this code would leak
+references. We add it to the standard suite so the routine refleak-tests
+would trigger if it starts being uncleanable again.
+
+>>> import itertools
+>>> def leak():
+...     class gen:
+...         def __iter__(self):
+...             return self
+...         def __next__(self):
+...             return self.item
+...     g = gen()
+...     head, tail = itertools.tee(g)
+...     g.item = head
+...     return head
+>>> it = leak()
+
+Make sure to also test the involvement of the tee-internal teedataobject,
+which stores returned items.
+
+>>> item = next(it)
+
+
+
+This test leaked at one point due to generator finalization/destruction.
+It was copied from Lib/test/leakers/test_generator_cycle.py before the file
+was removed.
+
+>>> def leak():
+...    def gen():
+...        while True:
+...            yield g
+...    g = gen()
+
+>>> leak()
+
+
+
+This test isn't really generator related, but rather exception-in-cleanup
+related. The coroutine tests (above) just happen to cause an exception in
+the generator's __del__ (tp_del) method. We can also test for this
+explicitly, without generators. We do have to redirect stderr to avoid
+printing warnings and to doublecheck that we actually tested what we wanted
+to test.
+
+>>> from test import support
+>>> class Leaker:
+...     def __del__(self):
+...         def invoke(message):
+...             raise RuntimeError(message)
+...         invoke("del failed")
+...
+>>> with support.catch_unraisable_exception() as cm:
+...     l = Leaker()
+...     del l
+...
+...     cm.unraisable.object == Leaker.__del__
+...     cm.unraisable.exc_type == RuntimeError
+...     str(cm.unraisable.exc_value) == "del failed"
+...     cm.unraisable.exc_traceback is not None
+True
+True
+True
+True
+
+
+These refleak tests should perhaps be in a testfile of their own,
+test_generators just happened to be the test that drew these out.
+
+"""
+
+__test__ = {"tut":      tutorial_tests,
+            "pep":      pep_tests,
+            "email":    email_tests,
+            "fun":      fun_tests,
+            "syntax":   syntax_tests,
+            "conjoin":  conjoin_tests,
+            "weakref":  weakref_tests,
+            "coroutine":  coroutine_tests,
+            "refleaks": refleaks_tests,
+            }
+
+def load_tests(loader, tests, pattern):
+    tests.addTest(doctest.DocTestSuite())
+    return tests
+
+
+if __name__ == "__main__":
+    unittest.main()
diff --git a/crates/ruff/src/linter.rs b/crates/ruff/src/linter.rs
index e93fd782f8..2af63d639d 100644
--- a/crates/ruff/src/linter.rs
+++ b/crates/ruff/src/linter.rs
@@ -393,6 +393,7 @@ pub fn lint_fix<'a>(
             // longer parseable on a subsequent pass, then we've introduced a
             // syntax error. Return the original code.
             if parseable && result.error.is_some() {
+                println!("{}", transformed);
                 #[allow(clippy::print_stderr)]
                 {
                     eprintln!(
diff --git a/foo.py b/foo.py
new file mode 100644
index 0000000000..853bbdbebc
--- /dev/null
+++ b/foo.py
@@ -0,0 +1,21 @@
+def f():
+    with Nested(m) as (foo):
+        pass
+
+def f():
+    # Issue #23192: Test that a lambda returning a generator behaves
+    # like the equivalent function
+    f = lambda: (yield 1)
+    def g(): return (yield 1)
+
+    # test 'yield from'
+    f2 = lambda: (yield from g())
+    def g2(): return (yield from g())
+
+    f3 = lambda: (yield from f())
+
+def f():
+    # Remove `toplevel`.
+    toplevel = tt = lexer.get_token()
+    if not tt:
+        break