[red-knot] - Flow-control for boolean operations (#13940)
## Summary
As python uses short-circuiting boolean operations in runtime, we should
mimic that logic in redknot as well.
For example, we should detect that in the following code `x` might be
undefined inside the block:
```py
if flag or (x := 1):
print(x)
```
## Test Plan
Added mdtest suit for boolean expressions.
---------
Co-authored-by: Carl Meyer <carl@astral.sh>
This commit is contained in:
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# Short-Circuit Evaluation
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## Not all boolean expressions must be evaluated
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In `or` expressions, if the left-hand side is truthy, the right-hand side is not evaluated.
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Similarly, in `and` expressions, if the left-hand side is falsy, the right-hand side is not
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evaluated.
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```py
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def bool_instance() -> bool:
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return True
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if bool_instance() or (x := 1):
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# error: [possibly-unresolved-reference]
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reveal_type(x) # revealed: Unbound | Literal[1]
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if bool_instance() and (x := 1):
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# error: [possibly-unresolved-reference]
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reveal_type(x) # revealed: Unbound | Literal[1]
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```
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## First expression is always evaluated
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```py
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def bool_instance() -> bool:
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return True
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if (x := 1) or bool_instance():
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reveal_type(x) # revealed: Literal[1]
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if (x := 1) and bool_instance():
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reveal_type(x) # revealed: Literal[1]
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```
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## Statically known truthiness
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```py
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if True or (x := 1):
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# TODO: infer that the second arm is never executed so type should be just "Unbound".
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# error: [possibly-unresolved-reference]
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reveal_type(x) # revealed: Unbound | Literal[1]
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if True and (x := 1):
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# TODO: infer that the second arm is always executed so type should be just "Literal[1]".
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# error: [possibly-unresolved-reference]
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reveal_type(x) # revealed: Unbound | Literal[1]
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```
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## Later expressions can always use variables from earlier expressions
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```py
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def bool_instance() -> bool:
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return True
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bool_instance() or (x := 1) or reveal_type(x) # revealed: Literal[1]
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# error: [unresolved-reference]
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bool_instance() or reveal_type(y) or (y := 1) # revealed: Unbound
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```
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## Nested expressions
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```py
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def bool_instance() -> bool:
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return True
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if bool_instance() or ((x := 1) and bool_instance()):
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# error: "Name `x` used when possibly not defined"
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reveal_type(x) # revealed: Unbound | Literal[1]
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if ((y := 1) and bool_instance()) or bool_instance():
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reveal_type(y) # revealed: Literal[1]
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# error: [possibly-unresolved-reference]
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if (bool_instance() and (z := 1)) or reveal_type(z): # revealed: Unbound | Literal[1]
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# error: [possibly-unresolved-reference]
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reveal_type(z) # revealed: Unbound | Literal[1]
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```
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