Preserve can_be_true and can_be_false when copying types #7991
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This pull request modifies the `last_known_value` eraser so it
always copies over the previously inferred value of the
`can_be_true` and `can_be_false` fields.
This isn't entirely typesafe -- for example, we can sometimes
accidentally infer the wrong result if we try reassigning variables
or call a mutating function :
def bad1(a: List[str], b: bool) -> bool:
x = a and b
a = ["foo"]
y: bool
return x or y # Should be an error
def bad2(a: List[str], b: bool) -> bool:
x = a and b
a.append("foo")
y: bool
return x or y # Should be an error
But this was apparently the old behavior/something mypy wasn't
previously able to detect, so I guess this is fine for now.
I also decided against modifying every `copy_modified` method in
all the type classes because of this reason: I wanted to limit
the spread of this potentially misleading additional inferred info.
Fixes python#7986, probably.
JukkaL
approved these changes
Nov 21, 2019
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Thanks!
Yes, the original behavior that this reverts to isn't type safe. Preserving the old behavior for now seems reasonable to avoid false positives. We can revisit this later and either try to fix the unsafety without introducing false positives, or decide that the false positives are acceptable.
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This pull request modifies the
last_known_valueeraser so it always copies over the previously inferred value of thecan_be_trueandcan_be_falsefields.This isn't entirely typesafe -- for example, we can sometimes accidentally infer the wrong result if we try reassigning variables or call a mutating function :
But this was apparently the old behavior/something mypy wasn't previously able to detect, so I guess this is fine for now.
I also decided against modifying every
copy_modifiedmethod in all the type classes because of this reason: I wanted to limit the spread of this potentially misleading additional inferred info.Fixes #7986, probably.