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the possibility that isinstance(some_var, (A, B)) causes mypy to forget about a some_var's type #10835
Description
Bug Report
Certain patterns of if isinstance(...) cause mypy to forget that a variable has a particular generic type.
To Reproduce
Run the following Python code through mypy with --strict:
from typing import TypeVar
# context:
T = TypeVar('T')
class A: pass
class B: pass
def function1(obj: T) -> T: # expected behaviour:
if isinstance(obj, A):
pass
elif isinstance(obj, B):
pass
return obj # no problem here
def function2(obj: T) -> T: # unexpected behaviour:
if isinstance(obj, (A, B)):
pass
return obj # mypy has forgotten that obj is of type TThe issue also reproduces with this alternate definition of function2:
def function2(obj: T) -> T: # unexpected behaviour:
if isinstance(obj, A) or isinstance(obj, B):
pass
return obj # mypy has forgotten that obj is of type TExpected Behavior
mypy should issue no errors.
Actual Behavior
<last line of function2>: error: Returning Any from function declared to return "T"
[no-any-return]
return obj # mypy has forgotten that 'obj' is of type 'T'
^
Found 1 error in 1 file (checked 1 source file)
Your Environment
- Mypy version used: mypy 0.910
- Mypy command-line flags:
--strict - Mypy configuration options from
mypy.ini(and other config files): no additional flags are necessary to reproduce the issue - Python version used: CPython 3.8.3 (from pyenv)
- Operating system and version: macOS 10.14
Activity
erictraut commentedon Jul 17, 2021
This appears to be the same issue as #10817.
finite-state-machine commentedon Jul 17, 2021
It could very well be the same root cause; given my lack of knowledge of mypy internals, I can't say. If there is a distinction, it's this: in #10817, it appears that the type of the variable is being positively inferred, overriding the already-known generic type. In this issue, no inference is possible, but the type somehow changes from the generic
TtoAny.finite-state-machine commentedon Jul 17, 2021
I also note that in this issue, testing against one class (at a time) is not sufficient to trigger the problem; testing against two classes is required. (See
function1, which behaves as expected.)finite-state-machine commentedon Dec 8, 2022
This appears to be resolved in 0.991.
https://mypy-play.net/?mypy=latest&python=3.10&gist=c04e5bbd46caaf91e8549f7552b2c074