Incompatible types in assignment (expression has type "BinaryIO", variable has type "TextIO")

[jdhao]

Bug Report

mypy reports correct reading of pickle files as error.

To Reproduce

Use the following code:

import json
import pickle

with open('test_ids.json') as f:
    my_ids = json.load(f)

for _id in my_ids:
    id_path = f'{_id}.pkl'

    with open(id_path, 'rb') as f:
        v2_feat = pickle.load(f)

Then use mypy to check the code: mypy test.py

Expected Behavior

No error occurs.

Actual Behavior

I see the following errors:

test.py:10: error: Incompatible types in assignment (expression has type "BinaryIO", variable has type "TextIO")
test.py:11: error: Argument 1 to "load" has incompatible type "TextIO"; expected "IO[bytes]"

Your Environment

• Mypy version used: 0.910
• Mypy command-line flags: none
• Mypy configuration options from mypy.ini (and other config files): none
• Python version used: 3.7.6
• Operating system and version: CentOS 7.4

[emmatyping]

mypy is technically correct here. Note that even after a context manager scope has been left, the variable f is still valid.

with open('test.txt') as f:
    pass
print(f) # works

Therefore, you are actually re-defining the variable f between uses of the context manager.

I do understand this is rather un-intuitive but I'm not sure if there is a good workaround for this pattern.

[erictraut]

I would expect this to work if you specify the --allow-redefinition flag, but it still generates an error.

[AlexWaygood]

Mypy no longer emits an error for this code (I believe this is a result of #12254).