Allow redefinition doesn't work

[Emerentius]

• Are you reporting a bug, or opening a feature request?
  Bug

• Please insert below the code you are checking with mypy,
  or a mock-up repro if the source is private. We would appreciate
  if you try to simplify your case to a minimal repro.

redefine_me = 1
redefine_me = ""

I've used this mypy.ini:

[mypy]
python_version = 3.5
allow_redefinition = True

but I've also tried it with mypy --allow-redefinition, both with and without the ini-file.

• What is the actual behavior/output?

testme.py:2: error: Incompatible types in assignment (expression has type "str", variable has type "int")

• What is the behavior/output you expect?
  No warning

• What are the versions of mypy and Python you are using?
  Do you see the same issue after installing mypy from Git master?

0.701 and 3.6.7. Still happens with master.

[ilevkivskyi]

This is expected behavior: --allow-redefinition only allows changing type after you have read the variable. For example:

x = 42
x = str(x)  # this is OK because 'x' was used

See also https://mypy.readthedocs.io/en/latest/command_line.html#miscellaneous-strictness-flags

[Emerentius]

The docs say nothing about the read requirement. They say: "only redefinitions within the same block and nesting depth as the original definition are allowed".

Even if this is expected to error, the message is misleading.

[JukkaL]

At least we should mention this in the documentation (#6963).

[abravalheri]

Hi guys, sorry to comment in this closed issue, but I ended up having a similar problem, I will try to describe it bellow:

from typing import AnyStr

def force_bytes(xml: AnyStr) -> bytes:
    if isinstance(xml, str):
        xml = bytes(xml, "utf-8")
    return xml

I run this example directly with mypy --allow-redefinition from the /tmp directory without any setup.cfg or mypi.ini around, and I got the following output:

% mypy --allow-redefinition test.py
test.py:6: error: Incompatible types in assignment (expression has type "bytes", variable has type "str")
test.py:7: error: Incompatible return value type (got "str", expected "bytes")
Found 2 errors in 1 file (checked 1 source file)

I tried running this on both (Python 3.6 + mypy 0.740) and (Python 3.8 + mypy from master)

As you can see in the example, I am consuming the variable, but I think mypy gets a bit confused because of the if statement.

Please let me know if you prefer that I open a different issue for this.

Thank you very much for mypy anyway, it is great and keeps getting better.

[ilevkivskyi]

@abravalheri This is a separate issue being tracked in #6232 and #6233.

[abravalheri]

😄 Thank you very much @ilevkivskyi for pointing me that out, I will follow those issues.