Mypy doesn't recognize None is checked when using or operator

[GlenNicholls]

Bug Report

When I use or to handle None, mypy doesn't recognize that the code handles the None case and issues errors about accessing attributes:

from typing import Optional

def func(arg: Optional[list] = None) -> list:
  arg = arg or []
  return arg.append(1)

I get a union-attr error for append. To fix this, I can do the following:

if arg is None:
  arg = []

However, I think this is wrong because the code is equivilent. Am I'm missing something here?

Expected Behavior

I would expect that when using arg or [], mypy would recognize that arg is never None after that line (assuming I don't override).

Actual Behavior

error: Item "None" of "Optional[list]" has no attribute "append" [union-attr]

Your Environment

• Mypy version used: 0.971
• Mypy configuration options from pyproject.toml:

  [tool.mypy]
  warn_unused_configs = true
  warn_unused_ignores = true
  install_types = true
  non_interactive = true
  check_untyped_defs = true
  show_error_codes = true
  show_error_context = true
  pretty = true
  color_output = true

• Python version used: 3.9
• Operating system and version: Ubuntu 20.04

[hauntsaninja]

I can't repro, the only error I get is main.py:5: error: "append" of "list" does not return a value [func-returns-value] which is correct (you need to do return arg).

[hauntsaninja]

https://mypy-play.net/?mypy=latest&python=3.10&flags=show-error-codes&gist=8e95bb7e4e869abd005c815fef230232