Mypy doesn't recognize dataclass attributes defined in __post_init__

[GlenNicholls]

Bug Report

I think this is somewhat related to #11923, not positive though.

Mypy doesn't seem to recognize dataclass attributes defined in __post_init__ and I'm not sure if this is expected/intentional:

import dataclasses
@dataclasses.dataclass 
class A:
  def __post_init__(self):
    self.a = dict(foo=["a"])

  def blah(self):
    self.a["foo"].append("a")

Expected Behavior

I would expect that this should be fine as the __post_init__ should have enough information for mypy to recognize that the attribute is defined.

Actual Behavior

In member "foo" of class "A":
error: "object" has no attribute "append"  [attr-defined]
            self.a["foo"].append("a")

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

[GlenNicholls]

Bleh, I can't reproduce this as it looks like there's something else going on. I'll re-open once I have a better MWE