Bug Report

After upgrading to 1.16.1 we are seeing occasional AssertionErrors being logged:

$ mypy
mypy 1.16.1 (compiled: yes)
Exception ignored in PyObject_HasAttr(); consider using PyObject_HasAttrWithError(), PyObject_GetOptionalAttr() or PyObject_GetAttr():
Traceback (most recent call last):
  File "mypy/nodes.py", line 615, in setter
AssertionError: 
... (repeated many times)

It seems like the code introduced in #19248 to fix #19224 makes an assert which is not always true. It seems like the worry expressed in a comment on that PR was justified ;).

To Reproduce

We haven't managed to find a reliable reproducer yet. Switching between branches in our repo and invoking mypy, sometimes triggers it, but sometimes doesn't.

Expected Behavior

No errors.

Actual Behavior

Errors.

Your Environment

• Mypy version used: 1.6.1
• Mypy command-line flags: dmypy run -- --cache-dir .mypy_cache/not_recently_touched <explicitly specified list of .py files>
• Mypy configuration options from mypy.ini (and other config files):

[tool.mypy]
plugins = [
  "pydantic.mypy",
  "pydantic.v1.mypy",
  "sqlalchemy.ext.mypy.plugin",
  "mypy.plugins.proper_plugin", 
  <a custom mypy plugin>
]
python_version = "3.13"
show_error_codes = true
strict = true

enable_error_code = [
  "ignore-without-code",
  "redundant-expr",
  "redundant-self",
  "truthy-bool",
  "truthy-iterable",
]
no_implicit_optional = true
warn_unreachable = true

[[tool.mypy.overrides]]
ignore_missing_imports = true
module = [
    "asyncpg.*",
]

• Python version used:

$ python --version
Python 3.13.3