Variable assignment on module reference reported as "Module has no attribute <variable_name>"

[timmwagener]

Please provide more information to help us understand the issue:

• Problem
  • I am using code to access module level variables through an alternative reference created on import time following this recipe. mypy reports an error on this, which I believe may be unnecessary.
• 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.

# a.py
import sys
this = sys.modules[__name__]
this.serializable_classes = {}  # a.py:15: error: Module has no attribute "serializable_classes"
setattr(this, "serializable_classes", {})  # alternative without mypy error

• What is the actual behavior/output?
  • error: Module has no attribute "serializable_classes"
• What is the behavior/output you expect?
  • No error. It seems mypy applies rules for accessing a name from a module, while it really is an assignment. Why would the previous presence of a variable name in a module namespace matter under these circumstances?
• What are the versions of mypy and Python you are using?

$ tox -e mypy
mypy installed: mypackage==0.0.1,mypy==0.711,mypy-extensions==0.4.1,typed-ast==1.4.0
mypy run-test-pre: PYTHONHASHSEED='349'
mypy run-test: commands[0] | python --version
Python 3.7.3

• What are the mypy flags you are using? (For example --strict-optional)

# mypy.ini
[mypy]
warn_return_any = True
warn_unused_configs = True
# and some import ignores below

• Possibly relates issues
  • mypy fails to recognise assignment as a form of module aliasing #1778

[timmwagener]

Just realizing, there may be no need to assign to this on import time. I guess this should still work:

import sys
this = sys.modules[__name__]
serializable_classes = {}

def set(key, value):
    this.serializable_classes[key] = value

This fixes the error in mypy, however it is still theoretically an unnecessary and misleading error when the syntax from above is used, isn't it?

[ilevkivskyi]

Mypy doesn't support monkey-patching modules and classes. Using setattr() (or a # type: ignore) is a possible work-around.