MyPy Incorrectly Flags Module Attribute Inside Format String

Bug Report

An unexpected [attr-defined] was reported when accessing module attributes inside of a format string.

To Reproduce

1. Create a file called constants.py with contents below:

FOO = "foo"

2. Create a file called main.py with contents below:

import constants

def bar():
    # type: () -> str
    print("{foo}".format(foo=constants.FOO))
    return "{constants.FOO}".format(constants=constants)

3. run python3 -m mypy constants.py main.py

main.py:6: error: Module has no attribute "FOO"
Found 1 error in 1 file (checked 2 source files)

Expected Behavior

I expected the format string "{constants.FOO}".format(constants=constants) to not throw an error, since constants.FOO is defined.

(Write what you thought would happen.)

Actual Behavior

The line print("{foo}".format(foo=constants.FOO)) reports no error (indicating that the value FOO exists in constants, but the line "{constants.FOO}".format(constants=constants) reports an error as if it does not.

Your Environment

• Mypy version used: 0.790
• Mypy command-line flags: python3 -m mypy constants.py main.py
• Mypy configuration options from mypy.ini (and other config files): No mypy.ini used
• Python version used: Python 3.8.5 (brew installed)
• Operating system and version: MacOS Catalina 10.15.6

Start with the constants.py and main.py reproducer and run python3 -m mypy constants.py main.py using the reported example. Trace how the format-string expression is analyzed, then verify that the module attribute inside the format string no longer produces an attr-defined error while the existing direct access remains valid.