Simplest list assignment fails

[elazarg]

Adding the following test crashes:

[case testListSimplest]
a = [1]

Results

  ...
  File ".../mypy/mypy/checker.py", line 2238, in lookup_qualified
    return n.names[parts[-1]]
KeyError: 'list'

[Michael0x2a]

This is due to a quirk of mypy's test framework. To speed up the tests and make debugging easier, what mypy's test framework will do is load up a very limited subset of Python's builtins, which, incidentally, doesn't lists.

To fix this, you can include one of the "builtins extension" files in test-data/unit/fixtures to augment the builtins by doing:

[case testListSimplest]
a = [1]
[builtins fixtures/list.py]

You can see examples of test cases doing this inside the check-list.text file. But yeah, it's not a perfect system and can be pretty confusing to debug the first time around -- for more context about this system/suggestions people have made, see #890 and #1675.