mypy crashes with arguments --ignore-missing-imports --follow-imports skip

[marcharper]

mypy --ignore-missing-imports --follow-imports skip -m my_module

(py3) user@...$ mypy --ignore-missing-imports --follow-imports skip -m axelrod
Traceback (most recent call last):
  File "/home/user/anaconda2/envs/py3/bin/mypy", line 6, in <module>
    main(__file__)
  File "/home/user/anaconda2/envs/py3/lib/python3.5/site-packages/mypy/main.py", line 42, in main
    res = type_check_only(sources, bin_dir, options)
  File "/home/user/anaconda2/envs/py3/lib/python3.5/site-packages/mypy/main.py", line 87, in type_check_only
    options=options)
  File "/home/user/anaconda2/envs/py3/lib/python3.5/site-packages/mypy/build.py", line 183, in build
    dispatch(sources, manager)
  File "/home/user/anaconda2/envs/py3/lib/python3.5/site-packages/mypy/build.py", line 1526, in dispatch
    process_graph(graph, manager)
  File "/home/user/anaconda2/envs/py3/lib/python3.5/site-packages/mypy/build.py", line 1652, in process_graph
    sccs = sorted_components(graph)
  File "/home/user/anaconda2/envs/py3/lib/python3.5/site-packages/mypy/build.py", line 1879, in sorted_components
    for ready in topsort(data):
  File "/home/user/anaconda2/envs/py3/lib/python3.5/site-packages/mypy/build.py", line 1988, in topsort
    for item in set.union(*data.values()) - set(data.keys()):
TypeError: descriptor 'union' of 'set' object needs an argument

Running the same command with individual files in the module works fine (no crash, type errors caught and reported as expected).

[JukkaL]

I can reproduce this:

$ touch b.py
$ mypy --ignore-missing-imports --follow-imports skip -m b

[gvanrossum]

Aha, the problem is that because of the --follow-imports=skip it doesn't want to load b so it ends with an entirely empty graph. The fix will have to involve ensuring that it does follow these kinds of "imports". Stay tuned.

[gvanrossum]

I think I've got a fix, but I need to devise a test.

[gvanrossum]

Closed by #2786.