Crash on double inheritance

[dmoisset]

While writing an iterable file-like class class MyFileLike(object) I was having trouble getting the Iterable protocol to be implicitly recognized. So I decided to add an explicit Iterable parent:

from typing import Iterator, Iterable

class MyFileLike(object, Iterable[str]):
    def readlines(self) -> Iterator[str]:
        return iter(["one", "two", "three"])

    __iter__ = readlines

m = MyFileLike()
it = iter(m)
for l in it: print(l)

That snippet crashes mypy:

Traceback (most recent call last):
  File "/home/machinalis/.virtualenvs/mypy-django/bin/mypy", line 6, in <module>
    exec(compile(open(__file__).read(), __file__, 'exec'))
  File "/home/machinalis/oss/mypy/scripts/mypy", line 6, in <module>
    main(__file__)
  File "/home/machinalis/oss/mypy/mypy/main.py", line 38, in main
    res = type_check_only(sources, bin_dir, options)
  File "/home/machinalis/oss/mypy/mypy/main.py", line 79, in type_check_only
    options=options)
  File "/home/machinalis/oss/mypy/mypy/build.py", line 180, in build
    dispatch(sources, manager)
  File "/home/machinalis/oss/mypy/mypy/build.py", line 1335, in dispatch
    process_graph(graph, manager)
  File "/home/machinalis/oss/mypy/mypy/build.py", line 1478, in process_graph
    process_stale_scc(graph, scc)
  File "/home/machinalis/oss/mypy/mypy/build.py", line 1551, in process_stale_scc
    graph[id].semantic_analysis()
  File "/home/machinalis/oss/mypy/mypy/build.py", line 1300, in semantic_analysis
    self.manager.semantic_analyzer.visit_file(self.tree, self.xpath)
  File "/home/machinalis/oss/mypy/mypy/semanal.py", line 244, in visit_file
    self.accept(d)
  File "/home/machinalis/oss/mypy/mypy/semanal.py", line 2462, in accept
    node.accept(self)
  File "/home/machinalis/oss/mypy/mypy/nodes.py", line 664, in accept
    return visitor.visit_class_def(self)
  File "/home/machinalis/oss/mypy/mypy/semanal.py", line 572, in visit_class_def
    defn.defs.accept(self)
  File "/home/machinalis/oss/mypy/mypy/nodes.py", line 727, in accept
    return visitor.visit_block(self)
  File "/home/machinalis/oss/mypy/mypy/semanal.py", line 1028, in visit_block
    self.accept(s)
  File "/home/machinalis/oss/mypy/mypy/semanal.py", line 2462, in accept
    node.accept(self)
  File "/home/machinalis/oss/mypy/mypy/nodes.py", line 765, in accept
    return visitor.visit_assignment_stmt(self)
  File "/home/machinalis/oss/mypy/mypy/semanal.py", line 1060, in visit_assignment_stmt
    s.rvalue.accept(self)
  File "/home/machinalis/oss/mypy/mypy/nodes.py", line 1116, in accept
    return visitor.visit_name_expr(self)
  File "/home/machinalis/oss/mypy/mypy/semanal.py", line 1906, in visit_name_expr
    n = self.lookup(expr.name, expr)
  File "/home/machinalis/oss/mypy/mypy/semanal.py", line 2250, in lookup
    return self.type[name]
  File "/home/machinalis/oss/mypy/mypy/nodes.py", line 1864, in __getitem__
    raise KeyError(name)
KeyError: 'readlines'

*** INTERNAL ERROR ***

myiter.py:7: error: Internal error -- please report a bug at https://github.com/python/mypy/issues

NOTE: you can use "mypy --pdb ..." to drop into the debugger when this happens.

The crash goes away if I remove the object reference and only inherit from Iterable[str].

[gvanrossum]

Yeah, this is an easy repro. There are too many similar crashes where apparently the mro is used after an error message about it has been produced. :-( In this case, the code first produced the error

error: Cannot determine consistent method resolution order (MRO) for "MyFileLike"

Errors are buffered so you don't see them when there's a crash. (Hmm... Maybe we could fix that!)