AttributeError: 'NoneType' object has no attribute 'bases'

[samuelcolvin]

with mypy-lang==0.3.1. I think this is in an imported module not my code as unset MYPYPATH stops the exception.

Traceback (most recent call last):
  File "/home/samuel/code/harrier/env/bin/mypy", line 6, in <module>
    main(__file__)
  File "/home/samuel/code/harrier/env/lib/python3.5/site-packages/mypy/main.py", line 54, in main
    type_check_only(sources, bin_dir, options)
  File "/home/samuel/code/harrier/env/lib/python3.5/site-packages/mypy/main.py", line 98, in type_check_only
    python_path=options.python_path)
  File "/home/samuel/code/harrier/env/lib/python3.5/site-packages/mypy/build.py", line 206, in build
    result = manager.process(initial_states)
  File "/home/samuel/code/harrier/env/lib/python3.5/site-packages/mypy/build.py", line 403, in process
    next.process()
  File "/home/samuel/code/harrier/env/lib/python3.5/site-packages/mypy/build.py", line 894, in process
    self.semantic_analyzer_pass3().visit_file(self.tree, self.tree.path)
  File "/home/samuel/code/harrier/env/lib/python3.5/site-packages/mypy/semanal.py", line 2341, in visit_file
    file_node.accept(self)
  File "/home/samuel/code/harrier/env/lib/python3.5/site-packages/mypy/nodes.py", line 171, in accept
    return visitor.visit_mypy_file(self)
  File "/home/samuel/code/harrier/env/lib/python3.5/site-packages/mypy/traverser.py", line 34, in visit_mypy_file
    d.accept(self)
  File "/home/samuel/code/harrier/env/lib/python3.5/site-packages/mypy/nodes.py", line 673, in accept
    return visitor.visit_if_stmt(self)
  File "/home/samuel/code/harrier/env/lib/python3.5/site-packages/mypy/traverser.py", line 110, in visit_if_stmt
    o.else_body.accept(self)
  File "/home/samuel/code/harrier/env/lib/python3.5/site-packages/mypy/nodes.py", line 526, in accept
    return visitor.visit_block(self)
  File "/home/samuel/code/harrier/env/lib/python3.5/site-packages/mypy/traverser.py", line 38, in visit_block
    s.accept(self)
  File "/home/samuel/code/harrier/env/lib/python3.5/site-packages/mypy/nodes.py", line 673, in accept
    return visitor.visit_if_stmt(self)
  File "/home/samuel/code/harrier/env/lib/python3.5/site-packages/mypy/traverser.py", line 110, in visit_if_stmt
    o.else_body.accept(self)
  File "/home/samuel/code/harrier/env/lib/python3.5/site-packages/mypy/nodes.py", line 526, in accept
    return visitor.visit_block(self)
  File "/home/samuel/code/harrier/env/lib/python3.5/site-packages/mypy/traverser.py", line 38, in visit_block
    s.accept(self)
  File "/home/samuel/code/harrier/env/lib/python3.5/site-packages/mypy/nodes.py", line 485, in accept
    return visitor.visit_class_def(self)
  File "/home/samuel/code/harrier/env/lib/python3.5/site-packages/mypy/semanal.py", line 2350, in visit_class_def
    for type in tdef.info.bases:
AttributeError: 'NoneType' object has no attribute 'bases'

[JukkaL]

It would be helpful if you could narrow this crash down to a small code fragment. It's not obvious how tdef.info can be None during the 3rd semantic analysis pass. This seems related to a class that is defined within an if statement at the top level of a file.

[gvanrossum]

Also possibly try if this is fixed with the head from the repo?

[samuelcolvin]

ok, I've tried with head and got the same problem.

I've tracked it down to the libsass library and specifically to importing its _sass.cpython-35m-x86_64-linux-gnu.so file.

Calling mypy for the following script will give the error above:

import _sass

[gnprice]

Thanks for reporting this crash! This slightly hacky repro works for me, sidestepping the question of getting MYPYPATH right:

env=$(mktemp -d)
virtualenv -p python3.4 "$env"
. "$env"/bin/activate
pip install libsass
cd "$env"/lib/python3.4/site-packages
mypy sass.py

I get a traceback that looks just like yours.

I haven't succeeded in reproducing it with that one-line version -- I suspect the trigger is something else in that sass.py file. The traceback also suggests that to me, because it looks like it's a few layers deep in the AST of the file.

We have a mechanism for catching internal errors and reporting the source file and line that triggered them, but this one seems to have slipped past it. I'll file a bug for that. That'll probably be the easiest way to spot exactly what's causing this crash.

[gnprice]

Ha, actually on looking at the node types indicated in that traceback, I narrowed it down to a class definition inside a conditional block. There aren't many of those, and it turns out to be a line in six, not anything in libsass itself. Here's a reduced test case:

import sys

PY3 = sys.version_info[0] == 3

if PY3:
    pass
else:
    class X(object):
        pass

Same traceback.

[gvanrossum]

Thanks for the repro, Greg! This should be easy enough to fix, I expect it's in the slightly hacky code to deal with conditional definitions. I'm guessing it's not properly handing the case where one of the branches leaves the class undefined.

[gvanrossum]

Interestingly this is sensitive to the name of the PY3 variable, which is built into mypy as a known constant (with value dependent on pyversion), and if PY3: <block1> else: <block2> marks one or the other block unreachable. The first and second pass skip unreachable blocks. But the third pass doesn't do this. I think I'll add that.