Pyreverse regression after #857 (astroid 2.5)

[doublethefish]

Steps to reproduce

1. Checkout pylint's source (which contains pyreverse)
2. cd <pylint checkout>
3. Run source .tox/py39/bin/activate or similar (you may need to run a tox session first)
4. Ensure you have astroid ac2b173 or later
5. Ensure you have installed astroid (python3 -m pip install -e <path-to-astroid>) as dependencies may be different
6. Run pyreverse --output png --project test tests/data

Current behaviour

A ModuleNotFoundError exception is raised.

$ pyreverse --output png --project test tests/data
parsing tests/data/__init__.py...
parsing /opt/contrib/pylint/pylint/tests/data/suppliermodule_test.py...
parsing /opt/contrib/pylint/pylint/tests/data/__init__.py...
parsing /opt/contrib/pylint/pylint/tests/data/clientmodule_test.py...
Traceback (most recent call last):
  File "/opt/contrib/pylint/pylint/.tox/py39/bin/pyreverse", line 8, in <module>
    sys.exit(run_pyreverse())
  File "/opt/contrib/pylint/pylint/.tox/py39/lib/python3.9/site-packages/pylint/__init__.py", line 39, in run_pyreverse
    PyreverseRun(sys.argv[1:])
  File "/opt/contrib/pylint/pylint/.tox/py39/lib/python3.9/site-packages/pylint/pyreverse/main.py", line 201, in __init__
    sys.exit(self.run(args))
  File "/opt/contrib/pylint/pylint/.tox/py39/lib/python3.9/site-packages/pylint/pyreverse/main.py", line 219, in run
    diadefs = handler.get_diadefs(project, linker)
  File "/opt/contrib/pylint/pylint/.tox/py39/lib/python3.9/site-packages/pylint/pyreverse/diadefslib.py", line 236, in get_diadefs
    diagrams = DefaultDiadefGenerator(linker, self).visit(project)
  File "/opt/contrib/pylint/pylint/.tox/py39/lib/python3.9/site-packages/pylint/pyreverse/utils.py", line 210, in visit
    self.visit(local_node)
  File "/opt/contrib/pylint/pylint/.tox/py39/lib/python3.9/site-packages/pylint/pyreverse/utils.py", line 207, in visit
    methods[0](node)
  File "/opt/contrib/pylint/pylint/.tox/py39/lib/python3.9/site-packages/pylint/pyreverse/diadefslib.py", line 162, in visit_module
    self.linker.visit(node)
  File "/opt/contrib/pylint/pylint/.tox/py39/lib/python3.9/site-packages/pylint/pyreverse/utils.py", line 210, in visit
    self.visit(local_node)
  File "/opt/contrib/pylint/pylint/.tox/py39/lib/python3.9/site-packages/pylint/pyreverse/utils.py", line 207, in visit
    methods[0](node)
  File "/opt/contrib/pylint/pylint/.tox/py39/lib/python3.9/site-packages/pylint/pyreverse/inspector.py", line 257, in visit_importfrom
    relative = astroid.modutils.is_relative(basename, context_file)
  File "/opt/contrib/pylint/astroid/astroid/modutils.py", line 581, in is_relative
    parent_spec = importlib.util.find_spec(name, from_file)
  File "/usr/local/Cellar/python@3.9/3.9.2_4/Frameworks/Python.framework/Versions/3.9/lib/python3.9/importlib/util.py", line 94, in find_spec
    parent = __import__(parent_name, fromlist=['__path__'])
ModuleNotFoundError: No module named 'pylint.tests'

Expected behaviour

No exception should be raised. Prior to #857 no exception was raised.

$ pyreverse --output png --project test tests/data
parsing tests/data/__init__.py...
parsing /opt/contributing/pylint/tests/data/suppliermodule_test.py...
parsing /opt/contributing/pylint/tests/data/__init__.py...
parsing /opt/contributing/pylint/tests/data/clientmodule_test.py...

python -c "from astroid import __pkginfo__; print(__pkginfo__.version)" output

2.6.0-dev0 (cab9b08)

[doublethefish]

Some notes:

• The ModuleNotFoundError exception is thrown by importlib.util.find_spec. Before python 3.7 this was an AttributeError
• modutils.is_relative() is the call-site for the find_spec call
• It seems that is_ralative() is trying to find a parent mod-spec for the anchor from_file parameter so that it can check to see if the modname is contained by it
• Neither is_relative nor its tests, explicitly handle on-disk paths (that is, paths for which os.path.exists() returns True) vs virtual-paths (exists() == False). The seems to be important to find_spec which raises if the package's __path__ attribute isn't found (which it won't be for virtual paths).
• The existing unit tests for is_realtive() use a range of modules to check against a set of module paths via __path__[0]. Somehow these pass and it is not clear why/how

One workaround fix, that feels like a hack until I understand the problem better, is for is_relative() to handle the ModuleNotFoundError/AttributeError exceptions and them as signifying that the parent_spec is being not-yet found. However, that requires that the loop-logic is fixed for linux-like systems, otherwise you end up in an infinite loop with len(file_path)>0 always being true for / and file_path = os.path.dirname('/') always returning /.

I have written some new unit tests for this issue and extended the existing ones, but there is some fundamental logic underpinning the use of importlib.util.find_spec that I am not smart enough to understand. For example, why do the existing unit-tests not all use the same modname parameter? Is it to work around the import caching in sys.modules? Should we take that into account?

My unit tests look at both virtual and ondisk paths (because of the if not os.path.isdir(from_file): line in is_realtive(), and the docs for find_spec). They also look at both absolute and relative paths explicitly. Finally, they also use system modules and assert that they are already in the import cache.

.. and I thought this fix was going to be easy.

[hippo91]

@doublethefish thanks for your report and investigation.
I can reproduce it and i confirm that #857 is the origin of the issue.

[DudeNr33]

It seems like this happens if an import inside a package which is at least two levels deep is processed.
For example, trying to run pyreverse on pylint itself will crash as soon as the import statements in checker modules in pylint.checkers.refactoring are processed.
importlib.util.find_spec(name, from_file) is called with name = checkers.refactoring.
find_spec will then split this up and try to import checkers, which fails because the path to inside the pylint package (i.e. pylint/pylint from the repo root) is normally not in the Pythonpath.