Using Python 3.7.2 on macOS 10.14 I get an unexpected exception when calling "importlib.util.find_spec('py')" after importing "py". find_spec works as expected before I import 'py'.

See the repl session below:

Python 3.7.2 (v3.7.2:9a3ffc0492, Dec 24 2018, 02:44:43) 
[Clang 6.0 (clang-600.0.57)] on darwin
Type "help", "copyright", "credits" or "license" for more information.
>>> import importlib.util
>>> importlib.util.find_spec("py")
ModuleSpec(name='py', loader=<_frozen_importlib_external.SourceFileLoader object at 0x10a953fd0>, origin='/Library/Frameworks/Python.framework/Versions/3.7/lib/python3.7/site-packages/py/__init__.py', submodule_search_locations=['/Library/Frameworks/Python.framework/Versions/3.7/lib/python3.7/site-packages/py'])
>>> import py
>>> importlib.util.find_spec("py")
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "/Library/Frameworks/Python.framework/Versions/3.7/lib/python3.7/importlib/util.py", line 111, in find_spec
    raise ValueError('{}.__spec__ is not set'.format(name)) from None
ValueError: py.__spec__ is not set

This is with py version 1.7.0 installed (pip install py)

This might be a bug in "py", which uses a vendored version of apipkg that replaces sys.modules["py"] with a lazy loading object. That copy of apipkg does not copy __spec__ into the lazy loading object.

If this is indeed consider to be a bug in py I'll file an issue with them.

Yep, that's a bug in py's module interface emulation - see the last paragraph in https://docs.python.org/3/reference/import.html#module-spec

I expected as much.

I've filed an issue with apipkg for this: pytest-dev/apipkg#13.

My next challenge is to find a way to work around this issue in my code.

BTW. Typing.io is a namespace added to sys.modules by the typing module that also does not have __spec__, and causes similar problems. I have an simple workaround for that on my side.

FYI, I opened bpo-35806 for typing.io.

bpo-35839 is follow-up enhancement request, proposing that we tweak the sys.modules handling in find_spec to ignore cache entries that don't have a __spec__ attribute set.