Steps to reproduce:

Create the following three files (or download the attached zip file, which contains these files):

main.py

    import foo
    from foo import a
    from foo import b

    print(foo.b.my_function())


foo/a.py

    import importlib.util
    import sys

    # implementation copied from https://github.com/python/cpython/blob/master/Doc/library/importlib.rst#implementing-lazy-imports
    def lazy_import(name):
        spec = importlib.util.find_spec(name)
        loader = importlib.util.LazyLoader(spec.loader)
        spec.loader = loader
        module = importlib.util.module_from_spec(spec)
        sys.modules[name] = module
        loader.exec_module(module)
        return module

    b = lazy_import("foo.b")


foo/b.py

    def my_function():
        return "my_function"

and then run main.py



Expected results

my_function should be printed to the terminal


Actual results

The following traceback is printed to the terminal

    Traceback (most recent call last):
      File "F:\Documents\lazy_import\main.py", line 6, in <module>
        print(foo.b.my_function())
    AttributeError: module 'foo' has no attribute 'b'

If you comment out "from foo import a" from main.py, then the traceback doesn't occur and my_function gets printed.  Alternatively, if you move "from foo import a" after "from foo import b", then the traceback doesn't occur and my_function gets printed.  Adding "foo.b = b" before "print(foo.b.my_function())" will also fix the traceback.


A colleague of mine originally ran into this bug when writing unit tests for lazily imported code, since mock.patch("foo.b.my_function") triggers the same AttributeError.  I've reproduced this on Windows using both Python 3.8.3 and Python 3.9.0, and my colleague was using Python 3.8.3 on Mac.