Simple example of how to use importlib to create a loader

[Paul Moore]

I'm trying to write my own loader using importlib. And frankly, I'm getting nowhere. I'm struggling to understand precisely which methods of the various ABCs I need to implement, and in some cases what they should do.

Could anyone point me to a simple example (say, something that implements zip imports using the zipfile module - that was what I initially tried to get working as a proof of concept).

Where I've got to is the following stub implementation to try to log the sequence of calls:

from importlib.abc import PathEntryFinder, SourceLoader

class MyFinder(PathEntryFinder):
    def __init__(self, pathentry):
        """Create a finder for the given `pathentry`"""
        if pathentry == 'foo':
            return
        raise ImportError
    def find_loader(self, fullname):
        """Return a loader for the module `fullname`"""
        print("find_loader('{}')".format(fullname))
        return MyLoader(), ['foo']

class MyLoader(SourceLoader):
    def module_repr(self):
        print("module_repr()")
        return True
    def is_package(self, fullname):
        print("is_package('{}')".format(fullname))
        return True
    def get_data(self, path):
        """Return an open binary file object for `path`"""
        print("get_data('{}')".format(path))
        return b"print('hello from foo!')"
    def get_filename(self, fullname):
        """Return the filename for module `fullname`"""
        print("get_filename('{}')".format(fullname))
        return '/this/is/foo'

if __name__ == '__main__':
    import sys
    sys.path_hooks.append(lambda p: MyFinder(p))
    sys.path.insert(0, 'foo')
    import a
    import a.b
    from a.b.c import x


It prints out

>py .\loader.py
find_loader('a')
is_package('a')
get_filename('a')
get_data('/this/is/foo')
get_filename('a')
is_package('a')
hello from foo!
Traceback (most recent call last):
  File ".\loader.py", line 46, in <module>
    import a.b
ImportError: No module named 'a.b'

which is a good start, but I don't know why it isn't willing to check for 'a.b' (after all, I tell it that 'a' is a package).

I'm quite probably doing something trivially wrong here - after all, I'm not even *trying* to implement a sane view of a package structure here. But I'd have expected at least a find_loader('a.b') call...

Thanks for any help,
Paul.