Create the following tree:

    foo.py
    mypackage
        __init__.py

and get a loader for the non existent module 'mypackage.foo'.

$ mkdir tmp
$ cd tmp
$ >foo.py
$ mkdir mypackage
$ >mypackage/__init__.py
$ ./python
Python 3.4.0a0 (default:53a7c2226a2b, Nov  9 2012, 16:47:41) 
[GCC 4.3.2] on linux
Type "help", "copyright", "credits" or "license" for more information.
>>> import importlib
>>> importlib.find_loader('mypackage').get_filename()
'./mypackage/__init__.py'
>>> importlib.find_loader('mypackage.foo').get_filename()
'./foo.py'
>>>


find_loader should return None in this case.

And yes, find_loader returns None, when correctly invoked with the
path argument:

>>> importlib.find_loader('mypackage.foo', ['./mypackage/'])
>>>

Not necessarily.  The fact that there is nothing to load doesn't mean it isn't the right loader if there *was* something to load.  But I'll leave it to the import experts to say what the expected behavior is.  I'll admit that I can't figure it out from a quick perusal of the docs and PEP, so I'd say that regardless of what the correct behavior is there is at least a doc bug here.

Everything is working as expected; you left out the path argument::

  importlib.find_loader('package.foo', ['package'])

This works the way it does because otherwise we would have to stat every single time from the top level on down and that is extremely costly.

Maybe find_loader could check its parameters, notice that the name is a
dotted name, that path is None and in this case, not return a loader ?

That won't work as frozen and builtin modules don't care about the path.

> Not necessarily.  The fact that there is nothing to load doesn't
> mean it isn't the right loader if there *was* something to load.

But it is not even the right loader if there *was* something to load,
as get_filename() returns './foo.py' which is wrong even if
mypackage/foo.py had existed.  It should have returned
'./mypackage/foo.py' to be an acceptable loader for a would-be
mypackage.foo module.

If one would want to fix this, one way to do it could be to change the
following two methods of the PathFinder class such that:

    find_module() does not set path to sys.path when its path argument
    is None, so as to keep this information for _get_loader() to
    process it later

    _get_loader() sets path to sys.path when its path argument is None
    and returns a None loader when a loader has been found by a finder
    and:
        - this finder is a FileFinder instance
        - the _get_loader() path argument is None
        - the module name is a dotted name

Feel free to submit a patch, but I'm personally not motivated enough to try to change this on my own.

It might be more motivating if you (Xavier de Gaye) could let us know of a real-world problem caused by this behavior.

I was bitten by this behavior while, new to the importlib library, I
was trying to understand if one has to call recursively find_loader
for a dotted module name (in the way it must be done when using
imp.find_module), since the documentation on find_loader is not
clear. My test environment happened to be at that time:

    foo.py
    mypackage
        __init__.py
        foo.py

and at first I could not understand why find_loader('mypackage.foo')
was returning a loader, while find_loader('logging.handlers') was
returning None.

It is fine with me to consider that this behavior does not have
to be changed and to close this discussion.

New changeset 358be7742377 by Brett Cannon in branch '3.3':
Issue #16489: Make it clearer that importlib.find_loader() requires
http://hg.python.org/cpython/rev/358be7742377

New changeset ba1d7447bd1b by Brett Cannon in branch 'default':
Merge fix for #16489 from 3.3
http://hg.python.org/cpython/rev/ba1d7447bd1b

I clarified the wording in 3.3 and default. I also opened issue #16492 to add a keyword-only argument to find_loader() to have it import parent packages for you if you so desire.

Thanks, this is great!