How do I mock ZipFile's namelist?

I have a method in Python that looks like this:

def page_count(self):
    """Return the number of pages in the file."""
    if self.file == None:
        raise ComicFile.FileNoneError()

    if not os.path.isfile(self.file):
        raise ComicFile.FileNotFoundError()

    with ZipFile(self.file) as zip:
        members = zip.namelist()
        pruned = self.prune_dirs(members)
        length = len(pruned)
        return length

I'm trying to write a unit test for this (I've already tested prune_dirs), and so for this is what I have:

@unittest.mock.patch('comicfile.ZipFile')
def test_page_count(self, mock_zip_file):
    # Store as tuples to use as dictionary keys.
    members_dict = {('dir/', 'dir/file1', 'dir/file2'):2,
                    ('file1.jpg', 'file2.jpg', 'file3.jpg'):3
    }

    # Make the file point to something to prevent FileNoneError.
    self.comic_file.file = __file__

    for file_tuple, count in members_dict.items():
        mock_zip_file.return_value.namelist = list(file_tuple)
        self.assertEqual(count, self.comic_file.page_count())

When I run this test, I get the following:

F..ss....
======================================================================
FAIL: test_page_count (test_comicfile.TestPageCount)
----------------------------------------------------------------------
Traceback (most recent call last):
  File "/usr/local/Cellar/python3/3.5.1/Frameworks/Python.framework/Versions/3.5/lib/python3.5/unittest/mock.py", line 1157, in patched
    return func(*args, **keywargs)
  File "/Users/chuck/Dropbox/Projects/chiv/chiv.cbstar/test_comicfile.py", line 86, in test_page_count
    self.assertEqual(count, self.comic_file.page_count())
AssertionError: 2 != 0

----------------------------------------------------------------------
Ran 9 tests in 0.010s

FAILED (failures=1, skipped=2)

OK, so self.comic_file.page_count() is returning 0. I tried placing the following line after members = zip.namelist() in page_count.

print('\nmembers -> ' + str(members))

During the test, I get this:

members -> <MagicMock name='ZipFile().__enter__().namelist()' id='4483358280'>

I'm quite new to unit testing and am quite nebulous on using unittest.mock, but my understanding is that mock_zip-file.return_value.namelist = list(file_tuple) should have made it so that the namelist method of the ZipFile class would return each of the file_tuple contents in turn. What it is doing I have no idea.

I think what I'm trying to do here is clear, but I can't seem to figure out how to override the namelist method so that my unit test is only testing this one function instead of having to deal with ZipFile as well.