How to mock os.walk in python with a temporary filesystem?

No. os.walk() is constructed entirely around os.listdir(), with assistance of os.path.islink() and os.path.isdir(). These are essentially system calls, so you’d have to mock your filesystem at the system level. Unless you want to write a FUSE plugin this is not going to be easy to mock.

All os.walk() needs to return is a list of tuples, really. Unless you are testing manipulating the dirs component, it couldn’t be more simple:

with mock.patch('os.walk') as mockwalk:
    mockwalk.return_value = [
        ('/foo', ('bar',), ('baz',)),
        ('/foo/bar', (), ('spam', 'eggs')),
    ]

This would mock the following directory structure:

/foo
 ├── baz
 └── bar 
     ├── spam
     └── eggs