Use os.path.split. It is a system independent way to split paths. Note that this only splits into (head, tail). To get all the individual parts, you need to recursively split head or use str.split using os.path.sep as the separator.

Using regular expression (re module documentation):

>>> import re
>>> print(re.match(r'[^\\]+\\[^\\]+', r'C:\my_dir\repo\branch').group())
C:\my_dir

>>> re.findall(r'[^\\]+\\[^\\]+|.+', r'C:\my_dir\repo\branch')
['C:\\my_dir', '\\repo\\branch']

The OP specified "will work with Windows paths too". There are a few wrinkles with Windows paths.

Firstly, Windows has the concept of multiple drives, each with its own current working directory, and 'c:foo' and 'c:\\foo' are often not the same. Consequently it is a very good idea to separate out any drive designator first, using os.path.splitdrive(). Then reassembling the path (if required) can be done correctly by drive + os.path.join(*other_pieces)

Secondly, Windows paths can contain slashes or backslashes or a mixture. Consequently, using os.sep when parsing an unnormalised path is not useful.

More generally:

The results produced for 'foo' and 'foo/' should not be identical.

The loop termination condition seems to be best expressed as "os.path.split() treated its input as unsplittable".

Here's a suggested solution, with tests, including a comparison with @Spacedman's solution

import os.path

def os_path_split_asunder(path, debug=False):
    parts = []
    while True:
        newpath, tail = os.path.split(path)
        if debug: print repr(path), (newpath, tail)
        if newpath == path:
            assert not tail
            if path: parts.append(path)
            break
        parts.append(tail)
        path = newpath
    parts.reverse()
    return parts

def spacedman_parts(path):
    components = [] 
    while True:
        (path,tail) = os.path.split(path)
        if not tail:
            return components
        components.insert(0,tail)

if __name__ == "__main__":
    tests = [
        '',
        'foo',
        'foo/',
        'foo\\',
        '/foo',
        '\\foo',
        'foo/bar',
        '/',
        'c:',
        'c:/',
        'c:foo',
        'c:/foo',
        'c:/users/john/foo.txt',
        '/users/john/foo.txt',
        'foo/bar/baz/loop',
        'foo/bar/baz/',
        '//hostname/foo/bar.txt',
        ]
    for i, test in enumerate(tests):
        print "\nTest %d: %r" % (i, test)
        drive, path = os.path.splitdrive(test)
        print 'drive, path', repr(drive), repr(path)
        a = os_path_split_asunder(path)
        b = spacedman_parts(path)
        print "a ... %r" % a
        print "b ... %r" % b
        print a == b

and here's the output (Python 2.7.1, Windows 7 Pro):

Test 0: ''
drive, path '' ''
a ... []
b ... []
True

Test 1: 'foo'
drive, path '' 'foo'
a ... ['foo']
b ... ['foo']
True

Test 2: 'foo/'
drive, path '' 'foo/'
a ... ['foo', '']
b ... []
False

Test 3: 'foo\\'
drive, path '' 'foo\\'
a ... ['foo', '']
b ... []
False

Test 4: '/foo'
drive, path '' '/foo'
a ... ['/', 'foo']
b ... ['foo']
False

Test 5: '\\foo'
drive, path '' '\\foo'
a ... ['\\', 'foo']
b ... ['foo']
False

Test 6: 'foo/bar'
drive, path '' 'foo/bar'
a ... ['foo', 'bar']
b ... ['foo', 'bar']
True

Test 7: '/'
drive, path '' '/'
a ... ['/']
b ... []
False

Test 8: 'c:'
drive, path 'c:' ''
a ... []
b ... []
True

Test 9: 'c:/'
drive, path 'c:' '/'
a ... ['/']
b ... []
False

Test 10: 'c:foo'
drive, path 'c:' 'foo'
a ... ['foo']
b ... ['foo']
True

Test 11: 'c:/foo'
drive, path 'c:' '/foo'
a ... ['/', 'foo']
b ... ['foo']
False

Test 12: 'c:/users/john/foo.txt'
drive, path 'c:' '/users/john/foo.txt'
a ... ['/', 'users', 'john', 'foo.txt']
b ... ['users', 'john', 'foo.txt']
False

Test 13: '/users/john/foo.txt'
drive, path '' '/users/john/foo.txt'
a ... ['/', 'users', 'john', 'foo.txt']
b ... ['users', 'john', 'foo.txt']
False

Test 14: 'foo/bar/baz/loop'
drive, path '' 'foo/bar/baz/loop'
a ... ['foo', 'bar', 'baz', 'loop']
b ... ['foo', 'bar', 'baz', 'loop']
True

Test 15: 'foo/bar/baz/'
drive, path '' 'foo/bar/baz/'
a ... ['foo', 'bar', 'baz', '']
b ... []
False

Test 16: '//hostname/foo/bar.txt'
drive, path '' '//hostname/foo/bar.txt'
a ... ['//', 'hostname', 'foo', 'bar.txt']
b ... ['hostname', 'foo', 'bar.txt']
False

There's no single function that will do what you want...primarily because what you're asking doesn't make any sense from a path-manipulation standpoint (because discarding .. changes the path in a meaningful fashion).

You could do something along the lines of:

[x for x in path.split(os.path.sep) if x not in ['.', '..']]

Which gives you:

>>> path='/s//pam/../ham/./eggs'
>>> [x for x in path.split(os.path.sep) if x not in ['.', '..']]
['', 's', '', 'pam', 'ham', 'eggs']

And:

>>> path=r'C:\s\\pam\..\ham\.\eggs'
>>> [x for x in path.split(os.path.sep) if x not in ['.', '..']]
['C:', 's', '', 'pam', 'ham', 'eggs']

(As long as os.path.sep is \).

It's obviously not exactly what you're looking for, but maybe it points the way in the right direction.