How to force absolute path to be relative when joining?

What is the idiomatic way to turn an absolute path into a relative path when joining?

Inputs:

abs = r"\\server\share\data"
newroot = r"X:\project"

Desired result:

X:\project\server\share\data

Both os.path.join() and pathlib.Path().joinpath() trim everything after the drive letter:

>>> os.path.join(newroot, abs)
'\\\\server\\share\\data'

>>> pathlib.Path(newroot).joinpath(abs)
WindowsPath('//server/share/data')

I can make them work by stripping the leading slashes:

>>> os.path.join(newroot, abs[2:])
'X:\\project\\server\\share\\data'

but that means I need to wrap in a test to find out whether the incoming path is actually absolute and to count how many slashes to remove. This just feels like a place for me to create bugs.

Note: Windows syntax is shown but I imagine the problem is cross platform and best-practice answer is probably that way also.

Solution

What I'm using until a solid answer is found. It works for my situation but I wouldn't count on it to be problem free.

Concise:

def make_relative(path):
    '''Remove leading characters that make an absolute path'''
    while os.path.isabs(path):
        path = path[1:]
        if ':' in path[0]: path = path[1:]
    return path

Informative:

def make_relative(path, debug=False):
    '''Remove leading characters that make an absolute path'''
    msg = [f"\nIn: {repr(path)}"]
    while os.path.isabs(path):
        msg.append(f'stripping {path[0]}')
        path = path[1:]
        if ':' in path[0]:
            msg.append(f'stripping {path[0]}')
            path = path[1:]
    msg.append(f'Out: {repr(path)}')
    if debug:
        [print(x) for x in msg]
    return path

if __name__ == '__main__':
    abs = r"\\server\share\data"
    newroot = r"X:\project\test"

    make_relative(abs, debug=True)
    make_relative(newroot, debug=True)

Result:

In: '\\\\server\\share\\data'
stripping \
stripping \
Out: 'server\\share\\data'

In: 'X:\\project\\test'
stripping X
stripping :
stripping \
Out: 'project\\test'