How to circumvent the fallacy of Python's os.path.commonprefix?

Question

My problem is to find the common path prefix of a given set of files.

Literally I was expecting that \"os.path.commonprefix\" would do just that. Unfortunat

Answer 1

A robust approach is to split the path into individual components and then find the longest common prefix of the component lists.

Here is an implementation which is cross-platform and can be generalized easily to more than two paths:

import os.path
import itertools

def components(path):
    '''
    Returns the individual components of the given file path
    string (for the local operating system).

    The returned components, when joined with os.path.join(), point to
    the same location as the original path.
    '''
    components = []
    # The loop guarantees that the returned components can be
    # os.path.joined with the path separator and point to the same
    # location:    
    while True:
        (new_path, tail) = os.path.split(path)  # Works on any platform
        components.append(tail)        
        if new_path == path:  # Root (including drive, on Windows) reached
            break
        path = new_path
    components.append(new_path)

    components.reverse()  # First component first 
    return components

def longest_prefix(iter0, iter1):
    '''
    Returns the longest common prefix of the given two iterables.
    '''
    longest_prefix = []
    for (elmt0, elmt1) in itertools.izip(iter0, iter1):
        if elmt0 != elmt1:
            break
        longest_prefix.append(elmt0)
    return longest_prefix

def common_prefix_path(path0, path1):
    return os.path.join(*longest_prefix(components(path0), components(path1)))

# For Unix:
assert common_prefix_path('/', '/usr') == '/'
assert common_prefix_path('/usr/var1/log/', '/usr/var2/log/') == '/usr'
assert common_prefix_path('/usr/var/log1/', '/usr/var/log2/') == '/usr/var'
assert common_prefix_path('/usr/var/log', '/usr/var/log2') == '/usr/var'
assert common_prefix_path('/usr/var/log', '/usr/var/log') == '/usr/var/log'
# Only for Windows:
# assert common_prefix_path(r'C:\Programs\Me', r'C:\Programs') == r'C:\Programs'