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The documentation for os.path.commonprefix notes:
os.path.commonprefix(list)
Return the longest path prefix (taken character-by-character) that is a prefix of all paths in list. If list is empty, return the empty string (''). Note that this may return invalid paths because it works a character at a time.
And indeed:
>>> os.path.commonprefix(['/usr/bin', '/usr/bicycle'])
'/usr/bi'
This is IMHO useless behaviour for a function in the os.path namespace, I'd expect that os.path.commonprefix works with path elements (e.g. that the call above would have returned '/usr').