Get relative path from comparing two absolute paths
python
Solution
os.path.commonprefix() and os.path.relpath() are your friends:
>>> print os.path.commonprefix(['/usr/var/log', '/usr/var/security'])
'/usr/var'
>>> print os.path.commonprefix(['/tmp', '/usr/var']) # No common prefix: the root is the common prefix
'/'
You can thus test whether the common prefix is one of the paths, i.e. if one of the paths is a common ancestor:
paths = […, …, …]
common_prefix = os.path.commonprefix(list_of_paths)
if common_prefix in paths:
…
You can then find the relative paths:
relative_paths = [os.path.relpath(path, common_prefix) for path in paths]
You can even handle more than two paths, with this method, and test whether all the paths are all below one of them.
PS: depending on how your paths look like, you might want to perform some normalization first (this is useful in situations where one does not know whether they always end with '/' or not, or if some of the paths are relative). Relevant functions include os.path.abspath() and os.path.normpath().
PPS: as Peter Briggs mentioned in the comments, the simple approach described above can fail:
>>> os.path.commonprefix(['/usr/var', '/usr/var2/log'])
'/usr/var'
even though `/usr/var` is not a common prefix of the paths. Forcing all paths to end with '/' before calling `commonprefix()` solves this (specific) problem.
PPPS: as bluenote10 mentioned, adding a slash does not solve the general problem. Here is his followup question: How to circumvent the fallacy of Python's os.path.commonprefix?
PPPPS: starting with Python 3.4, we have pathlib, a module that provides a saner path manipulation environment. I guess that the common prefix of a set of paths can be obtained by getting all the prefixes of each path (with `PurePath.parents()`), taking the intersection of all these parent sets, and selecting the longest common prefix.
PPPPPS: Python 3.5 introduced a proper solution to this question: `os.path.commonpath()`, which returns a valid path.
Problem
Say, I have two absolute paths. I need to check if the location referring to by one of the paths is a descendant of the other. If true, I need to find out the relative path of the descendant from the ancestor. What's a good way to implement this in Python? Any library that I can benefit from?