Python Get relative path from comparing two absolute paths

Question

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

User · Answer

Edit   See jme s answer for the best way with Python3   Using pathlib  you have the following solution    Let s say we want to check if son is a descendant of parent  and both are Path objects  We can get a list of the parts in the path with list parent parts   Then  we just check that the begining of the son is equal to the list of segments of the parent    gt  gt  gt  lparent   list parent parts   gt  gt  gt  lson   list son parts   gt  gt  gt  if lson  len lparent      lparent   gt  gt  gt       parent is a parent of son      If you want to get the remaining part  you can just do   gt  gt  gt     join lson len lparent       It s a string  but you can of course use it as a constructor of an other Path object

User · Answer

Pure Python2 w o dep    def relpath cwd  path          Create a relative path for path from cwd  if possible        if sys platform     win32           cwd   cwd lower           path   path lower        cwd   os path abspath cwd  split os path sep       path   os path abspath path  split os path sep      eq until pos   None     for i in xrange min len  cwd   len  path             if  cwd i      path i               eq until pos   i         else              break     if eq until pos is None          return path     newpath         for i in xrange len  cwd eq until pos 1          newpath extend  path eq until pos 1        return os path join  newpath  if newpath else

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A write-up of jme s suggestion  using pathlib  in Python 3   from pathlib import Path parent   Path r  a b   son   Path r  a b c d                 if parent in son parents or parent  son      print son relative to parent     returns Path object equivalent to  c d

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Another option is   gt  gt  gt  print os path relpath   usr var log      usr var   log

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os path relpath      Return a relative filepath to path either from the current directory or from an optional start point     gt  gt  gt  from os path import relpath  gt  gt  gt  relpath   usr var log      usr var    log   gt  gt  gt  relpath   usr var log      usr var sad        log    So  if relative path starts with      - it means that the second path is not descendant of the first path   In Python3 you can use PurePath relative to   Python 3 5 1  default  Jan 22 2016  08 54 32   gt  gt  gt  from pathlib import Path   gt  gt  gt  Path   usr var log   relative to   usr var log    PosixPath        gt  gt  gt  Path   usr var log   relative to   usr var    PosixPath  log     gt  gt  gt  Path   usr var log   relative to   etc    Traceback  most recent call last     File   lt stdin gt    line 1  in  lt module gt    File   usr local Cellar python3 3 5 1 Frameworks Python framework Versions 3 5 lib python3 5 pathlib py   line 851  in relative to      format str self   str formatted    ValueError    usr var log  does not start with   etc

User · Answer

os path commonprefix   and os path relpath   are your friends    gt  gt  gt  print os path commonprefix    usr var log     usr var security      usr var   gt  gt  gt  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    gt  gt  gt  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  39 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

[python] Python: Get relative path from comparing two absolute paths

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