comparing pathlib.Paths finding if one is a subdir of the other

Question

How can I find out if a pathlib.Path() is a subdir of another?

I can take the string of pathlib.PurePath(), but it only works if I have the whole path.

with zipfile.ZipFile(fname, 'w') as zip:
  for f in files:
    myrelroot = relroot # relative root for this zipfile
    if f"{pathlib.PurePath(relroot)}" not in f"{pathlib.PurePath(f)}":
      myrelroot = None # outside of zipfile so we do not get ..\\..
    try:
      zip.write(f, arcname=os.path.relpath(f, myrelroot))
    except FileNotFoundError as e:
      log.warning(f"file '{f}' not found: {e}")
    else:
      log.info(f"adding {f} to {fname}")

Answer 1

You can use the pathlib methods is_relative_to and relative_to to determine if 1 path is contained within another as well as what that relative path is.

>>> from pathlib import Path
>>> base = Path('/home/username')
>>> p1 = Path('/home/username/Downloads')
>>> p2 = Path('/usr/bin')
>>> p1.is_relative_to(base)
True
>>> p2.is_relative_to(base)
False

Then you can restructure your code to something like:

from pathlib import Path

fname = Path("some/path/to/zipfile.zip")
relroot = fname.parent

with zipfile.ZipFile(fname, 'w') as zip:
  for f in files:
    f = Path(f)
    if not f.exists(): # guard clause protects against non-existent files
        log.warning(f"file '{f}' not found: {e}")
        continue

    # default to use full path
    #   use relative f is a subdirectory of `relroot`
    arcname = f.resolve() 
    if f.is_relative_to(relroot):
        arcname = f.relative_to(relroot)
    
    zip.write(f, arcname=arcname)
    log.info(f"adding {f} to {fname}")