Convenience method for verifying if one path is ancestor of another

[jml]

I recently found myself wanting to check whether one path is the ancestor of another. It's a pain in the neck using os.path, so I checked out FilePath too see if it offered any help.

FilePath.segmentsFrom is very close to what I want. In fact, something like the following would be perfect:

def isParent(self, ancestor):
    try:
        self.segmentsFrom(ancestor)
    except ValueError:
        return False
    else:
        return True

Incidentally, why ValueError?

[glyph]

Probably a misguided attempt to fit into the idiom of the standard library, and use an existing exception. The rationalization would go something like: it is raised when ancestor is an invalid value. Sort of like the 'x not in list' and 'substring not found' ValueErrors from str.index and list.index.

Maybe there should be a more specific exception but it's unlikely it's worth it.

[jml]

OK, that makes sense. Mostly I like having more specific exceptions because they make it harder to accidentally suppress real errors, but here it's not such a big deal.

[glyph]

Perhaps segmentsFrom should take a "default" argument, which in your case could be "False"? (I could perhaps see wanting a default iterable as well).

[glyph]

Actually, I'll propose another implementation strategy on #3588 too...

[jml]

... with the default default implying raise ValueError? Yeah, that doesn't seem too bad. I'd rather something named isAncestor, since that will make the intent of the calling code clearer, but a default would be a reasonably convenient API.