pathlib.PurePath division raises TypeError instead of returning NotImplemented

[aiudirog]

BPO	34775
Nosy	@ned-deily, @berkerpeksag, @serhiy-storchaka, @tirkarthi, @aiudirog
PRs	bpo-34775: Return NotImplemented in PurePath division. #9509 [3.8] bpo-34775: Return NotImplemented in PurePath division. (GH-9509) #15172 bpo-36305: Fixes to path handling and parsing in pathlib #12361

^{Note: these values reflect the state of the issue at the time it was migrated and might not reflect the current state.}

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GitHub fields:

assignee = None
closed_at = <Date 2019-08-29.05:12:34.289>
created_at = <Date 2018-09-23.07:02:08.850>
labels = ['3.8', 'type-bug', 'library', '3.9']
title = 'pathlib.PurePath division raises TypeError instead of returning NotImplemented'
updated_at = <Date 2020-01-16.12:08:52.969>
user = 'https://github.com/aiudirog'

bugs.python.org fields:

activity = <Date 2020-01-16.12:08:52.969>
actor = 'kmaork'
assignee = 'none'
closed = True
closed_date = <Date 2019-08-29.05:12:34.289>
closer = 'ned.deily'
components = ['Library (Lib)']
creation = <Date 2018-09-23.07:02:08.850>
creator = 'Roger Aiudi'
dependencies = []
files = []
hgrepos = []
issue_num = 34775
keywords = ['patch']
message_count = 6.0
messages = ['326140', '326146', '326156', '349217', '350712', '350714']
nosy_count = 5.0
nosy_names = ['ned.deily', 'berker.peksag', 'serhiy.storchaka', 'xtreak', 'Roger Aiudi']
pr_nums = ['9509', '15172', '12361']
priority = 'normal'
resolution = 'fixed'
stage = 'resolved'
status = 'closed'
superseder = None
type = 'behavior'
url = 'https://bugs.python.org/issue34775'
versions = ['Python 3.8', 'Python 3.9']

[aiudirog]

PurePath.__truediv__ and __rtruediv__ raise a TypeError when passed something which is not an instance of string or PurePath. This prevents creating any sort of compatible class that doesn't inherit from the previously mentioned types.

[berkerpeksag]

Thanks for the report and for the PR. Could you give us a little bit more information about your use case? Couldn't you make the class you want to use implement the __fspath__ protocol?

    import pathlib as p

    class Spam:
        def __fspath__(self):
            return 'pathlib.py'

And we can use it like:

    >>> p.Path('Lib') / Spam()
    PosixPath('Lib/pathlib.py')
    >>> (p.Path('Lib') / Spam()).exists()
    True

(3.4 and 3.5 are in security-fix-only mode, so I removed them from the versions field.)

[aiudirog]

Using your above example, my use case is returning an instance of Spam instead of PurePath from the division operation. The Spam class would have extra properties and methods for dealing with a substructure of our file system that can exist in different places, so being able to use a normal Path to locate it later with the division operation would be useful. (I know I could have a method that would do that, but I personally think reading the division left to right is clearer.)

The current implementation makes this impossible and I found raising a TypeError to be inconsistent with how Python handles operator overloading in the standard library.

[serhiy-storchaka]

New changeset 4c69be2 by Serhiy Storchaka (aiudirog) in branch 'master':
bpo-34775: Return NotImplemented in PurePath division. (GH-9509)
4c69be2

[ned-deily]

New changeset 4adcaf8 by Ned Deily (Miss Islington (bot)) in branch '3.8':
bpo-34775: Return NotImplemented in PurePath division. (GH-9509) (GH-15172)
4adcaf8

[ned-deily]

The change was pushed for release in 3.8.0b4. I don't think we should backport it to 3.7 at this stage in its lifecycle as the old behavior has been around for a long time. Thanks, everyone!