pathlib '/' operator does not resolve Enums with str mixin as expected

[andrew-ni]

BPO	39081
Nosy	@brettcannon, @ethanfurman, @tirkarthi, @andrew-ni

^{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 2019-12-18.17:31:06.241>
created_at = <Date 2019-12-17.23:37:20.537>
labels = ['3.7', 'invalid', 'type-bug', 'library']
title = "pathlib '/' operator does not resolve Enums with str mixin as expected"
updated_at = <Date 2019-12-18.17:48:52.115>
user = 'https://github.com/andrew-ni'

bugs.python.org fields:

activity = <Date 2019-12-18.17:48:52.115>
actor = 'ethan.furman'
assignee = 'none'
closed = True
closed_date = <Date 2019-12-18.17:31:06.241>
closer = 'brett.cannon'
components = ['Library (Lib)']
creation = <Date 2019-12-17.23:37:20.537>
creator = 'andrewni'
dependencies = []
files = []
hgrepos = []
issue_num = 39081
keywords = []
message_count = 4.0
messages = ['358598', '358615', '358633', '358637']
nosy_count = 4.0
nosy_names = ['brett.cannon', 'ethan.furman', 'xtreak', 'andrewni']
pr_nums = []
priority = 'normal'
resolution = 'not a bug'
stage = 'resolved'
status = 'closed'
superseder = None
type = 'behavior'
url = 'https://bugs.python.org/issue39081'
versions = ['Python 3.7']

[andrew-ni]

import os
import pathlib
import enum

class MyEnum(str, enum.Enum):
    RED = 'red'

# this resolves to: '/Users/niandrew/MyEnum.RED'
# EXPECTED: '/Users/niandrew/red'
str(pathlib.Path.home() / MyEnum.RED)

# this resolves to: '/Users/niandrew/red'
os.path.join(pathlib.Path.home(), MyEnum.RED)

[tirkarthi]

As per the fspath PEP https://www.python.org/dev/peps/pep-0519/#c-api the below precedence is set. So for the enum inheriting from str the object itself is returned and MyEnum.RED.__str__ is used returning MyEnum.RED. You can remove inheritance from str and implement __fspath__ for your enum class to be used as per fspath protocol.

If the object is str or bytes, then allow it to pass through with
an incremented refcount. If the object defines __fspath__(), then
return the result of that method. All other types raise a TypeError.

# bpo-39081.py

import os
import pathlib
import enum

class MyEnum(enum.Enum):
    RED = 'red'

    def __fspath__(self):
        return self.name

print(os.fspath(MyEnum.RED))
print(pathlib.Path.home() / MyEnum.RED)

$ python3.8 bpo39081.py
RED
/Users/kasingar/RED

[brettcannon]

Karthikeyan is right and this is working as expected. If you want the semantics you're after you can either implement __fspath__ as was suggested or get the 'value' attribute of the enum when constructing your path.

[ethanfurman]

The other option is to continue to inherit from str, but override the __str__ method:

    class MyEnum(str, enum.Enum):
        #
        def __str__(self):
            return self.value