posixmodule.c:path_converter() returns an invalid exception message for broken PathLike objects

[ambv]

BPO	35942
Nosy	@ambv, @serhiy-storchaka, @pablogsal, @miss-islington
PRs	bpo-35942: Improve the error message if __fspath__ returns invalid types in path_converter #11831 [3.7] bpo-35942: Improve the error message if __fspath__ returns invalid types in path_converter (GH-11831) #11912 Fix syntax warnings in tests introduced in bpo-35942. #11934 Fix syntax warnings in tests introduced in bpo-15248. #11932 [3.7] Fix syntax warnings in tests introduced in bpo-35942. (GH-11934) #11937

^{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 2019-02-18.11:38:39.756>
created_at = <Date 2019-02-08.16:34:39.069>
labels = ['3.8', 'type-bug', '3.7']
title = 'posixmodule.c:path_converter() returns an invalid exception message for broken PathLike objects'
updated_at = <Date 2019-02-19.12:16:41.732>
user = 'https://github.com/ambv'

bugs.python.org fields:

activity = <Date 2019-02-19.12:16:41.732>
actor = 'miss-islington'
assignee = 'none'
closed = True
closed_date = <Date 2019-02-18.11:38:39.756>
closer = 'pablogsal'
components = []
creation = <Date 2019-02-08.16:34:39.069>
creator = 'lukasz.langa'
dependencies = []
files = []
hgrepos = []
issue_num = 35942
keywords = ['patch']
message_count = 6.0
messages = ['335094', '335756', '335804', '335809', '335935', '335939']
nosy_count = 4.0
nosy_names = ['lukasz.langa', 'serhiy.storchaka', 'pablogsal', 'miss-islington']
pr_nums = ['11831', '11912', '11934', '11932', '11937']
priority = 'normal'
resolution = 'fixed'
stage = 'resolved'
status = 'closed'
superseder = None
type = 'behavior'
url = 'https://bugs.python.org/issue35942'
versions = ['Python 3.7', 'Python 3.8']

[ambv]

>>> class K:
...   def __fspath__(self):
...     return 1
...
>>> import os
>>> os.stat(K())
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
TypeError: stat: path should be string, bytes, os.PathLike or integer, not int

This error message is internally inconsistent:

• it suggests that the error is about the path argument whereas it's in fact about the value returned from __fspath__()
• it hilariously states "should be integer, not int"
• it claims os.PathLike is fine as a return value from __fspath__() whereas it's not

I would advise removing the custom __fspath__() handling from path_converter and just directly using PyOS_FSPath which returns a valid error in this case (example from pypy3):

>>>> class K:
....   def __fspath__(self):
....     return 1
....
>>>> import os
>>>> os.open(K(), os.O_RDONLY)
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
TypeError: expected K.__fspath__() to return str or bytes, not int

[serhiy-storchaka]

The custom __fspath__() handling is used in path_converter for better error reporting. The error message can include names of the function and the argument. PyOS_FSPath() can not provides this information.

I agree that error reporting about incorrect type of the __fspath__() result should be improved.

[pablogsal]

New changeset 09fbcd6 by Pablo Galindo in branch 'master':
bpo-35942: Improve the error message if __fspath__ returns invalid types in path_converter (GH-11831)
09fbcd6

[miss-islington]

New changeset a01065a by Miss Islington (bot) in branch '3.7':
bpo-35942: Improve the error message if __fspath__ returns invalid types in path_converter (GH-11831)
a01065a

[serhiy-storchaka]

New changeset 8d01eb4 by Serhiy Storchaka in branch 'master':
Fix syntax warnings in tests introduced in bpo-35942. (GH-11934)
8d01eb4

[miss-islington]

New changeset a883490 by Miss Islington (bot) in branch '3.7':
Fix syntax warnings in tests introduced in bpo-35942. (GH-11934)
a883490