raise NotImplemented vs return NotImplemented

[srinivasreddy]

BPO	32464
Nosy	@mdickinson, @1st1, @srinivasreddy

^{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 2018-05-29.19:11:30.796>
created_at = <Date 2017-12-31.11:44:32.019>
labels = ['invalid', '3.7', '3.8', 'performance']
title = 'raise NotImplemented vs return NotImplemented'
updated_at = <Date 2018-05-29.19:11:30.795>
user = 'https://github.com/srinivasreddy'

bugs.python.org fields:

activity = <Date 2018-05-29.19:11:30.795>
actor = 'yselivanov'
assignee = 'none'
closed = True
closed_date = <Date 2018-05-29.19:11:30.796>
closer = 'yselivanov'
components = []
creation = <Date 2017-12-31.11:44:32.019>
creator = 'thatiparthy'
dependencies = []
files = []
hgrepos = []
issue_num = 32464
keywords = []
message_count = 3.0
messages = ['309277', '309278', '318070']
nosy_count = 3.0
nosy_names = ['mark.dickinson', 'yselivanov', 'thatiparthy']
pr_nums = []
priority = 'normal'
resolution = 'not a bug'
stage = 'resolved'
status = 'closed'
superseder = None
type = 'resource usage'
url = 'https://bugs.python.org/issue32464'
versions = ['Python 3.7', 'Python 3.8']

[srinivasreddy]

I ran these queries on cpython repo.

➜ cpython git:(master) ✗ grep -r . -e return --include=\*.py | grep NotImplemented | wc -l
196

➜ cpython git:(master) ✗ grep -r . -e raise --include=\*.py | grep NotImplemented | wc -l
295

I have always used raise NotImplemented or raise NotImplementedError. But when does it make sense to return NotImplemented?

[mdickinson]

Here's the documentation, which I think explains all this clearly: briefly, return NotImplemented and raise NotImplementedError are the normal usages. raise NotImplemented doesn't make sense, and shouldn't be used: it'll end up raising a TypeError, sine NotImplemented is neither a subclass nor an instance of BaseException.

NotImplemented: https://docs.python.org/3.7/library/constants.html#NotImplemented

NotImplementedError:
https://docs.python.org/3.7/library/exceptions.html#NotImplementedError

Almost all of the grep results you're seeing do follow the expected patterns (either return NotImplemented or raise NotImplementedError, but a quick grep does show some questionable uses of raise NotImplemented in the standard library:

MacBook-Pro:cpython mdickinson$ grep -r . -e raise   --include=\*.py  | grep "\bNotImplemented\b"
./Lib/asyncio/transports.py:    The implementation here raises NotImplemented for every method
./Lib/idlelib/debugger_r.py:        raise NotImplemented("dict_keys not public or pickleable")
./Lib/ssl.py:        raise NotImplemented("Can't dup() %s instances" %

The other way around also turns up an odd-looking return NotImplementedError (along with a false positive):

MacBook-Pro:cpython mdickinson$ grep -r . -e return   --include=\*.py  | grep "\bNotImplementedError\b"
./Lib/ctypes/test/test_simplesubclasses.py:            return NotImplementedError
./Lib/test/test_asyncio/test_transports.py:        self.assertRaises(NotImplementedError, transport.get_returncode)

[1st1]

Seems this issue isn't a bug and can be closed.