list's in-place add doesn't return NotImplemented when appropriate

[ethanfurman]

BPO	22778
Nosy	@rhettinger, @ethanfurman

^{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 = 'https://github.com/rhettinger'
closed_at = <Date 2014-10-31.22:20:20.183>
created_at = <Date 2014-10-31.22:07:49.806>
labels = ['type-bug', 'invalid']
title = "list's in-place add doesn't return NotImplemented when appropriate"
updated_at = <Date 2014-10-31.22:43:33.995>
user = 'https://github.com/ethanfurman'

bugs.python.org fields:

activity = <Date 2014-10-31.22:43:33.995>
actor = 'ethan.furman'
assignee = 'rhettinger'
closed = True
closed_date = <Date 2014-10-31.22:20:20.183>
closer = 'rhettinger'
components = []
creation = <Date 2014-10-31.22:07:49.806>
creator = 'ethan.furman'
dependencies = []
files = []
hgrepos = []
issue_num = 22778
keywords = []
message_count = 3.0
messages = ['230396', '230401', '230408']
nosy_count = 2.0
nosy_names = ['rhettinger', 'ethan.furman']
pr_nums = []
priority = 'normal'
resolution = 'not a bug'
stage = None
status = 'closed'
superseder = None
type = 'behavior'
url = 'https://bugs.python.org/issue22778'
versions = ['Python 3.5']

[ethanfurman]

--> s = []
--> s += 1
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
TypeError: 'int' object is not iterable

For comparison, when NotImplemented is appropriately returned the message looks like this:

--> s -= 1
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
TypeError: unsupported operand type(s) for -=: 'list' and 'int'

Which is certainly more helpful than "<blank> object is not iterable"

[rhettinger]

Please don't replicate existing issues.

Also, this behavior for lists is long-standing, tested, and designed by Guido. More than a little code relies on this behavior.

[ethanfurman]

My understanding was that if a different patch will be needed, it is not the same issue. Is that not correct?

Here's the test:

    def test_iadd(self):
        ...
        self.assertRaises(TypeError, u.__iadd__, None)

It would still pass, as returning NotImplemented will still result in a TypeError, just with a friendlier message.

If any code is depending on this, it is depending on the error message text, which is explicitly not guaranteed