ABC register doesn't check abstract methods

[rmccampbell7]

BPO	22487
Nosy	@bitdancer

^{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 2014-09-24.22:00:59.285>
created_at = <Date 2014-09-24.21:37:14.812>
labels = ['type-feature', 'invalid']
title = "ABC register doesn't check abstract methods"
updated_at = <Date 2014-09-24.22:00:59.283>
user = 'https://bugs.python.org/rmccampbell7'

bugs.python.org fields:

activity = <Date 2014-09-24.22:00:59.283>
actor = 'r.david.murray'
assignee = 'none'
closed = True
closed_date = <Date 2014-09-24.22:00:59.285>
closer = 'r.david.murray'
components = []
creation = <Date 2014-09-24.21:37:14.812>
creator = 'rmccampbell7'
dependencies = []
files = []
hgrepos = []
issue_num = 22487
keywords = []
message_count = 3.0
messages = ['227489', '227490', '227491']
nosy_count = 2.0
nosy_names = ['r.david.murray', 'rmccampbell7']
pr_nums = []
priority = 'normal'
resolution = 'not a bug'
stage = 'resolved'
status = 'closed'
superseder = None
type = 'enhancement'
url = 'https://bugs.python.org/issue22487'
versions = []

[rmccampbell7]

Is there a reason register() doesn't check for abstract methods, like subclassing does? Would it fail for some builtin classes? It seems that this would be a better guarantee that, say, something really is iterable when you check isinstance(Collections.Iterable, o), since someone could have called Collections.Iterable.register(o.__class__) without adding an __iter__ method to their class.

[rmccampbell7]

Obviously, I meant isinstance(o, Collections.Iterable).

[bitdancer]

Python is a consenting adults language. If you call register, we assume you know what you are doing. The isinstance check, on the other hand, does look in certain cases. So if you define __iter__ you don't have to call register to make isinstance(o, Iterable) be True.