DOC: inspect.ismethod returns different results on the same basic code between Python2.7 Python3.5

[TonyFlury]

BPO	27901
Nosy	@stevendaprano, @bitdancer, @TonyFlury, @iritkatriel, @akulakov

^{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 = None
created_at = <Date 2016-08-30.19:26:21.227>
labels = ['easy', 'type-feature', '3.10', 'docs']
title = 'DOC: inspect.ismethod returns different results on the same basic code between Python2.7 Python3.5'
updated_at = <Date 2021-08-22.13:09:09.377>
user = 'https://github.com/TonyFlury'

bugs.python.org fields:

activity = <Date 2021-08-22.13:09:09.377>
actor = 'mark.dickinson'
assignee = 'docs@python'
closed = False
closed_date = None
closer = None
components = ['Documentation']
creation = <Date 2016-08-30.19:26:21.227>
creator = 'anthony-flury'
dependencies = []
files = []
hgrepos = []
issue_num = 27901
keywords = ['easy']
message_count = 10.0
messages = ['273955', '273959', '273960', '273967', '273980', '273984', '273985', '273987', '388975', '395275']
nosy_count = 6.0
nosy_names = ['steven.daprano', 'r.david.murray', 'docs@python', 'anthony-flury', 'iritkatriel', 'andrei.avk']
pr_nums = []
priority = 'normal'
resolution = None
stage = 'resolved'
status = 'open'
superseder = None
type = 'enhancement'
url = 'https://bugs.python.org/issue27901'
versions = ['Python 3.10']

[TonyFlury]

Consider the following code in Python2.7 & Python3.5

    import inspect
    
    class a(object):
         def m(self):
             pass

in Python 2.7

inspect.ismethod(a.m) returns True

in Python 3.5

inspect.ismethod(a.m) returns False

Not sure which is correct, but I can see the Python3.5 result causing some issues with automatic code documenters.

I have code which will break under Python3.5 with this - my code performs static analysis of code, detecting functions, classes, attributes, and also traversing the mro to find inherited methods etc. Amongst other things this code identifies methods on classes, without instantiating those classes.

This may simply require a documentation change to explain the difference on Py3.5 - rather than a code change.

[mdickinson]

Note that Python 3 did away with unbound methods: in Python 3, a.m is simply a function, so there's little the inspect module can do to distinguish it from a regular top-level function.

https://docs.python.org/3/whatsnew/3.0.html#operators-and-special-methods

[bitdancer]

They are both correct. In 2.7 a class method is a method. In python3, a class method is a function.

As for the docs, the python3 docs say that it returns true for "a bound method on an object", while the python2 docs say "a bound or unbound method". A class method is not bound, and unbound method do not exist in python3. So the docs are correct as well. The notes about the difference could go into a porting guide, but I don't think they belong in the main docs.

To fix your code, just treat function objects on classes as what you are thinking of as methods. Because they are: any function assigned to a class becomes a bound method when invoked through an instance. I think that's even backward compatible, though I haven't checked.

[TonyFlury]

Not sure I agree with closing this. I am not convinced (as a reasonably seasoned developer) that the documentation is clear.

It may not be a bug in the code, resulting as it does from a change in the way methods are implemented in Python 3.5, but I do think that the documentation could be a lot clearer. It does read as if it is written for someone who already knows what the library does, rather than as a guide for someone trying to work through it for the first time.

I have changed this to a documentation issue - I hope that this is ok.

[stevendaprano]

I do think that the documentation could be a lot clearer.

Which specific documentation are you referring to? The "What's New" document from 3.0? That's a historical document, a snapshot of the past.

If you have a concern about the current documentation, can you tell us specifically where and what?

[bitdancer]

The docs do not so much assume you know what the library does, as they assume you know how python3 works. Which is to say, that bound methods are obtained from instances and that functions defined on a class are functions. That's documented in the data model docs, and the inspect docs in general assume you understand the python data model and are using inspect to probe it.

As Steven said, if you can point specifically to something you think can be improved we will consider it, but know that the python3 docs are in general written without respect to how python2 worked. There are very few references to python2 in the python3 docs, and this is by design. The story is different in the python2 docs, so conceivably there could be something inserted there about the python3 difference.

[bitdancer]

Perhaps we could add a cross link from 'bound method' to the 'instance methods' section of the data model docs.