An unclear definition in Doc/glossary.rst

[StevenHsuYL]

BPO	44651
Nosy	@terryjreedy, @mdickinson, @jdevries3133, @StevenHsuYL
PRs	bpo-44651: delete entry of "coercion" in Doc/glossary.rst #27226

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assignee = None
closed_at = <Date 2021-07-22.16:29:31.507>
created_at = <Date 2021-07-16.05:11:26.422>
labels = ['3.11', 'type-feature', '3.9', '3.10', 'docs']
title = 'An unclear definition in Doc/glossary.rst'
updated_at = <Date 2021-07-22.16:29:31.507>
user = 'https://github.com/StevenHsuYL'

bugs.python.org fields:

activity = <Date 2021-07-22.16:29:31.507>
actor = 'mark.dickinson'
assignee = 'docs@python'
closed = True
closed_date = <Date 2021-07-22.16:29:31.507>
closer = 'mark.dickinson'
components = ['Documentation']
creation = <Date 2021-07-16.05:11:26.422>
creator = 'StevenHsuYL'
dependencies = []
files = []
hgrepos = []
issue_num = 44651
keywords = ['patch']
message_count = 9.0
messages = ['397597', '397645', '397646', '397689', '397700', '397739', '397751', '397983', '397987']
nosy_count = 5.0
nosy_names = ['terry.reedy', 'mark.dickinson', 'docs@python', 'jack__d', 'StevenHsuYL']
pr_nums = ['27226']
priority = 'normal'
resolution = 'fixed'
stage = 'resolved'
status = 'closed'
superseder = None
type = 'enhancement'
url = 'https://bugs.python.org/issue44651'
versions = ['Python 3.9', 'Python 3.10', 'Python 3.11']

[StevenHsuYL]

In Doc/glossary.rst, the definition about "coercion" is as below:

"The implicit conversion of an instance of one type to another during an operation which involves two arguments of the same type."

However, in the example following this definition, it shows the arguments of "different" type in an adding operation, and one of them was converted to float for the operation.

Therefore, we should fix the definition of the coercion to as below:

"The implicit conversion of an instance of one type to another during an operation which involves two arguments of the different type."

Or am I realize this sentence wrong?
Thanks for review.

https://github.com/python/cpython/blob/main/Doc/glossary.rst

[mdickinson]

Agreed that "of the same type" part is confusing. I suspect the intended meaning was that the operation *expects* both arguments to have the same type, so if the actual arguments have different types then they have to be coerced to a common type. In the example you mention, the 3 is converted to 3.0 so that the actual addition is applied to two arguments of the same type. So I think the "same type" part *was* intentional, if confusing.

But coercion was more of a real thing in Python 2, where there was a __coerce__ special method and a coerce builtin function. It isn't really a thing any more in Python 3, except inasmuch as it means "implicit type conversion". And I can't find any uses of :term:`coercion` elsewhere in the docs. Maybe we should just delete this glossary entry?

If all we mean by coercion is implicit type conversion, then the "operation which involves two arguments" part already seems over-specific: math.sqrt implicitly converts its argument to a float, for example.

[mdickinson]

Searching further, none of the uses of "coerce" or "coercion" in the docs seem to be a good match for the definition in this glossary entry. For example, from ipaddress.rst:

By default, attempting to create a network object with host bits set will
result in :exc:`ValueError` being raised. To request that the
additional bits instead be coerced to zero, the flag ``strict=False`` can
be passed to the constructor

and from configparser.rst:

A convenience method which coerces the *option* in the specified *section* to an integer.

etc. I think we should just get rid of the glossary entry.