Look up __enter__ before __exit__ in the with statement documentation

[maggyero]

BPO	39037
Nosy	@gvanrossum, @brettcannon, @mdickinson, @ncoghlan, @maggyero, @miss-islington
PRs	bpo-39037: Look up __enter__ before __exit__ in the with statement documentation #17608 [3.8] bpo-39037: Fix lookup order of magic methods in with statement documentation (GH-17608) #17749

^{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 2019-12-30.11:07:49.335>
created_at = <Date 2019-12-13.15:19:37.772>
labels = ['easy', 'type-bug', '3.7', 'docs']
title = 'Look up __enter__ before __exit__ in the with statement documentation'
updated_at = <Date 2020-01-14.12:19:39.848>
user = 'https://github.com/maggyero'

bugs.python.org fields:

activity = <Date 2020-01-14.12:19:39.848>
actor = 'maggyero'
assignee = 'docs@python'
closed = True
closed_date = <Date 2019-12-30.11:07:49.335>
closer = 'maggyero'
components = ['Documentation']
creation = <Date 2019-12-13.15:19:37.772>
creator = 'maggyero'
dependencies = []
files = []
hgrepos = []
issue_num = 39037
keywords = ['patch', 'easy']
message_count = 10.0
messages = ['358333', '358334', '358335', '358346', '358399', '358416', '358419', '358432', '359020', '359021']
nosy_count = 7.0
nosy_names = ['gvanrossum', 'brett.cannon', 'mark.dickinson', 'ncoghlan', 'docs@python', 'maggyero', 'miss-islington']
pr_nums = ['17608', '17749']
priority = 'normal'
resolution = 'fixed'
stage = 'resolved'
status = 'closed'
superseder = None
type = 'behavior'
url = 'https://bugs.python.org/issue39037'
versions = ['Python 3.7']

[maggyero]

>>> class A: pass
    ... 
    >>> with A(): pass
    ... 
    Traceback (most recent call last):
      File "<stdin>", line 1, in <module>
    AttributeError: __enter__

I expected AttributeError: __exit__, since PEP-343 states (https://www.python.org/dev/peps/pep-0343/#specification-the-with-statement):

The details of the above translation are intended to prescribe the exact semantics. If either of the relevant methods are not found as expected, the interpreter will raise AttributeError, in the order that they are tried (exit, __enter__).

and the language documentation states (https://docs.python.org/3/reference/compound_stmts.html#the-with-statement):

The execution of the with statement with one “item” proceeds as follows:

1. The context expression (the expression given in the with_item) is evaluated to obtain a context manager.
2. The context manager’s __exit__() is loaded for later use.
3. The context manager’s __enter__() method is invoked.

[mdickinson]

See bpo-27100, which led to a deliberate change in behaviour to look up __enter__ before __exit__. It looks like the documentation (not the PEP text, which shouldn't be considered normative; just the reference manual) needs to be adjusted to match the new behaviour.

[maggyero]

Thanks @mark.dickinson for the link. Can I open a documentation PR for this?