Flush file after warning is written

[manueljacob]

BPO	41051
Nosy	@manueljacob
PRs	gh-85223: Flush file after warning is written #21000

^{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 2020-06-20.12:34:37.631>
labels = ['type-bug', 'expert-IO']
title = 'Flush file after warning is written'
updated_at = <Date 2020-09-27.00:06:07.034>
user = 'https://github.com/manueljacob'

bugs.python.org fields:

activity = <Date 2020-09-27.00:06:07.034>
actor = 'iritkatriel'
assignee = 'none'
closed = False
closed_date = None
closer = None
components = ['IO']
creation = <Date 2020-06-20.12:34:37.631>
creator = 'mjacob'
dependencies = []
files = []
hgrepos = []
issue_num = 41051
keywords = ['patch']
message_count = 1.0
messages = ['371934']
nosy_count = 1.0
nosy_names = ['mjacob']
pr_nums = ['21000']
priority = 'normal'
resolution = None
stage = 'patch review'
status = 'open'
superseder = None
type = 'behavior'
url = 'https://bugs.python.org/issue41051'
versions = []

[manueljacob]

Calling warnings.warn() will write to a file, but not flush it. On Python 3.9+, it won’t usually be a problem because the file is most likely stderr, which is always line-buffered. However, on older Python versions or if a different file is used, the current behavior unnecessarily delays the output of the warning. This is especially problematic if the warning is about buffering behavior itself, as e.g. caused by open('/tmp/test', 'wb', buffering=1).