Date formats in help messages of argparse

[SengerM]

BPO	44986
Nosy	@rhettinger, @Julian, @SengerM
PRs	bpo-44986: Fixed bug with date formats like %Y-%m-%d in help messages of argparse #27923
Files	deleteme.py

^{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 2021-08-23.23:22:20.648>
labels = ['type-feature', 'library', '3.11']
title = 'Date formats in help messages of argparse'
updated_at = <Date 2021-09-03.00:59:17.771>
user = 'https://github.com/SengerM'

bugs.python.org fields:

activity = <Date 2021-09-03.00:59:17.771>
actor = 'paul.j3'
assignee = 'none'
closed = False
closed_date = None
closer = None
components = ['Library (Lib)']
creation = <Date 2021-08-23.23:22:20.648>
creator = 'mail.de.senger'
dependencies = []
files = ['50232']
hgrepos = []
issue_num = 44986
keywords = ['patch']
message_count = 3.0
messages = ['400183', '400246', '400327']
nosy_count = 5.0
nosy_names = ['rhettinger', 'Julian', 'python-dev', 'paul.j3', 'mail.de.senger']
pr_nums = ['27923']
priority = 'normal'
resolution = None
stage = 'patch review'
status = 'open'
superseder = None
type = 'enhancement'
url = 'https://bugs.python.org/issue44986'
versions = ['Python 3.11']

[SengerM]

If the help message of an argument in argparse contains a date format, e.g. %Y-%m-%d, it crashes when printing the help after being invoked with the -h option. Uploaded an example.

[Julian]

This is documented already I believe:

https://docs.python.org/3/library/argparse.html#help

As the help string supports %-formatting, if you want a literal % to appear in the help string, you must escape it as %%.

[SengerM]

With this change there should be no need to escape anything, and it is still compatible with the old formatting. Plus, with the current implementation if you forget to escape something like %Y the error message is not helpful at all and only happens when invoked with -h option.