argparse docs: default for prog= in ArgumentParser() should be basename of argv[0], not argv[0], to match behaviour

[lf]

BPO	46228
Nosy	@rhettinger, @miss-islington, @lf-
PRs	bpo-46228: argparse docs: prog default is the basename of argv[0] #30298 [3.10] bpo-46228: argparse docs: prog default is the basename of argv[0] (GH-30298) #30339
Files	test.py: Test program from above

^{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 2022-01-02.20:19:59.565>
created_at = <Date 2022-01-02.19:50:38.724>
labels = ['type-bug', '3.9', '3.10', '3.11', 'library', 'docs']
title = 'argparse docs: default for prog= in ArgumentParser() should be basename of argv[0], not argv[0], to match behaviour'
updated_at = <Date 2022-02-02.16:56:39.827>
user = 'https://github.com/lf'

bugs.python.org fields:

activity = <Date 2022-02-02.16:56:39.827>
actor = 'paul.j3'
assignee = 'docs@python'
closed = True
closed_date = <Date 2022-01-02.20:19:59.565>
closer = 'rhettinger'
components = ['Documentation', 'Library (Lib)']
creation = <Date 2022-01-02.19:50:38.724>
creator = 'lf-'
dependencies = []
files = ['50536']
hgrepos = []
issue_num = 46228
keywords = ['patch']
message_count = 2.0
messages = ['409515', '409518']
nosy_count = 5.0
nosy_names = ['rhettinger', 'docs@python', 'paul.j3', 'miss-islington', 'lf-']
pr_nums = ['30298', '30339']
priority = 'normal'
resolution = 'fixed'
stage = 'resolved'
status = 'closed'
superseder = None
type = 'behavior'
url = 'https://bugs.python.org/issue46228'
versions = ['Python 3.9', 'Python 3.10', 'Python 3.11']

[lf]

Currently the documentation for argparse.ArgumentParser states that the default value of the prog[ram] argument is argv[0], however, this does not match the actual behaviour of the constructor. In reality, the constructor uses the basename of argv[0], as can be seen in the code here:

cpython/Lib/argparse.py

Lines 1733 to 1735 in e800dd1

	# default setting for prog
	if prog is None:
	prog = _os.path.basename(_sys.argv[0])

Here is a demo:

/tmp » cat test.py
import argparse

ap = argparse.ArgumentParser()
print(ap.prog)
ap.print_usage()
/tmp » python test.py
test.py
usage: test.py [-h]
/tmp » python ./test.py
test.py
usage: test.py [-h]
/tmp » mkdir test
/tmp » cd test
/tmp/test » python ../test.py
test.py
usage: test.py [-h]