python -m gzip handles error incorrectly

[rhpvorderman]

BPO	43316
Nosy	@methane, @miss-islington, @rhpvorderman
PRs	bpo-43316: gzip: CLI uses non-zero return code on error. #24647 [3.9] bpo-43316: gzip: CLI uses non-zero return code on error. (GH-24647) #24648 [3.8] bpo-43316: gzip: CLI uses non-zero return code on error. (GH-24647) #24649 bpo-43316: gzip: Fix sys.exit() usage. #24652 [3.9] bpo-43316: gzip: Fix sys.exit() usage. (GH-24652) #24653 [3.8] bpo-43316: gzip: Fix sys.exit() usage. (GH-24652) #24654

^{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 2021-02-26.02:37:07.748>
created_at = <Date 2021-02-24.14:34:00.824>
labels = ['type-bug', '3.8', '3.9', '3.10', '3.7', 'library']
title = 'python -m gzip handles error incorrectly'
updated_at = <Date 2021-02-26.02:37:07.747>
user = 'https://github.com/rhpvorderman'

bugs.python.org fields:

activity = <Date 2021-02-26.02:37:07.747>
actor = 'methane'
assignee = 'none'
closed = True
closed_date = <Date 2021-02-26.02:37:07.748>
closer = 'methane'
components = ['Library (Lib)']
creation = <Date 2021-02-24.14:34:00.824>
creator = 'rhpvorderman'
dependencies = []
files = []
hgrepos = []
issue_num = 43316
keywords = ['patch']
message_count = 9.0
messages = ['387622', '387647', '387650', '387659', '387660', '387698', '387700', '387702', '387703']
nosy_count = 3.0
nosy_names = ['methane', 'miss-islington', 'rhpvorderman']
pr_nums = ['24647', '24648', '24649', '24652', '24653', '24654']
priority = 'normal'
resolution = 'fixed'
stage = 'resolved'
status = 'closed'
superseder = None
type = 'behavior'
url = 'https://bugs.python.org/issue43316'
versions = ['Python 3.6', 'Python 3.7', 'Python 3.8', 'Python 3.9', 'Python 3.10']

[rhpvorderman]

Python -m gzip -d myfile will throw an error because myfile does not end in '.gz'. That is fair (even though a bit redundant, GzipFile contains a header check, so why bother checking the extension?).

The problem is how this error is thrown.

1. Error is printed to stdout instead of stderr
2. Tool exits with exit 0.

This is not the behaviour that is expected when using python -m gzip in a script.

The error is even codified in a test:

cpython/Lib/test/test_gzip.py

Line 776 in 1f43340

def test_decompress_infile_outfile_error(self):

    def test_decompress_infile_outfile_error(self):
        rc, out, err = assert_python_ok('-m', 'gzip', '-d', 'thisisatest.out')
        self.assertIn(b"filename doesn't end in .gz:", out)
        self.assertEqual(rc, 0)
        self.assertEqual(err, b'')

This should be assert_python_failure, out and err should be swapped, and exit code should be something different than 0.

From the zen of python: Errors should never pass silently.

I am willing to fix this in a PR, but first I would like some feedback on how to solve this exactly.

I propose raising a ValueError("can not determine output filename: 'myfile' does not end in '.gz'").

[methane]

I prefer sys.exit("message").

[rhpvorderman]

That sounds perfect, I didn't think of that. I will make a PR.