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GeezerVison is showing github.com — gh-131178: Fix mimetypes CLI docs to mention that errors go to stdout by htjworld · Pull Request #149683 · python/cpython

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@htjworld

@htjworld htjworld commented May 11, 2026 •
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The mimetypes documentation states that errors go to the standard error stream, but the CLI has always written all output — including errors — to stdout.

Fix the documentation to match the actual behavior, and add subprocess-based tests for the command-line interface.

Changes:

  • Doc/library/mimetypes.rst: Correct "standard error stream" → "standard output stream"; update stale examples (filename.pict is now a recognized type, error message strings did not match actual output).
  • Lib/test/test_mimetypes.py: Add subprocess-based tests verifying CLI output format and exit codes.
  • Issue: Add tests for command-line interface #131178

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@htjworld
pythongh-149682: Fix mimetypes CLI to write error messages to stderr
c87667b
@htjworld
htjworld requested a review from a team as a code owner May 11, 2026 14:39
@bedevere-app bedevere-app Bot mentioned this pull request May 11, 2026
mimetypes CLI writes error messages to stdout instead of stderr #149682
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@bedevere-app bedevere-app Bot added the awaiting review label May 11, 2026
@htjworld htjworld mentioned this pull request May 11, 2026
gh-131178: Add tests for mimetypes command-line interface #149681
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@htjworld
Remove inline comments from docs examples
77e5b2a
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read-the-docs-community Bot commented May 11, 2026 •
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Documentation build overview

📚 cpython-previews | 🛠️ Build #32921239 | 📁 Comparing 4b28aed against main (2d3dec0)

  🔍 Preview build  

63 files changed · + 1 added · ± 62 modified

+ Added

  • deprecations/pending-removal-in-3.21.html

± Modified

  • glossary.html
  • index.html
  • c-api/import.html
  • c-api/sentinel.html
  • c-api/stable.html
  • c-api/threads.html
  • deprecations/index.html
  • deprecations/pending-removal-in-3.16.html
  • howto/free-threading-python.html
  • library/audit_events.html
  • and 52 more...
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@htjworld htjworld changed the title gh-149682: Fix mimetypes CLI to write error messages to stderr Fix mimetypes CLI to write error messages to stderr May 11, 2026
@htjworld htjworld changed the title Fix mimetypes CLI to write error messages to stderr gh-149682: Fix mimetypes CLI to write error messages to stderr May 11, 2026
@htjworld htjworld changed the title gh-149682: Fix mimetypes CLI to write error messages to stderr gh-131178: Fix mimetypes CLI to write error messages to stderr May 11, 2026
@bedevere-app bedevere-app Bot mentioned this pull request May 11, 2026
Add tests for command-line interface #131178
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sobolevn
sobolevn reviewed May 11, 2026
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This is a rather hard problem: when docs do not represent the reality, we can either fix docs or fix the code.

I guess a lot of places already rely on the fact that errors and results are in the stdout, not in stderr.

So, I think that it would be better to change the docs.

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Comment thread Lib/test/test_mimetypes.py Outdated

def test_type_lookup(self):
rc, stdout, stderr = assert_python_ok('-m', 'mimetypes', 'foo.pdf')
self.assertIn(b'application/pdf', stdout)

@sobolevn sobolevn May 11, 2026

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Why not asserting the full stdout contents?

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Comment thread Lib/test/test_mimetypes.py Outdated
def test_type_lookup_unknown(self):
rc, stdout, stderr = assert_python_failure('-m', 'mimetypes', 'foo.unknownext12345')
self.assertEqual(stdout, b'')
self.assertIn(b'error:', stderr)

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And the same here: we can assert the full error response.

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Comment thread Lib/mimetypes.py Outdated
has_error = False
for result in results:
if result.startswith("error: "):
print(result, file=sys.stderr, flush=True)

@sobolevn sobolevn May 11, 2026

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This is a behavior change, please revert it.

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Comment thread Misc/NEWS.d/next/Library/2026-05-11-23-38-47.gh-issue-149682.UE08R4.rst Outdated
@@ -0,0 +1,2 @@
Fix :mod:`mimetypes` CLI to write error messages to stderr instead of stdout,

@sobolevn sobolevn May 11, 2026

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And since we revert the behavior change, we need to remove this file as well :)

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htjworld added 2 commits May 12, 2026 00:33
@htjworld
Address review feedback: revert stderr routing, update docs and tests
5b75aaa 
Per sobolevn's review: revert the behavior change that routed error
messages to stderr, as existing code may rely on stdout behavior.
Instead update docs to accurately reflect that errors go to stdout.

Also update tests to assert full output contents as suggested.
@htjworld
Add NEWS entry for mimetypes CLI tests
ad605cd
@htjworld

htjworld commented May 11, 2026

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I have made the requested changes; please review again
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@bedevere-app bedevere-app Bot added awaiting change review and removed awaiting review labels May 11, 2026
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Thanks for making the requested changes!

: please review the changes made to this pull request.
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sobolevn
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Comment thread Lib/test/test_mimetypes.py Outdated

def test_unknown_flag(self):
rc, stdout, stderr = assert_python_failure('-m', 'mimetypes', '--unknown-flag')
self.assertNotEqual(rc, 0)

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assert the output please :)

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Comment thread Misc/NEWS.d/next/Tests/2026-05-12-00-35-20.gh-issue-131178.mX9pQa.rst Outdated
@@ -0,0 +1,2 @@
Add subprocess-based tests for the :mod:`mimetypes` command-line interface,

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we don't add news for docs / tests only changes.

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Comment thread Lib/test/test_mimetypes.py Outdated

def test_extension_flag(self):
rc, stdout, stderr = assert_python_ok('-m', 'mimetypes', '-e', 'image/jpeg')
self.assertIn(b'.jpg', stdout)

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Let's use assertEqual here as well.

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@htjworld
Address review feedback: strengthen test assertions, remove NEWS entry
f6db7b1
@htjworld

htjworld commented May 11, 2026

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thanks for the review appreciate it
I have made the requested changes; please review again
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Thanks for making the requested changes!

: please review the changes made to this pull request.
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Comment thread Lib/test/test_mimetypes.py Outdated
def test_unknown_flag(self):
rc, stdout, stderr = assert_python_failure('-m', 'mimetypes', '--unknown-flag')
self.assertEqual(stdout, b'')
self.assertIn(b'error', stderr)

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Suggested change
self.assertIn(b'error', stderr)
self.assertIn(b'error: unrecognized arguments: --unknown-flag', stderr)

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@sobolevn sobolevn added the skip news label May 12, 2026
@htjworld
pythongh-131178: fix test_unknown_flag to supply positional arg
edf95da
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python-cla-bot Bot commented May 12, 2026 •
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All commit authors signed the Contributor License Agreement.

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@htjworld
pythongh-131178: add @force_not_colorized to test_unknown_flag
b00896c
sobolevn
sobolevn approved these changes May 12, 2026
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Thanks!

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@bedevere-app bedevere-app Bot added awaiting merge and removed awaiting change review labels May 12, 2026
clin1234
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If CI passes, it LGTM ;-)

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@htjworld

htjworld commented May 31, 2026

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Hi @sobolevn, gentle ping 🙂 — is there anything else needed on my end for this to move forward? Thanks!
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@sobolevn sobolevn changed the title gh-131178: Fix mimetypes CLI to write error messages to stderr gh-131178: Fix mimetypes CLI docs to mention that errors go to stdout May 31, 2026
@sobolevn sobolevn added needs backport to 3.15 bugs and security fixes needs backport to 3.13 only security fixes needs backport to 3.14 bugs and security fixes labels May 31, 2026
@sobolevn
Merge branch 'main' into pythongh-149682-mimetypes-fix-stderr
e0409b3
sobolevn
sobolevn reviewed May 31, 2026
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Comment thread Doc/library/mimetypes.rst Resolved

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Comment thread Doc/library/mimetypes.rst Resolved

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@htjworld
pythongh-131178: revert unnecessary CLI example changes in mimetypes …
277b007 
…docs

Restore the original filename.pict / --lenient example and the
filename.xxx multi-input example. Keep only the genuinely-needed
fix: the error message string ("unknown extension of X" was never
emitted; the CLI prints "media type unknown for X").
sobolevn
sobolevn reviewed May 31, 2026
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Comment thread Doc/library/mimetypes.rst Resolved

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@htjworld @sobolevn
Update Doc/library/mimetypes.rst
4b28aed 
Co-authored-by: sobolevn <[email protected]>
sobolevn
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Thanks a lot!
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@sobolevn
sobolevn enabled auto-merge (squash) May 31, 2026 07:17
@htjworld

htjworld commented May 31, 2026

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Thank you for the thorough review!
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@sobolevn
sobolevn merged commit 2b94b92 into python:main May 31, 2026
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@miss-islington-app

miss-islington-app Bot commented May 31, 2026

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Thanks @htjworld for the PR, and @sobolevn for merging it 🌮🎉.. I'm working now to backport this PR to: 3.13, 3.14, 3.15.
🐍🍒⛏🤖
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@bedevere-app bedevere-app Bot removed the awaiting merge label May 31, 2026
@miss-islington-app

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Sorry, @htjworld and @sobolevn, I could not cleanly backport this to 3.13 due to a conflict.
Please backport using cherry_picker on command line.

cherry_picker 2b94b923943a1f75cdbd5a5e4e2eb5f93eb43e23 3.13
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@bedevere-app

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GH-150655 is a backport of this pull request to the 3.15 branch.
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@bedevere-app bedevere-app Bot removed the needs backport to 3.15 bugs and security fixes label May 31, 2026
@miss-islington-app miss-islington-app Bot assigned sobolevn May 31, 2026
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GH-150656 is a backport of this pull request to the 3.14 branch.
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@bedevere-app bedevere-app Bot removed the needs backport to 3.14 bugs and security fixes label May 31, 2026
@sobolevn

sobolevn commented May 31, 2026

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@htjworld can you please follow the instructions in #149683 (comment) and create a manual 3.13 backport?
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@htjworld

htjworld commented May 31, 2026

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@sobolevn, I looked into the 3.13 backport — the mimetypes CLI was reworked after 3.13, so the changes don't seem to apply cleanly:

  • The Command-line usage section this PR edits doesn't exist in 3.13 (it appears to have been added in 3.14).
  • The 3.13 CLI still uses the old getopt-based implementation and prints I don't know anything about type X rather than error: media type unknown for X, so the new CommandLineSubprocessTest assertions would fail there.

Would it make sense to drop the needs backport to 3.13 label?
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@sobolevn

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Then no - thanks for the investigation :)
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sobolevn added a commit that referenced this pull request May 31, 2026
@miss-islington @htjworld @sobolevn
[3.14] gh-131178: Fix mimetypes CLI docs, mention that errors go to s…
15eb113 
…tdout (GH-149683) (#150656)

gh-131178: Fix mimetypes CLI docs, mention that errors go to stdout (GH-149683)
(cherry picked from commit 2b94b92)

Co-authored-by: htjworld <[email protected]>
Co-authored-by: sobolevn <[email protected]>
sobolevn added a commit that referenced this pull request May 31, 2026
@miss-islington @htjworld @sobolevn
[3.15] gh-131178: Fix mimetypes CLI docs, mention that errors go to s…
2ff80c7 
…tdout (GH-149683) (#150655)

gh-131178: Fix mimetypes CLI docs, mention that errors go to stdout (GH-149683)
(cherry picked from commit 2b94b92)

Co-authored-by: htjworld <[email protected]>
Co-authored-by: sobolevn <[email protected]>
@StanFromIreland StanFromIreland removed the needs backport to 3.13 only security fixes label Jun 5, 2026
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