Inconsistency with uppercase file extensions in MimeTypes.guess_type

[rodrigoparra]

BPO	20392
Nosy	@tjguk, @bitdancer, @asvetlov, @miss-islington, @iritkatriel, @kumaraditya303
PRs	bpo-20392: Fix inconsistency with uppercase file extensions in mimetypes.guess_type #30229 [3.9] bpo-20392: Fix inconsistency with uppercase file extensions in mimetypes.guess_type (GH-30229) #31903 [3.10] bpo-20392: Fix inconsistency with uppercase file extensions in mimetypes.guess_type (GH-30229) #31904
Files	case_guess_type.patch

^{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 2022-03-15.15:50:53.918>
created_at = <Date 2014-01-25.17:17:48.028>
labels = ['easy', 'type-bug', '3.9', '3.10', '3.11', 'library']
title = 'Inconsistency with uppercase file extensions in MimeTypes.guess_type'
updated_at = <Date 2022-03-15.15:50:53.917>
user = 'https://bugs.python.org/rodrigoparra'

bugs.python.org fields:

activity = <Date 2022-03-15.15:50:53.917>
actor = 'asvetlov'
assignee = 'none'
closed = True
closed_date = <Date 2022-03-15.15:50:53.918>
closer = 'asvetlov'
components = ['Library (Lib)']
creation = <Date 2014-01-25.17:17:48.028>
creator = 'rodrigo.parra'
dependencies = []
files = ['33703']
hgrepos = []
issue_num = 20392
keywords = ['patch', 'easy']
message_count = 6.0
messages = ['209218', '209332', '408460', '415243', '415252', '415258']
nosy_count = 7.0
nosy_names = ['tim.golden', 'r.david.murray', 'asvetlov', 'rodrigo.parra', 'miss-islington', 'iritkatriel', 'kumaraditya']
pr_nums = ['30229', '31903', '31904']
priority = 'normal'
resolution = 'fixed'
stage = 'resolved'
status = 'closed'
superseder = None
type = 'behavior'
url = 'https://bugs.python.org/issue20392'
versions = ['Python 3.9', 'Python 3.10', 'Python 3.11']

[rodrigoparra]

The functions looks up for the file extension in three maps: types_map, suffix_map and encodings_map.

Lookup in types_map is case insensitive (by calling lower() first).
Lookup in both suffix_map and encodings_map is case sensitive.

These can lead to some seemingly counterintuitive results, like:

a)
guess_type("foo.tar") == ("application/x-tar", None)
guess_type("foo.TAR") == ("application/x-tar", None)

b)
guess_type("foo.tgz") == ("application/x-tar", "gzip")
guess_type("foo.TGZ") == (None, None)

c)
guess_type("foo.tar.gz") == ("application/x-tar", "gzip")
guess_type("foo.TAR.GZ") == (None, None)

Lookup should be case insensitive at least for the suffix_map, in which case (b) would be solved. The submitted patch implements this change.

As for the encodings_map, I am not so sure, in particular because of the tar.Z extension. I found that the compress command expects the uppercase 'Z'. If someone is relying in the results of guess_type to call compress, errors could occur.

[bitdancer]

I'm tagging this for 3.5 instead, since there are backward compatibility concerns and the 3.4 RC will probably be a couple weeks from now.

[iritkatriel]

Reproduced on 3.11.

[asvetlov]

New changeset 5dd7ec5 by Kumar Aditya in branch 'main':
bpo-20392: Fix inconsistency with uppercase file extensions in mimetypes.guess_type (GH-30229)
5dd7ec5

[miss-islington]

New changeset 3c4f24f by Miss Islington (bot) in branch '3.10':
bpo-20392: Fix inconsistency with uppercase file extensions in mimetypes.guess_type (GH-30229)
3c4f24f

[miss-islington]

New changeset 32ae9ab by Miss Islington (bot) in branch '3.9':
bpo-20392: Fix inconsistency with uppercase file extensions in mimetypes.guess_type (GH-30229)
32ae9ab