mimetypes.guess_all_extensions potentially mutates list

[rmccampbell7]

BPO	35474
Nosy	@ambv, @serhiy-storchaka, @miss-islington
PRs	bpo-35474: Fix mimetypes.guess_all_extensions() potentially mutating list #28286 [3.10] bpo-35474: Fix mimetypes.guess_all_extensions() potentially mutating list (GH-28286) #28289 [3.9] bpo-35474: Fix mimetypes.guess_all_extensions() potentially mutating list (GH-28286) #28290

^{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 2021-09-13.17:25:10.378>
created_at = <Date 2018-12-12.19:06:57.748>
labels = ['type-bug', 'library', '3.9', '3.10', '3.11']
title = 'mimetypes.guess_all_extensions potentially mutates list'
updated_at = <Date 2021-09-13.17:25:10.376>
user = 'https://bugs.python.org/rmccampbell7'

bugs.python.org fields:

activity = <Date 2021-09-13.17:25:10.376>
actor = 'lukasz.langa'
assignee = 'none'
closed = True
closed_date = <Date 2021-09-13.17:25:10.378>
closer = 'lukasz.langa'
components = ['Library (Lib)']
creation = <Date 2018-12-12.19:06:57.748>
creator = 'rmccampbell7'
dependencies = []
files = []
hgrepos = []
issue_num = 35474
keywords = ['patch']
message_count = 6.0
messages = ['331715', '401632', '401640', '401717', '401718', '401719']
nosy_count = 4.0
nosy_names = ['lukasz.langa', 'serhiy.storchaka', 'rmccampbell7', 'miss-islington']
pr_nums = ['28286', '28289', '28290']
priority = 'normal'
resolution = 'fixed'
stage = 'resolved'
status = 'closed'
superseder = None
type = 'behavior'
url = 'https://bugs.python.org/issue35474'
versions = ['Python 3.9', 'Python 3.10', 'Python 3.11']

[rmccampbell7]

The mimetypes.guess_all_extensions function is defined as:

def guess_all_extensions(self, type, strict=True):
    type = type.lower()
    extensions = self.types_map_inv[True].get(type, [])
    if not strict:
        for ext in self.types_map_inv[False].get(type, []):
            if ext not in extensions:
                extensions.append(ext)
    return extensions

If any mime type exists in both the strict and non-strict types_map_inv and it is called with strict=False, then it will modify the strict list in-place which effects future calls even with strict=True. While this doesn't manifest as an error for me because the dictionaries are non-overlapping, it is a potential error; it is also vulnerable to people accidentally modifying the returned list. The list should be copied after the first lookup.

[serhiy-storchaka]

Also, mutating the result of guess_all_extensions() mutated the internal state of the MimeTypes object and affected the following calls of guess_all_extensions().

[serhiy-storchaka]

New changeset 97ea18e by Serhiy Storchaka in branch 'main':
bpo-35474: Fix mimetypes.guess_all_extensions() potentially mutating list (GH-28286)
97ea18e

[ambv]

New changeset 9cd8fb8 by Miss Islington (bot) in branch '3.9':
bpo-35474: Fix mimetypes.guess_all_extensions() potentially mutating list (GH-28286) (GH-28290)
9cd8fb8

[ambv]

New changeset 06c26f4 by Miss Islington (bot) in branch '3.10':
bpo-35474: Fix mimetypes.guess_all_extensions() potentially mutating list (GH-28286) (GH-28289)
06c26f4

[ambv]

Thanks, Serhiy! ✨ 🍰 ✨