[New-bugs-announce] [issue35474] mimetypes.guess_all_extensions potentially mutates list

Ryan McCampbell report at bugs.python.org
Wed Dec 12 14:06:57 EST 2018

New submission from Ryan McCampbell <rmccampbell7 at gmail.com>:

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:
    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.

components: Library (Lib)
messages: 331715
nosy: rmccampbell7
priority: normal
severity: normal
status: open
title: mimetypes.guess_all_extensions potentially mutates list
type: behavior

Python tracker <report at bugs.python.org>

More information about the New-bugs-announce mailing list