MultiDict converts byte values to int list since version 3.1.0

[ricky-provan]

Since version 3.1.0, when I create a MultiDict with a bytes value as input it stores the bytes value in the MultiDict as a list of integer values. Prior to 3.1.0, this would preserve the bytes value in the MultiDict.

To reproduce, run this snippet with werkzeug installed:

from werkzeug.datastructures import MultiDict

multi_dict = MultiDict({
    'bytes_value': b'abc',
    'str_value': 'abc'
})

print(multi_dict)

Running the above on version 3.1.0 prints:
MultiDict([('bytes_value', 97), ('bytes_value', 98), ('bytes_value', 99), ('str_value', 'abc')])

Running the snippet with version 3.0.6 prints:
MultiDict([('bytes_value', b'abc'), ('str_value', 'abc')])

I expected the behaviour from 3.0.6 to be preserved in this case

Environment:

• Python version: 3.11
• Werkzeug version: >= 3.1.0

[davidism]

I switched to isinstance(value, Collection) and not isinstance(value, str) instead of isinstance(value, (list, tuple)), in order to accept more types of iterable data. However, bytes, bytearray, memoryview, and probably other types that shouldn't be treated as multiple values, are also matched by that. I think I might switch back to the (list, tuple) check.