MP4 fails with string argument for freeform text ('----') tags

[Simon Lambourn]

mutagen.mp4.MP4 container does not accept a string value for freeform text tags, but expects (and returns) a bytes argument. This is inconsistent with python3 default of string values.

code:
>>> import mutagen.mp4.MP4 as MP4
>>> m = MP4()
>>> m['----:test:item'] = 'test value'
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "/usr/lib/python3/dist-packages/mutagen/_file.py", line 75, in __setitem__
    self.tags[key] = value
  File "/usr/lib/python3/dist-packages/mutagen/mp4/__init__.py", line 375, in __setitem__
    self._render(key, value)
  File "/usr/lib/python3/dist-packages/mutagen/mp4/__init__.py", line 391, in _render
    return render_func(self, key, value, *render_args)
  File "/usr/lib/python3/dist-packages/mutagen/mp4/__init__.py", line 632, in __render_freeform
    data += v
TypeError: can't concat str to bytes
>>> m['----:test:item'] = b'test value'
>>> m['----:test:item']
b'test value'
>>>

Expected behaviour: accept (and return) string values, with encoding to utf-8 if required (this is implied in some of the comments in the code).

ProblemType: Bug
DistroRelease: Ubuntu 19.04
Package: python3-mutagen 1.40.0-2
ProcVersionSignature: Ubuntu 5.0.0-20.21-generic 5.0.8
Uname: Linux 5.0.0-20-generic x86_64
ApportVersion: 2.20.10-0ubuntu27
Architecture: amd64
CurrentDesktop: ubuntu:GNOME
Date: Mon Jul 8 15:03:10 2019
Dependencies:

PackageArchitecture: all
SourcePackage: mutagen
UpgradeStatus: Upgraded to disco on 2019-04-25 (74 days ago)

[Simon Lambourn (simon-lambourn-o)]

PS. The use case for this is in storing and retrieving musicbrainz ID tags, which are stored as freeform tags.

[Christoph Reiter (lazka)]

Please report things upstream: https://github.com/quodlibet/mutagen/issues

[Simon Lambourn (simon-lambourn-o)]

Christoph - thank you for the pointer. I have reported it as you suggest: https://github.com/quodlibet/mutagen/issues/391