Support writing arbitrary key/value pairs to container metadata

[Michael-Equi]

Desired Behavior

Currently only the title is allowed to be written to a containers metadata. The behavior is tested in the following unit test:

PyAV/tests/test_encode.py

Line 74 in 86f66ac

assert input_.metadata.get("key") is None

I have confirmed ffmpeg does support this by using the following CLI command given the path to a libx264 encoded video.

subprocess.run(
        [
            "ffmpeg",
            "-y",  # Overwrite output files without asking
            "-i",
            input,
            "-c",
            "copy",
            "-copyts",  # Copy timestamps from input to output
            "-metadata",
            f"key={value}",
            "-movflags",
            "+use_metadata_tags",
            output,
        ],

Example API

Allow any arbitrary key/value pair to be written to the metadata in the same way title can be written to the metadata.

[dotysan]

In addition to adding metadata to the container. Let's also be able to add it to the streams.

• -metadata:s:v:0 source=ACMEcam
• -metadata:s:a:0 sound=Silence

[WyattBlue]

(newer) PyAV already supports arbitrary container metadata. For MP4, FFmpeg requires the use_metadata_tags muxer flag for nonstandard keys:

with av.open(
    output, mode="w", format="mp4", options={"movflags": "+use_metadata_tags"},
) as container:
    container.metadata["key"] = "value"

Make sure metadata is set before encoding or muxing begins, since the metadata is written with the container header.

Streams also expose a metadata dictionary:

stream = container.add_stream("libx264", rate=30)
stream.metadata["source"] = "ACMEcam"

Arbitrary stream metadata works with formats such as Matroska. MP4 only preserves certain stream-level metadata fields, even when use_metadata_tags is enabled, which is a limitation of FFmpeg’s MP4 muxer rather than PyAV.