Add OutputContainer.add_mux_stream() for codec-context-free streams

Adds `add_mux_stream(codec_name, rate=None, **kwargs)` to `OutputContainer`,
allowing users to create a stream with only `codecpar` set (codec id, type,
width, height, sample_rate) and no `CodecContext`. This is useful when muxing
pre-encoded packets from an external source where no encoding or decoding is
needed, separating the muxer role from the encoder role.

Also relaxes `start_encoding()` to allow any stream type without a codec
context (previously only data/attachment streams were permitted), and guards
`VideoStream`/`AudioStream` repr and `__getattr__` against `codec_context=None`.

Two missing fields (`AVMediaType type` on `AVCodecDescriptor`, and `width`,
`height`, `sample_rate` on `AVCodecParameters`) are added to the pxd
declarations so they can be accessed from Cython.

Closes #1970

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

Author: WyattBlue

av/audio/stream.py

@@ -6,13 +6,19 @@
 @cython.cclass
 class AudioStream(Stream):
     def __repr__(self):
+        if self.codec_context is None:
+            return f"<av.AudioStream #{self.index} audio/<nocodec> at 0x{id(self):x}>"
         form = self.format.name if self.format else None
         return (
             f"<av.AudioStream #{self.index} {self.name} at {self.rate}Hz,"
             f" {self.layout.name}, {form} at 0x{id(self):x}>"
         )
 
     def __getattr__(self, name):
+        if self.codec_context is None:
+            raise AttributeError(
+                f"'{type(self).__name__}' object has no attribute '{name}'"
+            )
         return getattr(self.codec_context, name)
 
     @cython.ccall

av/container/output.py

@@ -137,6 +137,84 @@ def add_stream(self, codec_name, rate=None, options: dict | None = None, **kwarg
 
         return py_stream
 
+    def add_mux_stream(
+        self, codec_name: str, rate=None, **kwargs
+    ) -> Stream:
+        """add_mux_stream(codec_name, rate=None)
+
+        Creates a new stream for muxing pre-encoded data without creating a
+        :class:`.CodecContext`. Use this when you want to mux packets that were
+        already encoded externally and no encoding/decoding is needed.
+
+        :param codec_name: The name of a codec.
+        :type codec_name: str
+        :param \\**kwargs: Set attributes for the stream (e.g. ``width``, ``height``,
+            ``time_base``).
+        :rtype: The new :class:`~av.stream.Stream`.
+
+        """
+        # Find the codec to get its id and type (try encoder first, then decoder).
+        codec: cython.pointer[cython.const[lib.AVCodec]] = (
+            lib.avcodec_find_encoder_by_name(codec_name.encode())
+        )
+        codec_descriptor: cython.pointer[cython.const[lib.AVCodecDescriptor]] = (
+            cython.NULL
+        )
+        if codec == cython.NULL:
+            codec = lib.avcodec_find_decoder_by_name(codec_name.encode())
+        if codec == cython.NULL:
+            codec_descriptor = lib.avcodec_descriptor_get_by_name(codec_name.encode())
+            if codec_descriptor == cython.NULL:
+                raise ValueError(f"Unknown codec: {codec_name!r}")
+
+        codec_id: lib.AVCodecID
+        codec_type: lib.AVMediaType
+        if codec != cython.NULL:
+            codec_id = codec.id
+            codec_type = codec.type
+        else:
+            codec_id = codec_descriptor.id
+            codec_type = codec_descriptor.type
+
+        # Assert that this format supports the requested codec.
+        if not lib.avformat_query_codec(
+            self.ptr.oformat, codec_id, lib.FF_COMPLIANCE_NORMAL
+        ):
+            raise ValueError(
+                f"{self.format.name!r} format does not support {codec_name!r} codec"
+            )
+
+        # Create stream with no codec context.
+        stream: cython.pointer[lib.AVStream] = lib.avformat_new_stream(
+            self.ptr, cython.NULL
+        )
+        if stream == cython.NULL:
+            raise MemoryError("Could not allocate stream")
+
+        stream.codecpar.codec_id = codec_id
+        stream.codecpar.codec_type = codec_type
+
+        if codec_type == lib.AVMEDIA_TYPE_VIDEO:
+            stream.codecpar.width = kwargs.pop("width", 0)
+            stream.codecpar.height = kwargs.pop("height", 0)
+            if rate is not None:
+                to_avrational(rate, cython.address(stream.avg_frame_rate))
+        elif codec_type == lib.AVMEDIA_TYPE_AUDIO:
+            if rate is not None:
+                if type(rate) is int:
+                    stream.codecpar.sample_rate = rate
+                else:
+                    raise TypeError("audio stream `rate` must be: int | None")
+
+        # Construct the user-land stream (no codec context).
+        py_stream: Stream = wrap_stream(self, stream, None)
+        self.streams.add_stream(py_stream)
+
+        for k, v in kwargs.items():
+            setattr(py_stream, k, v)
+
+        return py_stream
+
     def add_stream_from_template(
         self, template: Stream, opaque: bool | None = None, **kwargs
     ):
@@ -361,10 +439,10 @@ def start_encoding(self):
         # Finalize and open all streams.
         for stream in self.streams:
             ctx = stream.codec_context
-            # Skip codec context handling for streams without codecs (e.g. data/attachments).
+            # Skip codec context handling for streams without codecs
+            # (e.g. data, attachments, and mux-only streams).
             if ctx is None:
-                if stream.type not in {"data", "attachment"}:
-                    raise ValueError(f"Stream {stream.index} has no codec context")
+                pass
             else:
                 if not ctx.is_open:
                     for k, v in self.options.items():

av/container/output.pyi

@@ -39,6 +39,12 @@ class OutputContainer(Container):
         options: dict[str, str] | None = None,
         **kwargs,
     ) -> VideoStream | AudioStream | SubtitleStream: ...
+    def add_mux_stream(
+        self,
+        codec_name: str,
+        rate: Fraction | int | None = None,
+        **kwargs,
+    ) -> Stream: ...
     def add_stream_from_template(
         self, template: _StreamT, opaque: bool | None = None, **kwargs
     ) -> _StreamT: ...

av/video/stream.py

@@ -8,6 +8,8 @@
 @cython.cclass
 class VideoStream(Stream):
     def __repr__(self):
+        if self.codec_context is None:
+            return f"<av.VideoStream #{self.index} video/<nocodec> at 0x{id(self):x}>"
         return (
             f"<av.VideoStream #{self.index} {self.name}, "
             f"{self.format.name if self.format else None} {self.codec_context.width}x"
@@ -19,7 +21,10 @@ def __getattr__(self, name):
             raise AttributeError(
                 f"'{type(self).__name__}' object has no attribute '{name}'"
             )
-
+        if self.codec_context is None:
+            raise AttributeError(
+                f"'{type(self).__name__}' object has no attribute '{name}'"
+            )
         return getattr(self.codec_context, name)
 
     @cython.ccall

include/avcodec.pxd

@@ -206,6 +206,7 @@ cdef extern from "libavcodec/avcodec.h" nogil:
 
     cdef struct AVCodecDescriptor:
         AVCodecID id
+        AVMediaType type
         char *name
         char *long_name
         int props
@@ -470,6 +471,9 @@ cdef extern from "libavcodec/avcodec.h" nogil:
         AVCodecID codec_id
         uint8_t *extradata
         int extradata_size
+        int width
+        int height
+        int sample_rate
 
     cdef int avcodec_parameters_copy(
         AVCodecParameters *dst, const AVCodecParameters *src

tests/test_remux.py

@@ -1,3 +1,5 @@
+import io
+
 import av
 import av.datasets
 
@@ -31,3 +33,49 @@ def test_video_remux() -> None:
             packet_count += 1
 
         assert packet_count > 50
+
+
+def test_add_mux_stream_video() -> None:
+    """add_mux_stream creates a video stream without a CodecContext."""
+    input_path = av.datasets.curated("pexels/time-lapse-video-of-night-sky-857195.mp4")
+
+    buf = io.BytesIO()
+    with av.open(input_path) as input_:
+        in_stream = input_.streams.video[0]
+        width = in_stream.codec_context.width
+        height = in_stream.codec_context.height
+
+        with av.open(buf, "w", format="mp4") as output:
+            out_stream = output.add_mux_stream(
+                in_stream.codec_context.name, width=width, height=height
+            )
+            assert out_stream.codec_context is None
+            assert out_stream.type == "video"
+
+            out_stream.time_base = in_stream.time_base
+
+            for packet in input_.demux(in_stream):
+                if packet.dts is None:
+                    continue
+                packet.stream = out_stream
+                output.mux(packet)
+
+    buf.seek(0)
+    with av.open(buf) as result:
+        assert len(result.streams.video) == 1
+        assert result.streams.video[0].codec_context.width == width
+        assert result.streams.video[0].codec_context.height == height
+
+
+def test_add_mux_stream_no_codec_context() -> None:
+    """add_mux_stream streams have no codec context and repr does not crash."""
+    buf = io.BytesIO()
+    with av.open(buf, "w", format="mp4") as output:
+        video_stream = output.add_mux_stream("h264", width=1920, height=1080)
+        audio_stream = output.add_mux_stream("aac", rate=44100)
+
+        assert video_stream.codec_context is None
+        assert audio_stream.codec_context is None
+        # repr should not crash
+        assert "video/<nocodec>" in repr(video_stream)
+        assert "audio/<nocodec>" in repr(audio_stream)