How to: ℹ️ See help(yt_dlp.postprocessor.PostProcessor)
class MyCustomPP(yt_dlp.postprocessor.PostProcessor):
    def run(self, info):
        self.to_screen('Doing stuff')
        return [], info

with yt_dlp.YoutubeDL() as ydl:
    # ℹ️ "when" can take any value in yt_dlp.utils.POSTPROCESS_WHEN
    ydl.add_post_processor(MyCustomPP(), when='pre_process')
    ydl.download(URLS)
```

#### Use a custom format selector

```python
import yt_dlp

URLS = ['https://www.youtube.com/watch?v=BaW_jenozKc']

def format_selector(ctx):
    """ Select the best video and the best audio that won't result in an mkv.
    NOTE: This is just an example and does not handle all cases """

    # formats are already sorted worst to best
    formats = ctx.get('formats')[::-1]

    # acodec='none' means there is no audio
    best_video = next(f for f in formats
                      if f['vcodec'] != 'none' and f['acodec'] == 'none')

    # find compatible audio extension
    audio_ext = {'mp4': 'm4a', 'webm': 'webm'}[best_video['ext']]
    # vcodec='none' means there is no video
    best_audio = next(f for f in formats if (
        f['acodec'] != 'none' and f['vcodec'] == 'none' and f['ext'] == audio_ext))

    # These are the minimum required fields for a merged format
    yield {
        'format_id': f'{best_video["format_id"]}+{best_audio["format_id"]}',
        'ext': best_video['ext'],
        'requested_formats': [best_video, best_audio],
        # Must be + separated list of protocols
        'protocol': f'{best_video["protocol"]}+{best_audio["protocol"]}'
    }

ydl_opts = {
    'format': format_selector,
}

with yt_dlp.YoutubeDL(ydl_opts) as ydl:
    ydl.download(URLS)
```