[Embedding yt-dlp] Add options to redirect streams

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Checklist

• I’m requesting a feature unrelated to a specific site
• I’ve looked through the README
• I’ve verified that I’m running yt-dlp version 2022.09.01 (update instructions) or later (specify commit)
• I’ve searched the bugtracker for similar issues including closed ones. DO NOT post duplicates
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Provide a description that is worded well enough to be understood

I would like to use yt-dlp from within a Python script to get a list of the URLs of the videos in a playlist, preferably alongside the playlist’s metadata, without launching yt-dlp as a subprocess (i.e., via the Python interface). The command-line interface offers --flat-playlist --dump-single-json for this purpose, which maps to {"extract_flat": "in_playlist", "dump_single_json": True} in Python. However, this still prints the information to the command-line instead of a location where my Python script can access it.

In such situations, it would be useful to be able to pass in a stream object (e.g., an io.StringIO instance) to be written to instead of sys.stdout. Redirecting the log prints and error messages might also be useful for e.g. a Python GUI application that serves as a front-end to yt-dlp.

Implementing this seems easy (just document the options and modify the self._out_files = Namespace(...) assignment in YoutubeDL.py), but I wanted to post this first in case I am missing something. Also, do you think that out_stream, error_stream, screen_stream, and console_stream (after the names in the Namespace) are good names for the options?

Edit: I think dump_stream instead of out_stream and log_stream instead of screen_stream would be more telling, if I understood correctly what these streams are used for.

Off-topic:

While looking at the aforementioned assignment, I noticed that options that “dump to stdout,” like --dump-single-json, instead print to stderr if {"logtostderr": True} is passed. I find this confusing because these are not logging messages (you use {"quiet": True} to keep them out of stdout, although logtostderr will also affect those). Is this name kept for compatibility reasons? At least the documentation should be updated – it currently reads:

    logtostderr:       Log messages to stderr instead of stdout.

Also, where is “--dump-single-json implies --quiet” implemented?

Provide verbose output that clearly demonstrates the problem

• Run your yt-dlp command with -vU flag added (yt-dlp -vU <your command line>)
• Copy the WHOLE output (starting with [debug] Command-line config) and insert it below

Complete Verbose Output

No response

[pukkandan]

I think I dismissed extract_info() because I was misled by its docstring, which reads like playlist metadata will never be returned:

        Return a list with a dictionary for each video extracted.

Damn, it looks like the docstring wasn’t updated in years!

As another example, a command-line application written in Python that uses yt-dlp in the background might not want to print logging and error messages to the command line but instead write them to a log file.

https://github.com/yt-dlp/yt-dlp#adding-logger-and-progress-hook

Also, where is “–dump-single-json implies --quiet” implemented?

This only applies to CLI

I was just asking this to get a better understanding of the code.

https://github.com/yt-dlp/yt-dlp/blob/7657ec7ed6318dd66dd72cc100ba7bc5b911366e/yt_dlp/__init__.py#L741

[pukkandan]

The documentation mistakes have been fixed