Find YouTube user name for a channel - youtube-api

Question

I am trying to use the YouTube Data API to fetch channel data for particular channels. According to the documentation, I can use list (by channel ID) or list (by YouTube username)
Search by channel ID
https://youtube.googleapis.com/youtube/v3/channels?part=snippet&id={ChannelID}&key={APIKey}
// Get channel by YouTube username
https://youtube.googleapis.com/youtube/v3/channels?part=snippet&forUsername={Username}&key={APIKey}
The channel ID can be retrieved by going to YouTube in a browser and search for the channel by name and then when you select the channel; the channel ID is part of the URL like this:
https://www.youtube.com/channel/UCbTw29mcP12YlTt1EpUaVJw.
This particular channel is for Sean Allen and his YouTube URL is:
https://www.youtube.com/SeanAllen.
I would have thought that the YouTube username would be SeanAllen, but it is not because, when I use that term in the API request, I get 0 results.
However, in my own case, my channel ID is UCOWdR4sFkmolWkU2fg669Gg and the search by channel ID works to fetch my channel data. My YouTube URL is:
https://wwww.youtube.com/StewartLynch
and, if I use StewartLynch as the username, I can use that 2nd API request to fetch the same data.
So, my question is, how does one find the correct YouTube username for a channel? Apparently it is not what is used in the channel URL.

Answer 1

The issue you're raising is recurrent time and again on Stack Overflow. I myself already tackled all faces of it (just issue either of the following SO search terms: user:8327971 custom URL or user:8327971 forUsername).
The short story is as follows: don't rely much on YouTube user names.
User names are a legacy feature of the API v3: not every channel has one attached and no channel is required to have one attached. (See this official statement from Google staff from 2013-07-11.)
Here is what YouTube Data API returns for the two names -- SeanAllen and StewartLynch -- of your post:
$ python3 youtube-search.py --custom-url SeanAllen
UCbTw29mcP12YlTt1EpUaVJw
$ python3 youtube-search.py --user-name SeanAllen
UCRGhxM6u14Uv309cC0ywEqA
$ python3 youtube-search.py --custom-url StewartLynch
UCOWdR4sFkmolWkU2fg669Gg
$ python3 youtube-search.py --user-name StewartLynch
UCOWdR4sFkmolWkU2fg669Gg
As you can see, StewartLynch is both the custom URL and the user name of the channel of which ID is UCOWdR4sFkmolWkU2fg669Gg.
But, in case of SeanAllen, things are not that simple since there's a channel -- UCbTw29mcP12YlTt1EpUaVJw -- that has its custom URL set to this name; and, at the same time, there's a different channel -- UCRGhxM6u14Uv309cC0ywEqA -- that has its attached user name set to the this very same name.
This situation is very much perennial with YouTube: many channels out there have in common this property.
The final note, which closes your question:
how does one find the correct YouTube UserName for a channel?
The Channels.list API endpoint queried with the request parameter forUsername does return the correct user name of a given channel (if the respective channel has attached an user name).
The ambiguity stems from the fact that URLs of form:
https://www.youtube.com/NAME
may well represent either a channel of which user name is NAME or a channel of which custom URL is NAME. The two cases can be set apart only by the (not-completely reliable) procedure of searching for custom URLs (which is implemented already by the script youtube-search.py).
Above I make use of my public (MIT licensed) Python 3 script youtube-search.py; this script is able to search the API for custom URLs and respectively query the API for user names.
Note that youtube-search.py requires a valid API key to be passed to it as argument of the command line option --app-key or, otherwise, passed on as the environment variable YOUTUBE_DATA_APP_KEY. (Use the command line option --help for brief helping info.)