It would be very helpful to document what the "magic url" "http://testserver" means.

[brian-farrell]

After a day and a half of debugging my test, I've come to understand that using http://testserver as a base_url when instantiating a Client or AsyncClient tells the client to query the given app instance. Although use of this URL is shown on the Advanced Usage and Async Support pages, there is no discussion or indication of what http://testserver actually means. As such, I took the value to be an example that should be replaced with the actual value of my own server's URL. Of course, I understand now that http://testserver makes more sense. I just would have would preferred to understand this when I was reading through the documentation the first time.

The only reason that I discovered what was going on was because I happened upon this mention here:
fastapi/fastapi#1876 (comment)