Rewrite 'assert' statements as if/raise

[adamchainz]

Checklist

• Raised initially in discussion on Django ticket #32508
• This cannot be dealt with as a third party library. (We prefer new functionality to be in the form of third party libraries where possible.)
• I have reduced the issue to the simplest possible case.

Python's -O flag, enables "optimized mode" which, among other things, disables the assert statement. A naive user may activate it to try speed up their application. DRF has many uses of assert in its library code (not tests) to guard against bad conditions. Rewriting those assert statements as an if check and a raise of an appropriate exception would allow it to continue functioning in "optimized mode".

[tomchristie]

I'd probably prefer not. If a user is turning off assertions, then, well, let 'em turn off assertions.

But how about this, shall we get this discussion to a more concrete point first, by taking a single specific example of one of the assert statements in question, to help figure out what the benefits/trade-offs would look like in a particular concrete case?