Assertions unsafe in production code

I’ve noticed while reading graphene and graphene-django that assertions are used frequently to protect from operations with undefined results or things that should be impossible (but might still happen if used incorrectly).

In Python, assertions are not safe to use for this in general. This is because they can be disabled, and often are in production.

Instead of:

assert something is not None, "Something was unexpectedly None"

We should do this:

if something is not None:
    raise AssertionError("Something was unexpectedly None")

This is the recommended best practice in Python development where assertions are being used.

[jkimbo]

@danpalmer totally agree would this and would love to see a PR which takes us in this direction

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