Replace asserts with conditions

[4383]

Running the python interpreter with the optimization mode turned on (python -O, python -OO, set PYTHONOPTIMIZE=1), will remove all the assert statements from running code [1][2][3].

Dnspython heavily rely on assert to check things at runtime. That could be an issue, if the flag is enabled. All the dnspython check would be removed, leading to possible bugs.

This patch propose to replace assert statements with condition and by raising an AssertionError if the condition is met.

This patch won't change the logic and the behaviour of the checks previously made by using assert, however, these changes will remain actives at runtime, even if python optimization is enabled.

This patch do not replace assert in unit tests because they are not an issue.

[1] https://docs.python.org/3/using/cmdline.html?highlight=pythonoptimize#cmdoption-O
[2] https://docs.python.org/3/using/cmdline.html?highlight=pythonoptimize#cmdoption-OO
[3] https://docs.python.org/3/using/cmdline.html?highlight=pythonoptimize#envvar-PYTHONOPTIMIZE

[rthalley]

Dnspython is NOT using assert for error checking. Dnspython uses asserts for two basic reasons: 1) to say that something should always be true because of the design of dnspython, and if it isn't there's a bug in dnspython that needs to be fixed, and 2) to give extra help to mypy which can't figure out some things, e.g. we will "assert x is not None" sometimes not because x could be None at that point, but because mypy can't figure out that it can't be None. In both of these cases, dnspython should behave no differently in -O or -OO mode.

[4383]

So be it!
Thanks for your feedback.