Unravelling `if` statements

After the initial posting of my summary post about my syntactic sugar series, I received the following reply to one of my tweets:

Now I had already dealt with else (and elif) when it came to if, but the tweet did make me realize that I could actually unravel if itself!

The key insight is that if you put a break at the very end of a while statement, it will exit the loop unconditionally. That means if you make the conditional guard on the while loop be what the conditional guard would have been for an if statement, you end up with the same outcome!

Take the following example:

if A:
    B

By placing that break statement at the end of the block, you can translate that if statement into a while statement:

while A:
    B
    break

And since break was already unravelled, you can simply this even further:

_done = False

while not _done and A:
    B
    _done = True

The key observation is you must put the break-substituting circuit breaker first in the conditional guard to avoid executing the conditional guard inherited from the if statement itself. If you don't then you run the risk of side-effects in the if guard being triggered twice (and that would never happen in a normal if statement).

You can also use exceptions if you want to avoid using and:

class _Done(Exception):
    pass
    
try:
    while A:
        B
        raise _Done
except _Done:
    pass