Question

Occasionally I’ll have a situation where I’ve written some code and, based on its logic, a certain path is impossible. For example:

activeGames = [10, 20, 30]
limit = 4

def getBestActiveGameStat():
    if not activeGames: return None
    return max(activeGames)

def bah():
    if limit == 0: return "Limit is 0"

    if len(activeGames) >= limit:
        somestat = getBestActiveGameStat()
        if somestat is None:
            print "The universe has exploded"
        #etc...

What would go in the universe exploding line? If limit is 0, then the function returns. If len(activeGames) >= limit, then there must be at least one active game, so getBestActiveGameStat() can’t return None. So, should I even check for it?

The same also happens with something like a while loop which always returns in the loop:

def hmph():
    while condition:
        if foo: return "yep"
        doStuffToMakeFooTrue()

    raise SingularityFlippedMyBitsError()

Since I “know” it’s impossible, should anything even be there?

Answer 1

If len(activeGames) >= limit, then
there must be at least one active
game, so getBestActiveGameStat() can’t
return None. So, should I even check
for it?

Sometimes we make mistakes. You could have a program error now — or someone could create one later.

Those errors might result in exceptions or failed unit tests. But debugging is expensive; it’s useful to have multiple ways to detect errors.

A quickly written assert statement can express an expected invariant to human readers. And when debugging, a failed assertion can pinpoint an error quickly.

Sutter and Alexandrescu address this issue in “C++ Coding Standards.” Despite the title, their arguments and guidelines are are language agnostic.

Assert liberally to document internal assumptions and invariants
… Use assert or an equivalent liberally to document assumptions internal to a module … that must always be true and otherwise represent programming errors.

For example, if the default case in a switch statement cannot occur, add the case with assert(false).