if statements with or w/o else statements

[Mark McEahern]

On Mon, 2004-02-23 at 07:31, Bart Nessux wrote:
> Should an if statement have a corresponding else statement?

Syntactically, the answer is no--the else statement is optional.

Logically, though, consider this:

if preCondition:
  doSomething()

That is, e.g., you're testing for a pre-condition that, if false,
indicates failure.  In that case, NOT having an else statement is likely
to mask subtle bugs and it's probably better to have:

if preCondition:
  doSomething()
else:
  raise RuntimeError('X must be true.')

which itself can be expressed differently so that you don't need the
else, e.g.,

if not preCondition:
  raise RuntimeError(...)
doSomething()

;-)

Cheers,

// m