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Condition and Loops Style Notes

As additional syntax, the for and while statements permits an else clause. This is a suite of statements that are executed when the loop terminates normally. This suite is skipped if the loop is terminated by a break statement. The else clause on a loop might be used for some post-loop cleanup. This is so unlike other programming languages, that it is hard to justify using it.

An else clause always raises a small problem when it is used. It's never perfectly clear what conditions lead to execution of an else clause. The condition that applies has to be worked out from context. For instance, in if statements, one explicitly states the exact condition for all of the if and elif clauses. The logical inverse of this condition is assumed as the else condition. It is, unfortunately, left to the person reading the program to work out what this condition actually is.

Similarly, the else clause of a while statement is the basic loop termination condition, with all of the conditions on any break statements removed. The following kind of analysis can be used to work out the condition under which the else clause is executed.

while not BB:
    if C1: break
    if C2: break
else:
    assert BB and not C1 and not C2
assert BB or C1 or C2

Because this analysis can be difficult, it is best to avoid the use of else clauses in loop constructs.


 
 
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