[Python-Dev] [Python-checkins] cpython (3.2): Nudge readers towards a more accurate mental model for loop else clauses (original) (raw)

Stephen J. Turnbull stephen at xemacs.org
Fri Jun 8 09:29:29 CEST 2012

Note: reply-to set to python-ideas.

Nick Coghlan writes:

The inaccuracies in the analogy are why this is in the tutorial, not the language reference. All 3 else clauses are really their own thing.

Nick, for the purpose of the tutorial, actually there are 4 else clauses: you need to distinguish while from for. It was much easier for me to get confused about for. I think Terry is on to something here:

"An else clause used with a loop has two differences from an else clause used with an if statement. [...] [The] condition is tested repeatedly instead of just once. A loop else is the same in that it triggers when that one condition is false."

I think it would be a good idea to use an example where the loop is a while loop, not a for loop. In the case of the for loop, it's easy to confuse the implicit test used (ie, "item is true") with a test on the iterable ("iterable is not empty"). With a while loop, that's not true. I don't have trouble understanding that the while suite is executed "while" the condition is true:

for n in range(2, 10): ... x = 2 ... while x < n: ... if n % x == 0: ... print(n, 'equals', x, '*', n//x) ... break ... x = x + 1 ... else: ... # loop fell through without finding a factor ... print(n, 'is a prime number') ...

Of course it's useful to point out here that use of a while loop is bad style, and the "for x in range(2,n)" version is preferred, having exactly the same semantics for the else clause.

Also, I find rewriting the if/else as

if condition:
    do_when_true()
else:
    do_when_false()

as

while condition:
    do_when_true()
    break             # do only once, and skip else!
else:
    do_when_false()

to be quite mnemonic for understanding what the while/else construct really does.

[The loop else] is subordinate to just one condition instead of possibly many. (In for statements, the condition is implicit but still there.)

FWIW, this issue doesn't affect my understanding, as I think of if/elif/else as "indentation-optimized tail-nested" ifs anyway.

I'm not sure if anybody else will feel as I do, so I don't offer a patch here.

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