[Python-ideas] for/else statements considered harmful

Alice Bevan–McGregor alice at gothcandy.com
Thu Jun 7 01:20:07 CEST 2012


Was teaching a new user to Python the ropes a short while ago and ran 
into an interesting headspace problem: the for/else syntax fails the 
obviousness and consistency tests.  When used in an if/else block the 
conditional code is executed if the conditional passes, and the else 
block is executed if the conditional fails.  Compared to for loops 
where the for code is repeated and the else code executed if we 
"naturally fall off the loop".  (The new user's reaction was "why the 
hoek would I ever use for/else?")

I forked Python 3.3 to experiment with an alternate implementation that 
follows the logic of pass/fail implied by if/else: (and to refactor the 
stdlib, but that's a different issue ;)

    for x in range(20):
        if x > 10: break
        pass # we had no values to iterate
        pass # we naturally fell off the loop

It abuses finally (to avoid tying up a potentially common word as a 
reserved word like "done") but makes possible an important distinction 
without having to perform potentially expensive length calculations 
(which may not even be possible!) on the value being iterated: that is, 
handling the case where there were no values in the collection or 
returned by the generator.

Templating engines generally implement this type of structure.  Of 
course this type of breaking change in semantics puts this idea firmly 
into Python 4 land.

I'll isolate the for/else/finally code from my fork and post a patch 
this week-end, hopefully.

	— Alice.

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