A quirk/gotcha of for i, x in enumerate(seq) when seq is empty

Ethan Furman ethan at stoneleaf.us
Fri Feb 24 04:49:01 CET 2012


Steven D'Aprano wrote:
> On Thu, 23 Feb 2012 16:30:09 -0800, Alex Willmer wrote:
> 
>> This week I was slightly surprised by a behaviour that I've not
>> considered before. I've long used
>>
>> for i, x in enumerate(seq):
>>    # do stuff
>>
>> as a standard looping-with-index construct. In Python for loops don't
>> create a scope, so the loop variables are available afterward. I've
>> sometimes used this to print or return a record count e.g.
>>
>> for i, x in enumerate(seq):
>>    # do stuff
>> print 'Processed %i records' % i+1
>>
>> However as I found out, if seq is empty then i and x are never created.
> 
> This has nothing to do with enumerate. It applies to for loops in 
> general: the loop variable is not initialised if the loop never runs. 
> What value should it take? Zero? Minus one? The empty string? None? 
> Whatever answer Python choose would be almost always wrong, so it refuses 
> to guess.
> 
> 
>> The above code will raise NameError. So if a record count is needed, and
>> the loop is not guaranteed to execute the following seems more correct:
>>
>> i = 0
>> for x in seq:
>>     # do stuff
>>     i += 1
>> print 'Processed %i records' % i
> 
> What fixes the problem is not avoiding enumerate, or performing the 
> increments in slow Python instead of fast C, but that you initialise the 
> loop variable you care about before the loop in case it doesn't run.
> 
> i = 0
> for i,x in enumerate(seq):
>     # do stuff
> 
> is all you need: the addition of one extra line, to initialise the loop 
> variable i (and, if you need it, x) before hand.

Actually,

i = -1

or his reporting will be wrong.

~Ethan~



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