A quirk/gotcha of for i, x in enumerate(seq) when seq is empty
Ethan Furman
ethan at stoneleaf.us
Thu Feb 23 22:49:01 EST 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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