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

Fri Feb 24 02:10:39 EST 2012

On 24/02/2012 03:49, Ethan Furman wrote:
> Steven D'Aprano wrote:
>> On 2012年2月23日 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~

Methinks an off by one error :)
-- 
Cheers.
Mark Lawrence.