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

Peter Otten __peter__ at web.de
Fri Feb 24 09:44:05 CET 2012


Ethan Furman wrote:

> 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.

Yes, either

i = -1
for i, x in enumerate(seq):
    ...
print "%d records" % (i+1)

or

i = 0
for i, x in enumerate(seq, 1):
    ...
print "%d records" % i




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