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

Peter Otten __peter__ at web.de
Fri Feb 24 13:44:15 CET 2012


Rick Johnson wrote:

> On Feb 23, 6:30 pm, Alex Willmer <a... at moreati.org.uk> wrote:
>> [...]
>> 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
> 
> You could employ the "else clause" of "for loops" to your advantage;

>>>> for x in []:
> ...     print x
> ... else:
> ...     print 'Empty Iterable'
> Empty Iterable
> 
>>>> for i,o in enumerate([]):
> ...     print i, o
> ... else:
> ...     print 'Empty Iterable'
> Empty Iterable

No:


>>> for i in []:
...     pass
... else:
...     print "else"
...
else
>>> for i in [42]:
...     pass
... else:
...     print "else"
...
else
>>> for i in [42]:
...     break
... else:
...     print "else"
...
>>>

The code in the else suite executes only when the for loop is left via 
break. A non-empty iterable is required but not sufficient.



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