checking if a list is empty

[Philip Semanchuk]

On May 6, 2011, at 5:57 PM, scattered wrote:

> On May 6, 2:36 am, Jabba Laci <jabba.l... at gmail.com> wrote:
>> Hi,
>> 
>> If I want to check if a list is empty, which is the more pythonic way?
>> 
>> li = []
>> 
>> (1) if len(li) == 0:
>> ...
>> or
>> (2) if not li:
>> ...
>> 
>> Thanks,
>> 
>> Laszlo
> 
> is there any problem with
> 
> (3) if li == []:
> 
> ?

What if it's not a list but a tuple or a numpy array? Often I just want to iterate through an element's items and I don't care if it's a list, set, etc. For instance, given this function definition --

def print_items(an_iterable):
    if not an_iterable:
        print "The iterable is empty"
    else:
        for item in an_iterable:
            print item

I get the output I want with all of these calls:
print_items( list() )
print_items( tuple() )
print_items( set() )
print_items( numpy.array([]) )

Given this slightly different definition, only the  first call gives me the output I expect: 

def print_items(an_iterable):
    if an_iterable == []:
        print "The iterable is empty"
    else:
        for item in an_iterable:
            print item


I find I use the the former style ("if not an_iterable") almost exclusively.


bye
Philip