Question

Is there a better way to write the following:

   row_counter = 0
   for item in iterable_sequence:
      # do stuff with the item

      counter += 1

   if not row_counter:
      # handle the empty-sequence-case

Please keep in mind that I can’t use len(iterable_sequence) because 1) not all sequences have known lengths; 2) in some cases calling len() may trigger loading of the sequence’s items into memory (as the case would be with sql query results).

The reason I ask is that I’m simply curious if there is a way to make above more concise and idiomatic. What I’m looking for is along the lines of:

for item in sequence:
   #process item
*else*:
   #handle the empty sequence case

(assuming “else” here worked only on empty sequences, which I know it doesn’t)

Answer 1

for item in iterable:
    break
else:
    # handle the empty-sequence-case here

Or

item = next(iterator, sentinel)
if item is sentinel:
   # handle the empty-sequence-case here   

In each case one item is consumed if it is present.

---

An example of empty_adapter()‘s implementation mentioned in the comments:

def empty_adaptor(iterable, sentinel=object()):
    it = iter(iterable)
    item = next(it, sentinel)
    if item is sentinel:
       return None # empty
    else:
       def gen():
           yield item
           for i in it:
               yield i
       return gen()

You could use it as follows:

it = empty_adaptor(some_iter)
if it is not None: 
   for i in it:
       # handle items
else:
   # handle empty case

Introducing special case for an empty sequence for a general case seems wrong. There should be a better solution for a domain specific problem.