Why does python use 'else' after for and while loops?

Question

I understand how this construct works:

for i in range(10):     print(i)      if i == 9:         print("Too big - I'm giving up!")         break else:     print("Completed successfully") 

But I don't understand why else is used as the keyword here, since it suggests the code in question only runs if the for block does not complete, which is the opposite of what it does! No matter how I think about it, my brain can't progress seamlessly from the for statement to the else block. To me, continue or continuewith would make more sense (and I'm trying to train myself to read it as such).

I'm wondering how Python coders read this construct in their head (or aloud, if you like). Perhaps I'm missing something that would make such code blocks more easily decipherable?

Answer 1

A common construct is to run a loop until something is found and then to break out of the loop. The problem is that if I break out of the loop or the loop ends I need to determine which case happened. One method is to create a flag or store variable that will let me do a second test to see how the loop was exited.

For example assume that I need to search through a list and process each item until a flag item is found and then stop processing. If the flag item is missing then an exception needs to be raised.

Using the Python for...else construct you have

for i in mylist:     if i == theflag:         break     process(i) else:     raise ValueError("List argument missing terminal flag.") 

Compare this to a method that does not use this syntactic sugar:

flagfound = False for i in mylist:     if i == theflag:         flagfound = True         break     process(i)  if not flagfound:     raise ValueError("List argument missing terminal flag.") 

In the first case the raise is bound tightly to the for loop it works with. In the second the binding is not as strong and errors may be introduced during maintenance.

Answer 2

It's a strange construct even to seasoned Python coders. When used in conjunction with for-loops it basically means "find some item in the iterable, else if none was found do ...". As in:

found_obj = None for obj in objects:     if obj.key == search_key:         found_obj = obj         break else:     print('No object found.') 

But anytime you see this construct, a better alternative is to either encapsulate the search in a function:

def find_obj(search_key):     for obj in objects:         if obj.key == search_key:             return obj 

Or use a list comprehension:

matching_objs = [o for o in objects if o.key == search_key] if matching_objs:     print('Found {}'.format(matching_objs[0])) else:     print('No object found.') 

It is not semantically equivalent to the other two versions, but works good enough in non-performance critical code where it doesn't matter whether you iterate the whole list or not. Others may disagree, but I personally would avoid ever using the for-else or while-else blocks in production code.

See also [Python-ideas] Summary of for...else threads