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         


        

Answer 1

I agree, it's more like an 'elif not [condition(s) raising break]'.

I know this is an old thread, but I am looking into the same question right now, and I'm not sure anyone has captured the answer to this question in the way I understand it.

For me, there are three ways of "reading" the else in For... else or While... else statements, all of which are equivalent, are:

1. else == if the loop completes normally (without a break or error)
2. else == if the loop does not encounter a break
3. else == else not (condition raising break) (presumably there is such a condition, or you wouldn't have a loop)

So, essentially, the "else" in a loop is really an "elif ..." where '...' is (1) no break, which is equivalent to (2) NOT [condition(s) raising break].

I think the key is that the else is pointless without the 'break', so a for...else includes:

for:
    do stuff
    conditional break # implied by else
else not break:
    do more stuff

So, essential elements of a for...else loop are as follows, and you would read them in plainer English as:

for:
    do stuff
    condition:
        break
else: # read as "else not break" or "else not condition"
    do more stuff

As the other posters have said, a break is generally raised when you are able to locate what your loop is looking for, so the else: becomes "what to do if target item not located".

Example

You can also use exception handling, breaks, and for loops all together.

for x in range(0,3):
    print("x: {}".format(x))
    if x == 2:
        try:
            raise AssertionError("ASSERTION ERROR: x is {}".format(x))
        except:
            print(AssertionError("ASSERTION ERROR: x is {}".format(x)))
            break
else:
    print("X loop complete without error")

Result

x: 0
x: 1
x: 2
ASSERTION ERROR: x is 2
----------
# loop not completed (hit break), so else didn't run

Example

Simple example with a break being hit.

for y in range(0,3):
    print("y: {}".format(y))
    if y == 2: # will be executed
        print("BREAK: y is {}\n----------".format(y))
        break
else: # not executed because break is hit
    print("y_loop completed without break----------\n")

Result

y: 0
y: 1
y: 2
BREAK: y is 2
----------
# loop not completed (hit break), so else didn't run

Example

Simple example where there no break, no condition raising a break, and no error are encountered.

for z in range(0,3):
     print("z: {}".format(z))
     if z == 4: # will not be executed
         print("BREAK: z is {}\n".format(y))
         break
     if z == 4: # will not be executed
         raise AssertionError("ASSERTION ERROR: x is {}".format(x))
else:
     print("z_loop complete without break or error\n----------\n")

Result

z: 0
z: 1
z: 2
z_loop complete without break or error
----------