Peculiar uses for python's 'else' keyword

I’ve been asked by a few people recently to explain the different uses for the else keyword in python.
python, for a reasons I do not understand, decided to overload the else keyword in ways most people never think of.

The spec isn’t too friendly to beginners either. This is a partial piece of the python grammar specification, for symbols that accept the else keyword, as it is read by the parser generator and used to parse Python source files:

if_stmt: 'if' test ':' suite ('elif' test ':' suite)* ['else' ':' suite]

while_stmt: 'while' test ':' suite ['else' ':' suite]

for_stmt: 'for' exprlist 'in' testlist ':' suite ['else' ':' suite]

try_stmt: ('try' ':' suite
           ((except_clause ':' suite)+
            ['else' ':' suite]
            ['finally' ':' suite] |
           'finally' ':' suite))

test: or_test ['if' or_test 'else' test] | lambdef

That’s kind of cryptic, right?

This blog post is primarily aimed at beginners, and covers:

The post has no ordering. You can pick-n-choose the ones you’re not familiar with.

if … else [ one liner ]

<value> if <condition> else <value if condition is False>

for example, instead of writing this:

age = 27

if age >= 18;
   print("adult")
else:
   print("kid")

# adult

we can do the same in one line:

age = 27
print("adult" if age >= 18 else "kid")
# adult

for | while … else

for and while loops take an optional else suite, which executes if the loop iteration completes normally. In other words, the else block will be executed only if no break & return were used, and no exception has been raised.

for <value> in <iterable>:
   # a code block with things
else:
  # runs only if the iteration finished without interruption (no break)

The following code randomizes five numbers, and prints them if they are not divisible by three. If that was the case for all of the numbers, it also print a message saying so.

from random import randint

for _ in range(5):
    n = randint(0, 10)
    if not n % 3:
        break
    print(n, end=' ')
else:
    print("non of the numbers are divisible by three")

# 1 5 10

what is ‘for..else’ good for?

A common use case is to implement search loops:

condition_is_met = False

for value in data:
    if meets_condition(value):
        condition_is_met = True
        break

if not condition_is_met:
    # condition did not meet. do something about it.

Using the else keyword, we can cut a few lines. It makes the code slimmer and more concise. I like it.

for value in data:
    if meets_condition(value):
        break
else:
    # condition did not meet. do something about it.

Because many people aren’t aware of the for...else syntax, I usually add a comment that explains when the else block is executed.

what is ‘while…else’ good for?

Lets recap on the syntax first -

while <condition>:
   # a code block with things
else:
  # runs only if the iteration finished without interruption (no break)

A common skeleton for code processing code:


ran_to_completion = True

while value < threshold:
    if not process_value(value):
        # something went wrong 
        ran_to_completion = False
        break
    value = update(value)

if ran_to_completion:
    # loop ended naturally, value passed threshold. 
    handle_threshold_reached()

Again, we can remove the flag by leveraging the else keyword:

while value < threshold:
    if not process_value(value):
        # something went wrong 
        break
    value = update(value)
else:
    # loop ended naturally, value passed threshold. 
    handle_threshold_reached()

try-catch-else-finally

try-catch-finally take an optional else suite, which executes if no exception were raised inside the try block -

try:
    # a code block that might raise an exception
except <exception-type>:
    # a code block that executes if an exception of type <exception-type> is raised
else:
    # a code block that executes if no exceptions were raised in the try block
finally:
    # a code block that always executes

what is it good for?

The following code is, unfortunately, common-place:

no_error = False
try:
    # do something
    no_error = True
except ...:
    # error handling

if no_error:
    # do something if no error has occurred

Adding a flag at the end of the try block is weird and non pythonic in my opinion. The else keyword really shines here and makes the code more readable:

try:
    # do something
except ...:
    # error handling
else:
    # do something if no error has occurred