Or else:

Thu 12 March 2020 by Moshe Zadka

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The underappreciated else keyword in Python has three distinct uses.

if/else

On an if statement, else will contain code that runs if the condition is false.

if anonymize:
    print("Hello world")
else:
    print("Hello, name")

This is probably the least surprising use.

loop/else

The easiest to explain is while/else: it works the same as if/else, and runs when the condition is false.

However, it does not run if the loop was broken out of using break or an exception: it serves as something that runs on normal loop termination.

for/else functions in the same way: it runs on normal loop termination, and not if the loop was broken out of using a break.

For example, searching for an odd element in a list:

for x in numbers:
    if x % 2 == 1:
        print("Found", x)
        break
else:
    print("No odd found")

This is a powerful way to avoid sentinel values.

try/except/else

When writing code that might raise an exception, we want to be able to catch it -- but we want to avoid catching unanticipated exceptions. This means we want to protect as little code with try as possible, but still have some code that runs only in the normal path.

try:
    before, after = things
except ValueError:
    part1 = things[0]
    part2 = 0
    after = 0
else:
    part1, part2 = before

This means that if things does not have two items, this is a valid case we can recover from. However, if it does have two items, the first one must also have two items. If this is not the case, this snippet will raise ValueError.

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