Short Circuit Evaluation

In short: Short-circuit evaluation means a logical expression stops as soon as its result is determined. With AND, if the first operand is false the rest is skipped; with OR, if the first is true the rest is skipped, which is useful for guarding against errors.

Short-circuit evaluation is a strategy most programming languages (including Python 2.7) use to avoid unnecessary work. For example, say we had a conditional like this:

if it_is_friday and it_is_raining: print "board games at my place!"

Let's say it_is_friday is false. Because Python 2.7 short-circuits evaluation, it wouldn't bother checking the value of it_is_raining—it knows that either way the condition is false and we won't print the invitation to board game night.

We can use this to our advantage. For example, say we have a check like this:

if friends['Becky'].is_free_this_friday(): invite_to_board_game_night(friends['Becky'])

What happens if 'Becky' isn't in our friends dictionary? We'll get a KeyError when we run friends['Becky'].

Instead, we could first confirm that Becky and I are still on good terms:

if 'Becky' in friends and friends['Becky'].is_free_this_friday(): invite_to_board_game_night(friends['Becky'])

This way, if 'Becky' isn't in friends, Python will ignore the rest of the conditional and avoid throwing the KeyError.

This is all hypothetical, of course. It's not like things with Becky are weird or anything. We're totally cool. She's still in my friends dictionary for sure and I hope I'm still in hers and Becky if you're reading this I just want you to know you're still in my friends dictionary.

Frequently Asked Questions

What is short-circuit evaluation?

When evaluating a boolean expression, the program stops as soon as the outcome is known: 'false AND anything' is false and 'true OR anything' is true, so the second operand is never evaluated.

Why is short-circuit evaluation useful?

It avoids unnecessary work and guards against errors; for example, 'node != null && node.value == x' won't dereference a null node.

Which operators short-circuit?

The logical AND (&&) and logical OR (||). The bitwise operators & and | do not short-circuit; they always evaluate both sides.

Last updated: June 17, 2026

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