Short-circuit evaluation

Short-circuit evaluation is a strategy most programming languages (including Python 3.6) 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 3.6 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.