Membership Testing

When working with data structures, one of the most common questions is "Does this value exist in this data structure?". Python comes to the rescue here with two operators: in and not in.

Testing Membership in Sequences

Recall that the sequence types in Python are list and tuples. While they can contain any data type, the order of the elements is guaranteed.

Testing membership using the in and not in operators combined with an if statement returns a truthy value (boolean) to indicate membership:

fav_books = ['mastery', 'the signal and the noise', 'the organized mind']

if 'mastery' in fav_books:
    print("One of your favourites!")
else:
    print("Maybe something new to consider?")

Returns:

One of your favourites!

Using not in works the opposite:

fav_books = ['mastery', 'the signal and the noise', 'the organized mind']

if 'discipline is destiny' not in fav_books:
    print("Have your read this one? Maybe you should!")
else:
    print("Ye ole` favourites once again!")

Would result in:

Have your read this one? Maybe you should!

Testing Membership in Dictionaries

Dictionaries (the dict data structure) present a little bit of a special case in testing membership, the caveat being you must remember that by default, the test runs against the key of the dictionary, not the entire key/value pair, nor against the value itself.

dream_car = {
    "model": "Pinto",
    "make": "Ford",
    "year": 1971,
    "mileage": 400,
}

if 'year' in dream_car:
    print("Good to know the year of your dream car!")
else:
    print("What year is your dream car?")

Would output:

Good to know the year of your dream car!

If you'd like to test to see if a specific value is in a dictionary, remmember to use the dict.values() method:

dream_car = {
    "model": "Pinto",
    "make": "Ford",
    "year": 1971,
    "mileage": 400,
}

if 'Pinto' not in dream_car.values():
    print("Your dream car isn't a Pinto???!??")
else:
    print("Nothing quite as sweet as a lovely Pinto!")

Which would return:

Nothing quite as sweet as a lovely Pinto!