Changing and Adding Values

The values in a dictionary can change. You assign a new value to a key the same way as to a variable, with =:

player = {"name": "Ninja", "level": 1, "lives": 3}

player["level"] = 2
print(player)

{'name': 'Ninja', 'level': 2, 'lives': 3}

You can also calculate the new value from the old one, as with variables: player["lives"] = player["lives"] - 1, or shorter, player["lives"] -= 1.

Note the difference: when you create a dictionary, you write a pair inside the curly braces with a colon ("level": 1). When you change a value later, you use square brackets and = (player["level"] = 2).

If you assign to a key that the dictionary doesn't have yet, Python adds a new pair at the end:

player = {"name": "Ninja", "level": 1, "lives": 3}

player["weapon"] = "sword"
print(player)

{'name': 'Ninja', 'level': 1, 'lives': 3, 'weapon': 'sword'}

Each key can be in a dictionary only once. When you assign to a key that is already there, Python doesn't add a second pair with the same key; it replaces the value. That's handy, but it also means that nothing warns you about a typo: player["Level"] = 2 doesn't change "level". It adds a new key, "Level", and Ninja stays at level 1.

You can also start with an empty dictionary, {}, and add pairs to it one by one. And len(), which you know from lists and text, tells you how many pairs a dictionary has:

player = {}
player["name"] = "Ninja"
player["level"] = 1
print(player)
print(len(player))

{'name': 'Ninja', 'level': 1}
2

To remove a pair, use .pop() with its key, as you use it with an index in a list: player.pop("level") removes the pair with the key "level" and returns its value.