Changing data in the dictionary — Python: Dictionaries and Sets

Python dictionaries are mutable. If we want to add a new key-value pair, we do not need the .append or other separate list methods. Usual assigning is enough:

d = {}  # An empty dictionary
d["a"] = 100
print(d)  # => {'a': 100}
d["b"] = 200
d["a"] = 0
print(d)  # => {'a': 0, 'b': 200}

Here you can see that assigning a value to a new key looks the same as setting values to existing keys — it is convenient. Removing items from the dictionary is done using the pop method. The dictionary is already more like a list in this case. Only we use the key instead of the index:

d = {'a': 1, 'b': 2}
d.pop('a')  # 1
d  # {'b': 2}
d.pop('BANG')  # KeyError: 'BANG'

The example shows that Python will throw an exception if you try to extract a value from a non-existent key. However, we can call the pop method with a default value. In this case, if there is no key in the dictionary, we will get this value with no exception thrown:

d = {'a': 1, 'b': 2}
d.pop('BANG', None)
d.pop('BANG', 42)  # 42