types-dict.md

Dictionary (dict) collection

Dictionaries are unordered, mutable (changeable) collections of data. They are defined by using curly brackets {} and separating the key-value pairs with a comma ,. The type in Python is called dict. For example:

x = {"name": "John", "age": 36}
print(x)  # {'name': 'John', 'age': 36}
print(type(x))  # <class 'dict'>

We can access the values in a dict by using the key of the value, for example:

x = {"name": "John", "age": 36}
print(x["name"])  # John

We can change the values in a dict, for example:

x = {"name": "John", "age": 36}
x["age"] = 40
print(x)  # {'name': 'John', 'age': 40}

But we can't add or remove values from a dict by using the index of the value, for example:

x = {"name": "John", "age": 36}
x[0] = 40  # TypeError: 'dict' object does not support item assignment

We can add or remove values from a dict by using the key of the value, for example:

x = {"name": "John", "age": 36}
x["city"] = "Oslo"
print(x)  # {'name': 'John', 'age': 36, 'city': 'Oslo'}
x.pop("age")
print(x)  # {'name': 'John', 'city': 'Oslo'}

Keys

The keys in a dict must be unique, somewhat similar to a set. If we try to add a key that already exists, the value will be overwritten, for example:

x = {"name": "John", "age": 36}
x["age"] = 40
print(x)  # {'name': 'John', 'age': 40}

Trying to access a key that doesn't exist will result in a KeyError, for example:

x = {"name": "John", "age": 36}
print(x["city"])  # KeyError: 'city'

We can use the in keyword to check if a key exists in a dict, for example:

x = {"name": "John", "age": 36}
print("name" in x)  # True
print("city" in x)  # False

And we can get all the keys in a dict by using the keys() method, for example:

x = {"name": "John", "age": 36}
print(x.keys())  # dict_keys(['name', 'age'])

But when iterating (looping) over a dict we will get the keys by default, for example:

x = {"name": "John", "age": 36}
for key in x:
    print(key)
# name
# age

Note: The keys in a dict are unordered, so the order of the keys may change when iterating over a dict.

Types of keys

The keys in a dict can be any immutable data type, for example, str, int, tuple, but not list or set, for example:

x = {"name": "John", "age": 36}
print(x)  # {'name': 'John', 'age': 36}
x[1] = "Hello"
print(x)  # {'name': 'John', 'age': 36, 1: 'Hello'}
x[(1, 2, 3)] = "World"
print(x)  # {'name': 'John', 'age': 36, 1: 'Hello', (1, 2, 3): 'World'}
x[[1, 2, 3]] = "World"  # TypeError: unhashable type: 'list'

Values

The values in a dict can be any data type, for example, even another dict, for example:

x = {"name": "John", "age": 36, "info": {"city": "Oslo", "country": "Norway"}}
print(x)  # {'name': 'John', 'age': 36, 'info': {'city': 'Oslo', 'country': 'Norway'}}

We can access the values in a dict by using the key of the value, for example:

x = {"name": "John", "age": 36}
print(x["name"])  # John

But we can't do the reverse, we can't access the key of a value, for example:

x = {"name": "John", "age": 36}
print(x["John"])  # KeyError: 'John'

We can get all the values in a dict by using the values() method, for example:

x = {"name": "John", "age": 36}
print(x.values())  # dict_values(['John', 36])

Items

We can get all the key-value pairs in a dict by using the items() method, for example:

x = {"name": "John", "age": 36}
print(x.items())  # dict_items([('name', 'John'), ('age', 36)])

Sometimes, it will be useful to iterate (loop) over the key-value pairs in a dict, for example:

x = {"name": "John", "age": 36}
for key, value in x.items():
    print(key, value)
# name John
# age 36

Useful methods

Method	Description	Example
clear()	Removes all the elements from the dictionary	x.clear()
copy()	Returns a copy of the dictionary	x.copy()
get()	Returns the value of the specified key, if it exists, otherwise None or another default value	x.get("name", "anonymous")
pop()	Removes the element with the specified key (and returns its value)	name = x.pop("name")
update()	Updates the dictionary with the specified key-value pairs	x.update({"name": "John", "age": 36})