Dictionaries

Dictionaries are an unordered collection of key-value pairs. They are similar to lists, but instead of accessing elements by their index, they are accessed by their key. They are created using curly braces {}. Items are separated by commas ,, and key-value pairs are separated by colons :.

a = {'a': 1, 'b': 2, 'c': 3}

print(a)
# > {'a': 1, 'b': 2, 'c': 3}

Accessing Items

Items in a dictionary can be accessed using their key. When accessing a key that doesn't exist, a KeyError is raised.

a = {'a': 1, 'b': 2, 'c': 3}

print(a['a'])
# > 1

print(a['b'])
# > 2

print(a['d'])
# > KeyError: 'd'

Adding Items

Items can be added to a dictionary by assigning a value to a new key.

a = {'a': 1, 'b': 2, 'c': 3}

a['d'] = 4

print(a)
# > {'a': 1, 'b': 2, 'c': 3, 'd': 4}

Removing Items

Items can be removed from a dictionary using the del keyword.

a = {'a': 1, 'b': 2, 'c': 3}

del a['a']

print(a)
# > {'b': 2, 'c': 3}

Note: Personal Taste: Personally I prefer using the pop() method to remove items from a dictionary, as it returns the value of the key that was removed. It is also more composable than the del keyword.

Dictionary Methods

There are several methods that can be used on dictionaries, the most common ones are:

keys(): Returns a list of all the keys in the dictionary.
values(): Returns a list of all the values in the dictionary.
items(): Returns a list of all the key-value pairs in the dictionary.
get(): Returns the value of a key, or a default value if the key doesn't exist.
pop(): Removes a key-value pair from the dictionary and returns the value.
clear(): Removes all key-value pairs from the dictionary.
update(): Updates the dictionary with the key-value pairs from another dictionary.
copy(): Returns a copy of the dictionary.
setdefault(): Returns the value of a key, or sets it to a default value if it doesn't exist.

a = {'a': 1, 'b': 2, 'c': 3}

print(a.keys())
# > dict_keys(['a', 'b', 'c'])

print(a.values())
# > dict_values([1, 2, 3])

print(a.items())
# > dict_items([('a', 1), ('b', 2), ('c', 3)])

print(a.get('a'))
# > 1

print(a.get('d'))
# > None

print(a.get('d', 4))
# > 4

print(a.pop('a'))
# > 1

print(a)
# > {'b': 2, 'c': 3}

print(a.setdefault('b'))
# > 2

print(a.setdefault('d', 4))
# > None

print(a)
# > {'b': 2, 'c': 3, 'd': 4}

a.clear()
print(a)
# > {}

a = {'a': 1, 'b': 2, 'c': 3}
b = {'d': 4, 'e': 5, 'f': 6}

a.update(b)
print(a)
# > {'a': 1, 'b': 2, 'c': 3, 'd': 4, 'e': 5, 'f': 6}

c = a.copy()
print(c)
# > {'a': 1, 'b': 2, 'c': 3, 'd': 4, 'e': 5, 'f': 6}

c.pop('a')
print(c)
# > {'b': 2, 'c': 3, 'd': 4, 'e': 5, 'f': 6}

print(a)
# > {'a': 1, 'b': 2, 'c': 3, 'd': 4, 'e': 5, 'f': 6}

# You can also use `|` to merge two dictionaries.
a = {'a': 1, 'b': 2, 'c': 3}
b = {'d': 4, 'e': 5, 'f': 6}

c = a | b
print(c)
# > {'a': 1, 'b': 2, 'c': 3, 'd': 4, 'e': 5, 'f': 6}

Dictionary Comprehensions

Dictionary comprehensions are similar to list comprehensions, but instead of creating a list, they create a dictionary.

a = {x: x**2 for x in range(5)}

print(a)
# > {0: 0, 1: 1, 2: 4, 3: 9, 4: 16}

Iterating Over Dictionaries

Iterating over a dictionary will iterate over its keys.

a = {'a': 1, 'b': 2, 'c': 3}

for key in a:
    print(key)

# > a
# > b
# > c

To iterate over the values, you can use the values() method.

a = {'a': 1, 'b': 2, 'c': 3}

for value in a.values():
    print(value)

# > 1
# > 2
# > 3

To iterate over the key-value pairs, you can use the items() method.

a = {'a': 1, 'b': 2, 'c': 3}

for key, value in a.items():
    print(key, value)

# > a 1
# > b 2
# > c 3

Dictionary Membership

You can check if a key exists in a dictionary using the in keyword.

a = {'a': 1, 'b': 2, 'c': 3}

print('a' in a)
# > True

print('d' in a)
# > False

Python 101: Introduction