Python Dictionaries

In Python, a dictionary is an ordered (from Python > 3.7) collection of key: value pairs.

The main operations on a dictionary are storing a value with some key and extracting the value given the key. It is also possible to delete a key:value pair with del.

Example Dictionary:

my_cat = { 'size': 'fat', 'color': 'gray', 'disposition': 'loud' }

Set key, value using subscript operator `[]`

>>> my_cat = { ... 'size': 'fat', ... 'color': 'gray', ... 'disposition': 'loud', ... } >>> my_cat['age_years'] = 2 >>> print(my_cat) ... # {'size': 'fat', 'color': 'gray', 'disposition': 'loud', 'age_years': 2}

Get value using subscript operator `[]`

In case the key is not present in dictionary KeyError is raised.

>>> my_cat = { ... 'size': 'fat', ... 'color': 'gray', ... 'disposition': 'loud', ... } >>> print(my_cat['size']) ... # fat >>> print(my_cat['eye_color']) # Traceback (most recent call last): # File "<stdin>", line 1, in <module> # KeyError: 'eye_color'

values()

The values() method gets the values of the dictionary:

>>> pet = {'color': 'red', 'age': 42} >>> for value in pet.values(): ... print(value) ... # red # 42

keys()

The keys() method gets the keys of the dictionary:

>>> pet = {'color': 'red', 'age': 42} >>> for key in pet.keys(): ... print(key) ... # color # age

There is no need to use .keys() since by default you will loop through keys:

>>> pet = {'color': 'red', 'age': 42} >>> for key in pet: ... print(key) ... # color # age

items()

The items() method gets the items of a dictionary and returns them as a Tuple:

>>> pet = {'color': 'red', 'age': 42} >>> for item in pet.items(): ... print(item) ... # ('color', 'red') # ('age', 42)

Using the keys(), values(), and items() methods, a for loop can iterate over the keys, values, or key-value pairs in a dictionary, respectively.

>>> pet = {'color': 'red', 'age': 42} >>> for key, value in pet.items(): ... print(f'Key: {key} Value: {value}') ... # Key: color Value: red # Key: age Value: 42

get()

The get() method returns the value of an item with the given key. If the key doesn’t exist, it returns None:

>>> wife = {'name': 'Rose', 'age': 33} >>> f'My wife name is {wife.get("name")}' # 'My wife name is Rose' >>> f'She is {wife.get("age")} years old.' # 'She is 33 years old.' >>> f'She is deeply in love with {wife.get("husband")}' # 'She is deeply in love with None'

You can also change the default None value to one of your choice:

>>> wife = {'name': 'Rose', 'age': 33} >>> f'She is deeply in love with {wife.get("husband", "lover")}' # 'She is deeply in love with lover'

Adding items with setdefault()

It’s possible to add an item to a dictionary in this way:

>>> wife = {'name': 'Rose', 'age': 33} >>> if 'has_hair' not in wife: ... wife['has_hair'] = True

Using the setdefault method, we can make the same code more short:

>>> wife = {'name': 'Rose', 'age': 33} >>> wife.setdefault('has_hair', True) >>> wife # {'name': 'Rose', 'age': 33, 'has_hair': True}

Removing Items

pop()

The pop() method removes and returns an item based on a given key.

>>> wife = {'name': 'Rose', 'age': 33, 'hair': 'brown'} >>> wife.pop('age') # 33 >>> wife # {'name': 'Rose', 'hair': 'brown'}

popitem()

The popitem() method removes the last item in a dictionary and returns it.

>>> wife = {'name': 'Rose', 'age': 33, 'hair': 'brown'} >>> wife.popitem() # ('hair', 'brown') >>> wife # {'name': 'Rose', 'age': 33}

del()

The del() method removes an item based on a given key.

>>> wife = {'name': 'Rose', 'age': 33, 'hair': 'brown'} >>> del wife['age'] >>> wife # {'name': 'Rose', 'hair': 'brown'}

clear()

Theclear() method removes all the items in a dictionary.

>>> wife = {'name': 'Rose', 'age': 33, 'hair': 'brown'} >>> wife.clear() >>> wife # {}

Checking keys in a Dictionary

>>> person = {'name': 'Rose', 'age': 33} >>> 'name' in person.keys() # True >>> 'height' in person.keys() # False >>> 'skin' in person # You can omit keys() # False

Checking values in a Dictionary

>>> person = {'name': 'Rose', 'age': 33} >>> 'Rose' in person.values() # True >>> 33 in person.values() # True

Pretty Printing

>>> import pprint >>> wife = {'name': 'Rose', 'age': 33, 'has_hair': True, 'hair_color': 'brown', 'height': 1.6, 'eye_color': 'brown'} >>> pprint.pprint(wife) # {'age': 33, # 'eye_color': 'brown', # 'hair_color': 'brown', # 'has_hair': True, # 'height': 1.6, # 'name': 'Rose'}

Merge two dictionaries

For Python 3.5+:

>>> dict_a = {'a': 1, 'b': 2} >>> dict_b = {'b': 3, 'c': 4} >>> dict_c = {**dict_a, **dict_b} >>> dict_c # {'a': 1, 'b': 3, 'c': 4}