In this tutorial, we’re gonna look at Python Dictionary data type: what it is, how to create, add, access, remove item and Python built-in methods.
Related Post: Python List functions
Python Dictionary data type
Python Dictionary is a collection of items. Unlike List, Dictionary doesn’t have indexes but keys. A key with its associated value is called a key-value pair.
Dictionary is covered by braces ({}):
>>> customer = {'name': 'Jack', 'age': 42, 'location': 'US'}
# keys: 'name', 'age', 'location'
# values: 'Jack', 42, 'US'
>>> customer['name']
'Jack'
>>> customer['age']
42
>>> customer['location']
'US'
Create a Python Dictionary
Usual way
We can use integer values as keys, just like List for indexes, but they do not have to start at 0 and can be any number:
>>> mPair = {123: 'One Two Three', 42: 'The Answer', 1: [1, 2, 3]}
>>> mPair[123]
'One Two Three'
>>> mPair[42]
'The Answer'
>>> mPair[1]
[1, 2, 3]
Dictionary can be created with mixed keys:
# keys must be immutable types (string, number or tuple with immutable elements)
>>> mDic = {'first': 1, 42: 'The Answer', ('Jack', 25): 'My Friend'}
>>> mDic['first']
1
>>> mDic[42]
'The Answer'
>>> mDic[('Jack',25)]
'My Friend'
Use dict() contructor
We can also create a Dictionary with dict() built-in function:
>>> mList = [('first', 1), (42, 'The Answer')]
>>> mDict = dict(mList)
>>> mDict
{42: 'The Answer', 'first': 1}
If the keys are simple strings:
>>> dict(jack=12, adam=3, katherin=26)
{'katherin': 26, 'adam': 3, 'jack': 12}
Use Python Dictionary Comprehension
>>> {'key_'+ str(i): i**2 for i in (1, 2, 3, 4, 5)}
{'key_1': 1, 'key_4': 16, 'key_3': 9, 'key_2': 4, 'key_5': 25}
Access items from Python Dictionary
Usual way
Use key inside square brackets:
>>> customer = {'name': 'Jack', 'age': 42, 'location': 'US'}
# keys: 'name', 'age', 'location'
>>> customer['name']
'Jack'
>>> customer['age']
42
>>> customer['location']
'US'
>>> customer['company']
Traceback (most recent call last):
File "", line 1, in
KeyError: 'company'
Use get() method
We can use get() method that returns None instead of KeyError when the key is not found:
>>> customer = {'name': 'Jack', 'age': 42, 'location': 'US'}
# keys: 'name', 'age', 'location'
>>> customer.get('name')
'Jack'
>>> customer.get('age')
42
>>> customer.get('location')
'US'
>>> customer.get('company')
>>>
Access all keys/values – Iterate a Python Dictionary
>>> customer = {'name': 'Jack', 'age': 42, 'location': 'US'}
# access all keys with keys() method
>>> customer.keys()
dict_keys(['name', 'location', 'age'])
>>> for key in customer.keys():
... print(key)
...
name
location
age
# access all values with values() method
>>> customer.values()
dict_values(['Jack', 'US', 42])
>>> for value in customer.values():
... print(value)
...
Jack
US
42
# access all key-value pairs with items() method
>>> customer = {'name': 'Jack', 'age': 42, 'location': 'US'}
>>> customer.items()
dict_items([('name', 'Jack'), ('location', 'US'), ('age', 42)])
>>> for key, value in customer.items():
... print('{}: {}'.format(key, value))
...
name: Jack
location: US
age: 42
Add item to Python Dictionary
– The simplest way is using a new key and assign a value:
>>> customer = {'name': 'Jack', 'age': 42, 'location': 'US'}
>>> customer['company'] = 'ozenero'
>>> customer
{'name': 'Jack', 'location': 'US', 'age': 42, 'company': 'ozenero'}
– Or we can use update() method:
>>> customer = {'name': 'Jack', 'age': 42, 'location': 'US'}
>>> customer.update({'company': 'ozenero'})
>>> customer
{'name': 'Jack', 'location': 'US', 'age': 42, 'company': 'ozenero'}
Update an item of Python Dictionary
Similar to add an item, but instead of using a new key, we update with the old key:
>>> customer = {'name': 'Jack', 'location': 'US', 'age': 42, 'company': 'ozenero'}
>>> customer['company'] = 'ozenero Technology'
>>> customer
{'name': 'Jack', 'location': 'US', 'age': 42, 'company': 'ozenero Technology'}
# use update() method
>>> customer = {'name': 'Jack', 'location': 'US', 'age': 42, 'company': 'ozenero'}
>>> customer.update({'company': 'ozenero Technology'})
>>> customer
{'name': 'Jack', 'location': 'US', 'age': 42, 'company': 'ozenero Technology'}
Remove an item from Python Dictionary
Pop item by key
pop() method to remove item by its key:
>>> customer = {'name': 'Jack', 'location': 'US', 'age': 42, 'company': 'ozenero'}
>>> customer.pop('company')
'ozenero'
>>> customer
{'name': 'Jack', 'location': 'US', 'age': 42}
Pop arbitrary item
popitem() method to remove arbitrary item:
>>> customer = {'name': 'Jack', 'location': 'US', 'age': 42, 'company': 'ozenero'}
>>> customer.popitem()
('name', 'Jack')
>>> customer
{'location': 'US', 'age': 42, 'company': 'ozenero'}
*Note: From version 3.7: LIFO order. In prior versions, popitem() for arbitrary item.
Remove all items
We can keep the dictionary and remove all items with clear() method:
>>> customer = {'name': 'Jack', 'location': 'US', 'age': 42, 'company': 'ozenero'}
>>> customer = {'name': 'Jack', 'age': 42, 'location': 'US'}
>>> customer.clear()
>>> customer
{}
Use del statement
– del statement to remove specific item (by key):
>>> customer = {'name': 'Jack', 'location': 'US', 'age': 42, 'company': 'ozenero'}
>>> del customer['company']
>>> customer
{'name': 'Jack', 'location': 'US', 'age': 42}
– delete the dictionary completely (this is different from clear() method):
>>> customer = {'name': 'Jack', 'location': 'US', 'age': 42}
>>> del customer
>>> customer
Traceback (most recent call last):
File "", line 1, in
NameError: name 'customer' is not defined
Check if a key/value exists in Python Dictionary
We can know whether a key/value is or is not in Dictionary using in and not in operators:
>>> customer = {'name': 'Jack', 'age': 42, 'location': 'US'}
>>> 'age' in customer.keys()
True
>>> 'Jack' in customer.values()
True
>>> 'ozenero' in customer.values()
False
>>> 'ozenero' not in customer.values()
True
>>> 'location' not in customer.keys()
False
# 'in customer' is short version of 'in customer.keys()'
>>> 'location' in customer
True
Set default value for a key in Python Dictionary
Sometimes we want to set default value for a key that doesn’t already have a value. It’s something like this:
>>> customer = {'name': 'Jack', 'age': 42}
>>> if 'location' not in customer:
... customer['location'] = 'US'
setdefault() method can make this happen in just one line of code:
>>> customer = {'name': 'Jack', 'age': 42}
>>> customer.setdefault('location','US')
'US'
>>> customer
{'age': 42, 'location': 'US', 'name': 'Jack'}
>>> customer.setdefault('location','Canada')
'US'
>>> customer
{'age': 42, 'location': 'US', 'name': 'Jack'}
Python Dictionary built-in functions
– all(): returns True if all keys are true, or if dictionary is empty.
– any(): returns True if any key is true, returns False if dictionary is empty.
– len(): returns number of items in dictionary.
– sorted(): returns a new sorted list of keys in the dictionary.
>>> customer = {'name': 'Jack', 'age': 42, 'location': 'US'}
>>> len(customer)
3
>>> sorted(customer)
['age', 'location', 'name']
