A dictionary is a data type that holds key-value pairs. These key-value pairs are called items. You will sometimes hear dictionaries referred to as associative arrays, hashes, or hash tables.
Dictionaries are created using comma-separated items between curly braces. The item starts with a key, is then followed by a colon, and is concluded with a value. The format is dictionary_name = {key_1: value_1, key_N: value_N} .
To create an empty dictionary use: dictionary_name = {}.
Items in a dictionary can be accessed by key. To do so, enclose the key in a bracket immediately following the dictionary name. The format is dictionary_name[key] .
There are a couple of important points while using dictionary keys
- More than one entry per key is not allowed ( no duplicate key is allowed)
- While the values in the dictionary can be of any type the keys must be immutable like numbers or strings.
- Dictionary keys are case sensitive – If you were to create a similarly named key but you use a different case then these are treated as different keys in Python.
Let’s look at an example
dict = {'Name': 'Bill', 'Age': 59, 'Country': 'UK'}
Accessing Values in Dictionary
To access an element in a dictionary, you can use square brackets along with the key to obtain its value.
Let’s take a look at this
dict = {'Name': 'Bill', 'Age': 59, 'Country': 'UK'}
print ("dict['Name']: ", dict['Name'])
print ("dict['Age']: ", dict['Age'])
print ("dict['Country']: ", dict['Country'])
When run you will see something like this
MicroPython v1.9.2-34-gd64154c73 on 2017-09-01; micro:bit v1.0.1 with nRF51822 Type "help()" for more information. >>> >>> dict['Name']: Bill dict['Age']: 59 dict['Country']: UK
Update A Dictionary
To update a dictionary you can simply add a new entry or a key-value pair, modify an existing entry, or delete an existing entry.
Let’s see an example of this.
dict = {'Name': 'Bill', 'Age': 59, 'Country': 'UK'}
print ("dict['Name']: ", dict['Name'])
print ("dict['Age']: ", dict['Age'])
print ("dict['Country']: ", dict['Country'])
dict['Age'] = 60; # update existing entry
dict['Country'] = 'USA'; # update existing entry
dict['City'] = 'New York'; #add a new entry
print ("dict['Name']: ", dict['Name'])
print ("dict['Age']: ", dict['Age'])
print ("dict['Country']: ", dict['Country'])
print ("dict['City']: ", dict['City'])
When run this would look like this
MicroPython v1.9.2-34-gd64154c73 on 2017-09-01; micro:bit v1.0.1 with nRF51822 Type "help()" for more information. >>> >>> dict['Name']: Bill dict['Age']: 59 dict['Country']: UK dict['Name']: Bill dict['Age']: 60 dict['Country']: USA dict['City']: New York
Check if a Key Exists
To check if a specified key is present in a dictionary you use the in keyword.
Let’s see an example of this.
dict = {'Name': 'Bill', 'Age': 59, 'Country': 'UK'}
print ("dict['Name']: ", dict['Name'])
print ("dict['Age']: ", dict['Age'])
print ("dict['Country']: ", dict['Country'])
if "Country" in dict:
print("Key is in the dictionary")
Length of a Dictionary
To determine how many items a dictionary has you can use the len() method.
Let’s see an example of this.
dict = {'Name': 'Bill', 'Age': 59, 'Country': 'UK'}
print ("dict['Name']: ", dict['Name'])
print ("dict['Age']: ", dict['Age'])
print ("dict['Country']: ", dict['Country'])
print(len(dict))
Removing Items from a Dictionary
There are several methods to remove items from a dictionary:
You can use the pop() method to remove the item with the key name that you specify
dict = {'Name': 'Bill', 'Age': 59, 'Country': 'UK'}
print ("dict['Name']: ", dict['Name'])
print ("dict['Age']: ", dict['Age'])
print ("dict['Country']: ", dict['Country'])
dict.pop("Country")
print(dict)
You can use the popitem() method to remove the last inserted item from Python 3.7.0 onwards, previous to this version of python it would remove a random item
dict = {'Name': 'Bill', 'Age': 59, 'Country': 'UK'}
print ("dict['Name']: ", dict['Name'])
print ("dict['Age']: ", dict['Age'])
print ("dict['Country']: ", dict['Country'])
dict.popitem()
print(dict)
The del keyword can either remove the item with the specified key name or it can delete the dictionary completely
dict = {'Name': 'Bill', 'Age': 59, 'Country': 'UK'}
print ("dict['Name']: ", dict['Name'])
print ("dict['Age']: ", dict['Age'])
print ("dict['Country']: ", dict['Country'])
del dict["Country"]
print(dict)
and to delete the entire dictionary, if you try and print or access this after you delete the entire dictionary you will see an error
dict = {'Name': 'Bill', 'Age': 59, 'Country': 'UK'}
print ("dict['Name']: ", dict['Name'])
print ("dict['Age']: ", dict['Age'])
print ("dict['Country']: ", dict['Country'])
del dict
You can use the clear() method to empty a dictionary.
dict = {'Name': 'Bill', 'Age': 59, 'Country': 'UK'}
print ("dict['Name']: ", dict['Name'])
print ("dict['Age']: ", dict['Age'])
print ("dict['Country']: ", dict['Country'])
dict.clear()
print(dict)
Copying a dictionary
There are a couple of methods that allow you to make a copy of a dictionary.
You can use the copy() method and the built-in method dict()
dict1 = {'Name': 'Bill', 'Age': 59, 'Country': 'UK'}
print ("dict1['Name']: ", dict1['Name'])
print ("dict1['Age']: ", dict1['Age'])
print ("dict1['Country']: ", dict1['Country'])
dict2 = dict1.copy()
print(dict2)
dict3 = dict(dict1)
print(dict3)
Run this and you will see something like this
MicroPython v1.9.2-34-gd64154c73 on 2017-09-01; micro:bit v1.0.1 with nRF51822
Type "help()" for more information.
>>>
>>> dict['Name']: Bill
dict['Age']: 59
dict['Country']: UK
{'Name': 'Bill', 'Country': 'UK'}