Python dictionary (15 easy examples with syntax)

In this tutorial we will learn about Python Dictionary data type. Dictionaries, sometimes referred to as associative arrays in other languages, are data structures that hold data or information in a key-value order. Dictionaries allow you to access whatever value you want, using the much easier to remember key.

 

What are Python dictionaries

• A dictionary is a collection of unordered data, which is stored in key-value pairs.
• What is meant by “unordered” is the way it is stored in memory.
• It is not accessible through an index, rather it is accessed through a key.
• Lists are known as ordered data collections because each item is assigned a specific location.
• Dictionaries work like a real-life dictionary, where the key is the word and the values are the definition.
• Dictionaries are useful for working with large data, mapped data, CSV files, APIs, sending or receiving data, and much more.

 

Python dictionary types

Dictionaries, unlike lists, are indexed using keys, which are usually strings. There are two kinds of dictionaries that you can use in Python:

• the default dict, which is unordered, and
• a special kind of dictionary called an OrderedDict.

The difference is that the keys in the default dictionary are stored in an unordered manner, whereas an OrderedDict stores key-value pairs in the order of insertion.

 

Comparison between different Python Data Types

Below table shows a summary of the differences between each data types:

 

Create Python dictionary

You can create python dictionary in two ways. The first way is to simply assign an empty dictionary to a variable by using curly brackets, like so:

dictionary = {}

The second way is to use the dict() function to return a new dictionary:

dictionary = dict()

In both cases, a dictionary object will be created.

 

Exanple-1: Create python dictionary using {}

In this example we will create a dictionary using {}. For the sake of simplicity I have distributed the content of cars into different lines but you can put all the keys and their values in single line as well.

#!/usr/bin/env python3

# dictionary
cars = {
        'hyundai': "i10",
        'maruti': "swift",
        'honda': "city"
        }

print(cars)

Output from this script:

~]# python3 dictionary-example-1.py
{'hyundai': 'i10', 'maruti': 'swift', 'honda': 'city'}

 

Example-2: Create python dictionary using dict()

In this example we will use dict() to create a python dictionary.

#!/usr/bin/env python3

# dictionary
cars = dict(
        hyundai: "i10",
        maruti: "swift",
        honda: "city"
    )

print(cars)

Output from this script:

~]# python3 dictionary-example-1.py
{'hyundai': 'i10', 'maruti': 'swift', 'honda': 'city'}

 

How to check object type

Method-1: Using isinstance()

The isinstance() function is used to check the type of an object. It takes two arguments, the first one being the object being inspected, and the second being the class that we want to type-check against; for example, int, str, dict, list, and so on.

For example, in this python script I have a dictionary object. But we will check the type of the object using isinstance():

#!/usr/bin/env python3

# dictionary
mydict1 = {'ab':1, 'bc':2, 'cd':3}

print(isinstance(mydict1, dict))

Let's execute this script

~]# python3 dictionary-example-1.py
True

So this confirms that mydict1 is a dictionary.

 

Method-2: Using type()

Alternatively we can also use type() to get the type of the object, I will update my code:

#!/usr/bin/env python3

# dictionary
mydict1 = {'ab':1, 'bc':2, 'cd':3}
print(type(mydict1))

Output from this script:

~]# python3 dictionary-example-1.py
<class 'dict'>

So this also confirms that mydict1 is a dictionary.

 

Add or append key value to existing Python dictionary

Example-1: Basic method to add new key value pair to python dictionary

To add or append a new key:value pair to existing dictionary we must use

dictionary[KEY]=vVALUE

where dictionary is your variable which contains the actual dictionary. Replace KEY and VALUE with your key:value pair which you want to add to the dictionary

Let's execute this script:

~]# python3 dictionary-example-2.py
Before adding: {'hyundai': 'i10', 'maruti': 'swift', 'honda': 'city'}
After adding: {'hyundai': 'i10', 'maruti': 'swift', 'honda': 'city', 'ford': 'ecosport'}

So a new key:value pair i.e. ford:ecosport was added to our cars dictionary.

 

Example-2: Append new key:value pair using dict.update()

The .update() method on dictionaries, is used to insert new key-value pairs into a dictionary, or update the value of an existing one. In this script we will append ford:ecoport using dict.update() to our existing dictionary cars

Output from the script:

 ~]# python3 dictionary-example-2.py
Before adding: {'hyundai': 'i10', 'maruti': 'swift', 'honda': 'city'}
After adding: {'hyundai': 'i10', 'maruti': 'swift', 'honda': 'city', 'ford': 'ecosport'}

 

Modify data in Python dictionary

Example-1: Basic method to replace value of existing key in the dictionary

In this example we will replace the value of one of the existing key in the dictionary. We will replace the value of honda:city to honda:jazz where jazz will be the new value for honda

Output from this script:

~]# python3 dictionary-example-3.py
Before modifying: {'hyundai': 'i10', 'maruti': 'swift', 'honda': 'city'}
After modifying: {'hyundai': 'i10', 'maruti': 'swift', 'honda': 'jazz'}

So the value of honda is modified to 'jazz'

 

Example-2: Modify key value using dict.update()

In the previous example we used .update() function to add new key value to existing dictionary, we can also use .update() to modify the value of any existing key in the dictionary. For example I will use dict.update() to modify the value of 'honda' to 'jazz'

Output from this script:

 ~]# python3 dictionary-example-2.py
Before modifying: {'hyundai': 'i10', 'maruti': 'swift', 'honda': 'city'}
After modifying: {'hyundai': 'i10', 'maruti': 'swift', 'honda': 'jazz'}

 

How to read data from Python dictionary

In this section we will access the dictionary values using their keys:

Example-1: Using key reference

We can use the key to get the mapping value details. In this example we will read the value of all the keys present in cars dictionary:

Output from this script:

~]# python3 dictionary-example-2.py
i10
swift
city

But if a key is not defined then you may get KeyError. I updated my script to lookup for key "ford" which doesn't exist and hence the script failed:

~]# python3 dictionary-example-2.py
Traceback (most recent call last):
  File "dictionary-example-2.py", line 10, in 
    print(cars['ford'])
KeyError: 'ford'

In such case we should use get() function which we will use in our next example:

 

Example-2: Using get() function

The get() function returns None if an item does not exist. You can also use the get() function to specify what should be returned when no value exists.

#!/usr/bin/env python3

# dictionary
cars = {
        'hyundai': "i10",
        'maruti': "swift",
        'honda': "city"
        }

print('This key is present: ', cars.get('hyundai'))

## prints 'None' when key is not defined
print('This key is not defined: ', cars.get('test'))

## prints custom message when key is not defined
print(cars.get('test', 'Key test is not defined'))

Output from this script:

~]# python3 dictionary-example-4.py
This key is present:  i10
This key is not defined:  None
Key test is not defined

 

How to iterate through Python dictionary

I already covered this part in my previous chapter where I used for loop to iterate over dictionary. But let's discuss this in more detail here:

Example-1: Use for loop to iterate over dictionary

The simplest way to iterate through dictionaries is to use a for loop. Use a for loop as follows, in order to get a dictionary's keys:

#!/usr/bin/env python3

# dictionary
cars = {
        'hyundai': "i10",
        'maruti': "swift",
        'honda': "city"
        }

for item in cars:
    print(item)

Output from this script:

~]# python3 dictionary-example-5.py
hyundai
maruti
honda

 

Example-2: Iterate over keys or values using keys() function in Python dictionary

You can also explicitly iterate through only the keys or the values of a dictionary by calling the keys() method, which returns a list of keys, or the values() method, which returns a list of values in the dictionary.

Use the keys() method as follows, in order to print the keys:

#!/usr/bin/env python3

# dictionary
cars = {
        'hyundai': "i10",
        'maruti': "swift",
        'honda': "city"
        }

print('Using .keys() to get the list of keys in cars dictionary')
for item in cars.keys():
    print(item)

print('Using .values() to get the list of values in cars dictionary')
for item in cars.values():
    print(item)

Output from this script:

 

Example-3: Get both keys and value at the same time from Python dictionary

In the previous example we got the keys and values using different methods in separate code, we can also iterate over a dictionary and get both keys and values at the same time:

#!/usr/bin/env python3

# dictionary
cars = {
        'hyundai': "i10",
        'maruti': "swift",
        'honda': "city"
        }

for key, value in cars.items():
    print('The key is: ', key, 'and mapping value is: ', value)

Here we are using two variables with dict.items to iterate over both key and the value and then print them on the console.

Output from this script:

 

How to check if a key exist in Python dictionary

In previous example we saw that when a key is not present and if we try to access them using print then we get KeyError. We can use get() function which we used earlier or in keyword.

You can use the in keyword to check whether a particular key exists in a dictionary, without iterating through it. This works the same way as it does in lists, and you will get back a Boolean value of True if the key exists, and False if it doesn't:

In this script we use in keyword to check if a particular key exist in the Python dictionary:

Output from this script:

 ~]# python3 dictionary-example-6.py
True
False

We can also use this in if condition to check existence of dictionary and perform some task accordingly:

if 'hyundai' in cars:
    print('Found')
else:
    print('Not Found')

 

How to remove all the data from Python dictionary with dict.clear()

The clear method is used to remove all keys from a Python dictionary.

#!/usr/bin/env python3

# dictionary
cars = {
        'hyundai': "i10",
        'maruti': "swift",
        'honda': "city"
        }

cars.clear() ## clear the data from cars dictionary
print(cars) ## print the content of cars dictionary

Here cars.clear() will remove all the keys and values from cars dictionary. Output from this script:

~]# python3 dictionary-example-8.py
{}

As you see our dictionary is empty.

 

How to remove single key value pair from Python dictionary

Example-1: Using del keyword

If you only want to remove one key-value pair, you can use the del keyword, for example we will delete maruti from our cars dictionary using del keyword:

#!/usr/bin/env python3

# dictionary
cars = {
        'hyundai': "i10",
        'maruti': "swift",
        'honda': "city"
        }

del cars['maruti'] ## delete maruti and mapping value
print(cars) ## print cars dictionary content

Output from this script:

 ~]# python3 dictionary-example-6.py
{'hyundai': 'i10', 'honda': 'city'}

As you see, maruti key and it's value was removed from the cars dictionary.

 

Example-2: Using dict.pop()

.pop() is more useful if your intention is to remove the key pair from the dictionary and store it in a different variable for further usage.

#!/usr/bin/env python3

# dictionary
cars = {
        'hyundai': "i10",
        'maruti': "swift",
        'honda': "city"
        }
var = cars.pop('honda') # remove honda key from cars
print(cars) # print cars dictionary
print(var) ## this will return value of honda

Output from this script:

~]# python3 dictionary-example-6.py
{'hyundai': 'i10', 'maruti': 'swift'}
city

So using pop() we removed honda from our dictionary and stored the value in different variable var, then we access the value of var

 

How to copy content from one Python dictionary to another

We can use dict.copy() to create shallow copies of Python dictionaries. In this example we copy the content of cars dictionary into cars_copy and later update the content of cars dictionary to make sure the same is not updated in cars_copy. So both the copies are not linked to each other in any way.

#!/usr/bin/env python3

# dictionary
cars = {
        'hyundai': "i10",
        'maruti': "swift",
        'honda': "city"
        }

cars_copy = cars.copy() ## copying cars content
cars.update({'honda':'jazz'}) ## update cars content
print('original:', cars) ## print cars content
print('duplicate: ', cars_copy) ## print cars_copy content

Output from the script:

~]# python3 dictionary-example-6.py
original: {'hyundai': 'i10', 'maruti': 'swift', 'honda': 'jazz'}
duplicate:  {'hyundai': 'i10', 'maruti': 'swift', 'honda': 'city'}

So our original dictionary was updated but the same is not reflected in the duplicate copied dictionary.

 

Using dict.popitem() in dictionary

The popitem() method pops and returns a random item from the Python dictionary. That item will no longer exist in the dictionary after that.

#!/usr/bin/env python3

# dictionary
cars = {
        'hyundai': "i10",
        'maruti': "swift",
        'honda': "city"
        }

print(cars.popitem()) # remove any random key value
print(cars) # print available content of cars

In this example I am using cars.popitem() which will randomly remove one key value pair from the cars dictionary. Next we will print the available data from our dictionary:

 ~]# python3 dictionary-example-6.py
('honda', 'city')
{'hyundai': 'i10', 'maruti': 'swift'}

 

Using dict.setdefault() in Python dictionary

The setdefault() method takes two arguments: a key to be searched for in the dictionary, and a value.

• If the key exists in the Python dictionary, its value will be returned.
• If the key does not exist, it will be inserted with the value provided in the second argument.
• If no second argument was passed, any insertion will be done with the value None.

 

Example-1: If key exists in the dictionary

Assuming the key exists in the Python dictionary which is used with setdefault()

#!/usr/bin/env python3

# dictionary
cars = {
        'hyundai': "i10",
        'maruti': "swift",
        'honda': "city"
        }

var = cars.setdefault("maruti")
print('cars dict: ', cars)
print('new variable ', var)

Output from this script:

~]# python3 dictionary-example-12.py
cars dict:  {'hyundai': 'i10', 'maruti': 'swift', 'honda': 'city'}
new variable  swift

In this case, the value is returned as-is, and the dictionary is left untouched. Passing the second argument in this case will have no effect, since a value already exists.

 

Example-2: When key doesn’t exist in the dictionary and value was provided with key

Let's look at another example, where the key does not exist in the dictionary, and a value was passed. In this case, the key-value pair will be added to the dictionary, and the value will be returned, as well:

#!/usr/bin/env python3

# dictionary
cars = {
        'hyundai': "i10",
        'maruti': "swift",
        'honda': "city"
        }

var = cars.setdefault("ford", "ecosport")
print('cars dict: ', cars)
print('new variable ', var)

Output from this script:

~]# python3 dictionary-example-13.py
cars dict:  {'hyundai': 'i10', 'maruti': 'swift', 'honda': 'city', 'ford': 'ecosport'}
new variable  ecosport

So I was trying to add value of ford key using setdefault but since it was not present, both key and value were added to the dictionary and then the new variable is able to return the value of new key

 

Example-3: When key doesn’t exist in the dictionary and value was “not” provided with key

Now, let's look at a final example, where the key does not exist in the dictionary, and no value was passed. In this case, the key will be added with a value of None. Nothing will be returned:

#!/usr/bin/env python3

# dictionary
cars = {
        'hyundai': "i10",
        'maruti': "swift",
        'honda': "city"
        }

var = cars.setdefault("ford")
print('cars dict: ', cars)
print('new variable ', var)

Output from this script:

~]# python3 dictionary-example-6.py
cars dict:  {'hyundai': 'i10', 'maruti': 'swift', 'honda': 'city', 'ford': None}
new variable  None

Since this time we did not provided value for ford key, the value is set as "None"

 

Ordered Dictionary in Python

So far, the dictionaries that we have created do not maintain the insertion order of the key-value pairs that are added. Ordered dictionaries are dictionaries that maintain the insertion order of keys. This means that when you are iterating through them, you will always access the keys in the order in which they were inserted.

The OrderedDict class is a dict subclass defined in the collections package that Python ships with. Creating an ordered dictionary is as easy as creating an instance of the OrderedDict class and passing in key-value pairs:

#!/usr/bin/env python3

from collections import OrderedDict

cars = OrderedDict(hyundai='i10', maruti='swift')
print(cars)

Output from this script:

~]# python3 dictionary-example-13.py
OrderedDict([('hyundai', 'i10'), ('maruti', 'swift')])

Everything about OrderedDict, except for it maintaining an internal key order, is similar to normal dictionaries

 

Conclusion

In this tutorial, we covered dictionaries and their types (the default, unordered dict, and the specialized OrderedDict). We also looked at attributes defined on dictionary objects and their use cases; for example, update and setdefault. Using these attributes, we learned how to iterate through dictionaries and modify them to achieve particular goals.

 

Further Readings

Python Dictionaries
Looping Techniques
Python Ordered Dictionary