How To Add to a Dictionary in Python

Introduction

Dictionary is a built-in Python data type. A dictionary is a sequence of key-value pairs. Dictionaries are mutable objects, however, dictionary keys are immutable and need to be unique within each dictionary. There’s no built-in add method, but there are several ways to add to and update a dictionary. In this article, you’ll use the Python assignment operator, the update() method, and the merge and update dictionary operators to add to and update Python dictionaries.

Four Methods to Add to the Python Dictionary

1. Using the Assignment Operator
2. Using the Update Method
3. Using the Merge Operator
4. Using the Update Operator

Add to Python Dictionary Using the = Assignment Operator

You can use the = assignment operator to add a new key to a dictionary:

dict[key] = value

If a key already exists in the dictionary, then the assignment operator updates, or overwrites, the value.

The following example demonstrates how to create a new dictionary and then use the assignment operator = to update a value and add key-value pairs:

dict_example = {'a': 1, 'b': 2}

print("original dictionary: ", dict_example)

dict_example['a'] = 100  # existing key, overwrite
dict_example['c'] = 3  # new key, add
dict_example['d'] = 4  # new key, add 

print("updated dictionary: ", dict_example)

The output is:

Output
original dictionary:  {'a': 1, 'b': 2}
updated dictionary:  {'a': 100, 'b': 2, 'c': 3, 'd': 4}

The output shows that the value of a is overwritten by the new value, the value of b is unchanged, and new key-value pairs are added for c and d.

Add to Python Dictionary Without Overwriting Values

Using the = assignment operator overwrites the values of existing keys with the new values. If you know that your program might have duplicate keys, but you don’t want to overwrite the original values, then you can conditionally add values using an if statement.

Continuing with the example from the preceding section, you can use if statements to add only new key-value pairs to the dictionary:

dict_example = {'a': 1, 'b': 2}

print("original dictionary: ", dict_example)

dict_example['a'] = 100  # existing key, overwrite
dict_example['c'] = 3  # new key, add
dict_example['d'] = 4  # new key, add 

print("updated dictionary: ", dict_example)

# add the following if statements

if 'c' not in dict_example.keys():
    dict_example['c'] = 300

if 'e' not in dict_example.keys():
    dict_example['e'] = 5

print("conditionally updated dictionary: ", dict_example)

The output is:

Output
original dictionary:  {'a': 1, 'b': 2}
updated dictionary:  {'a': 100, 'b': 2, 'c': 3, 'd': 4}
conditionally updated dictionary:  {'a': 100, 'b': 2, 'c': 3, 'd': 4, 'e': 5}

The output shows that, because of the if condition, the value of c didn’t change when the dictionary was conditionally updated.

Add to Python Dictionary Using the update() Method

You can append a dictionary or an iterable of key-value pairs to a dictionary using the update() method. The update() method overwrites the values of existing keys with the new values.

The following example demonstrates how to create a new dictionary, use the update() method to add a new key-value pair and a new dictionary, and print each result:

site = {'Website':'DigitalOcean', 'Tutorial':'How To Add to a Python Dictionary'}
print("original dictionary: ", site)

# update the dictionary with the author key-value pair
site.update({'Author':'Sammy Shark'})
print("updated with Author: ", site)

# create a new dictionary
guests = {'Guest1':'Dino Sammy', 'Guest2':'Xray Sammy'}

# update the original dictionary with the new dictionary
site.update(guests)
print("updated with new dictionary: ", site)

The output is:

Output
original dictionary:  {'Website': 'DigitalOcean', 'Tutorial': 'How To Add to a Python Dictionary'}
updated with Author:  {'Website': 'DigitalOcean', 'Tutorial': 'How To Add to a Python Dictionary', 'Author': 'Sammy Shark'}
updated with new dictionary:  {'Website': 'DigitalOcean', 'Tutorial': 'How To Add to a Python Dictionary', 'Author': 'Sammy Shark', 'Guest1': 'Dino Sammy', 'Guest2': 'Xray Sammy'}

The output shows that the first update adds a new key-value pair and the second update adds the key-value pairs from the guest dictionary to the site dictionary. Note that if your update to a dictionary includes an existing key, then the old value is overwritten by the update.

Add to Python Dictionary Using the Merge | Operator

You can use the dictionary merge | operator, represented by the pipe character, to merge two dictionaries and return a new dictionary object.

The following example demonstrates how to to create two dictionaries and use the merge operator to create a new dictionary that contains the key-value pairs from both:

site = {'Website':'DigitalOcean', 'Tutorial':'How To Add to a Python Dictionary', 'Author':'Sammy'}

guests = {'Guest1':'Dino Sammy', 'Guest2':'Xray Sammy'}

new_site = site | guests

print("site: ", site)
print("guests: ", guests)
print("new_site: ", new_site)

The output is:

Output
site:  {'Website': 'DigitalOcean', 'Tutorial': 'How To Add to a Python Dictionary', 'Author': 'Sammy'}
guests:  {'Guest1': 'Dino Sammy', 'Guest2': 'Xray Sammy'}
new_site:  {'Website': 'DigitalOcean', 'Tutorial': 'How To Add to a Python Dictionary', 'Author': 'Sammy', 'Guest1': 'Dino Sammy', 'Guest2': 'Xray Sammy'}

The two dictionaries were merged into a new dictionary object that contains the key-value pairs from both dictionaries.

If a key exists in both dictionaries, then the value from the second dictionary, or right operand, is the value taken. In the following example code, both dictionaries have a key called b:

dict1 = {'a':'one', 'b':'two'}
dict2 = {'b':'letter two', 'c':'letter three'}

dict3 = dict1 | dict2

print{"dict3: ", dict3}

The output is:

Output
dict3:  {'a': 'one', 'b': 'letter two', 'c': 'letter three'}

The value of key b was overwritten by the value from the right operand, dict2.

Add to Python Dictionary Using the Update |= Operator

You can use the dictionary update |= operator, represented by the pipe and equal sign characters, to update a dictionary in-place with the given dictionary or values.

Just like the merge | operator, if a key exists in both dictionaries, then the update |= operator takes the value from the right operand.

The following example demonstrates how to create two dictionaries, use the update operator to append the second dictionary to the first dictionary, and then print the updated dictionary:

site = {'Website':'DigitalOcean', 'Tutorial':'How To Add to a Python Dictionary', 'Author':'Sammy'}

guests = {'Guest1':'Dino Sammy', 'Guest2':'Xray Sammy'}

site |= guests

print("site: ", site)

The output is:

Output
site:  {'Website': 'DigitalOcean', 'Tutorial': 'How To Add to a Python Dictionary', 'Author': 'Sammy', 'Guest1': 'Dino Sammy', 'Guest2': 'Xray Sammy'}

In the preceding example, you didn’t need to create a third dictionary object, because the update operator modifies the original object. The output shows that the guests dictionary was appended to the original site dictionary.

Conclusion

In this article, you used different methods to add to and update a Python dictionary. Continue your learning with more Python tutorials.