Python Dictionary update() Method

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Definition

The dict.update() method updates a dictionary with a (key-value) pair element from another dictionary, or from an iterable of (key-value) pair elements.

Syntax

dict.update([dictionary/iterable])

Parameters

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  • The dict.update() method inputs either an iterable object of (key-value) pair elements (tuples in most cases), or another dictionary.
  • Also, if the Python dict.update() method is applied to a dictionary without any parameters passed, then no changes to the dictionary occur, so the dictionary remains the same.

💡 Side note: The dict.update() method inserts a specified (key-value) pair element into a dictionary if the key does not exist.

Return Value

  • The Python dict.update() method performs it’s update operation, but does not return any value (returns a None value).

Basic Example

Example using the Python dict.update() method to update the value of a key in a dictionary:

identities = {'id_1': 'jim', 'id_2': 'tammy', 
              'id_3': 'sarah', 'id_4': 'bob'}
change_id_3 = {'id_3': 'amada'}
 
identities.update(change_id_3)
print(identities)

Output:

{'id_1': 'jim', 'id_2': 'tammy', 'id_3': 'amada', 'id_4': 'bob'}

This example shows how to update a value of a particular key in a dictionary by passing another dictionary with the key and its changed value as a parameter to the dict.update() method.

Add Key Value Pair to Python Dictionary

The following example shows how to add (key-value) pair elements to a Python dictionary using the dict.update() method:

groceries = {}
apples = {'apples': 5}
oranges = {'oranges': 6}
peaches = {'peaches': 5}
 
groceries.update(apples)
groceries.update(oranges)
groceries.update(peaches)
 
print(groceries)
# {'apples': 5, 'oranges': 6, 'peaches': 5}

This example shows how to individually insert (key-value) pair elements into a dictionary.

Passing a Tuple to dict.update()

An example on how to pass a tuple to a Python dictionary using the dict.update() method:

store_items = {}
store_items.update(pens = 3, notebooks = 4, desks = 4, shelves = 6)
 
print(store_items)
# {'pens': 3, 'notebooks': 4, 'desks': 4, 'shelves': 6}

In the previous example, applying the dict.update() method to a dictionary with one (key-value) pair element is good when only one (key-value) pair element needs to be inserted into a dictionary,

But this operation becomes tedious if multiple (key-value) pair elements are required to be inserted into a dictionary. This example of passing a tuple to the Python dict.update() method is very useful because multiple (key-value) pair elements can be inserted into a dictionary, all at once.

Merge Two Nested Dictionaries with dict.update()

Example on how to merge two nested dictionaries using the Python dictionary method dict.update():

company_1 = {'id_1': {'name': 'john', 'profession': 'electrician'},
             'id_2': {'name': 'kim', 'profession': 'plumber'}}
 
company_2 = {'id_3': {'name': 'tammy', 'profession': 'mason'},
             'id_4': {'name': 'lily', 'profession': 'welder'}}
 
company_merge = company_1.update(company_2)
 
print(company_merge)

Output:

None

Attempted merging of nested dictionaries failed, resulting in a None value being returned. But you can see that the original dictionary in company_1 has changed:

print(company_1)
# {'id_1': {'name': 'john', 'profession': 'electrician'}, 
#  'id_2': {'name': 'kim', 'profession': 'plumber'}, 
#  'id_3': {'name': 'tammy', 'profession': 'mason'}, 
#  'id_4': {'name': 'lily', 'profession': 'welder'}}