How to update the value of a key in a dictionary in Python?

Python dictionary is an unordered collection of items. Each item of a dictionary has a key/value pair. And these are indexed by keys not with the index positions like other data types.

To update the value of a key in a dictionary we can simply update the value just by referencing the key directly, or we can use one of the dictionary method update() also.

In both ways, If an existing key is used in the expression, its associated value will be updated, otherwise a new key−value pair will be added in the dictionary object. Let’s learn about the update() method first.

Syntax

dict.update([other])	

Parameters

The update method takes either a dictionary or an iterable object of key/value pair as parameters.

Return Value

The method doesn’t return any output, instead it updates the Dictionary with newly associated values of the keys.

Using the update() method

Let’s take an example and see how the dict.update() method updates the value of a dictionary element based on key reference.

Example

d = {'A': 1, 'B': 5, 'C': 2}
print("Original Dictionary: ", d)

d.update({'A': 100})
print("Updated Dictionary: ",d)

Output

Original Dictionary:  {'A': 1, 'B': 5, 'C': 2}
Updated Dictionary:  {'A': 100, 'B': 5, 'C': 2}

The value 1 referring to the key “A” is changed to 100, here we specified {‘A’: 100} to the update method. Take another example and see what will happen if the reference key is not present in the dictionary object.

Example

We assigned an unused reference key to the update() method, so it has created the new key/value pair with assigned ones.

d = {'A': 1, 'B': 5, 'C': 2}
print("Original Dictionary: ", d)

d.update({'D': 10})
print("Updated Dictionary: ",d)

Output

Original Dictionary:  {'A': 1, 'B': 5, 'C': 2}
Updated Dictionary:  {'A': 1, 'B': 5, 'C': 2, 'D': 10}

Example

d = {'one':1, 'two':2,'three':3,'four':4}
print("Original Dictionary: ", d)

d.update({'one': '111', 'ten': 10})
print("Updated Dictionary: ",d)

Output

Original Dictionary:  {'one': 1, 'two': 2, 'three': 3, 'four': 4}
Updated Dictionary:  {'one': '111', 'two': 2, 'three': 3, 'four': 4, 'ten': 10}

In this above example, we have successfully updated the 2 key:value pairs using dict.update() method.

By directly assigning values

Without using any method we can simply assign dict[key] = new_value to update a value like the previous examples. Let’s take an example and see how to change the value of an existing key without using the method.

Example

d = {'one':1, 'two':2,'three':3,'four':4}
print("Original Dictionary: ", d)

d['one'] = '111'
print("Updated Dictionary: ",d)

Output

Original Dictionary:  {'one': 1, 'two': 2, 'three': 3, 'four': 4}
Updated Dictionary:  {'one': '111', 'two': 2, 'three': 3, 'four': 4}

The value referring key “one” is updated with a new value “111” successfully.