Add to a value in a Dictionary in Python

This tutorial will discuss how to add to a value in a dictionary in Python.

Suppose we have a dictionary that contains student data, where the name is the key (a string) and the age is the value (an integer). Like this,

# Student data dictionary with
# names as keys and ages as values
student_data = {
    "Varun Sharma": 21,
    "Renuka Singh": 19,
    "Sachin Roy": 20
}

Now, if we want to add a numeric value to the value field of a specific key, we need an integer value. For example, consider we have a dictionary of student data where the key is the name and the value is the age of the student. Now, if we want to add 5 to the age of student “Renuka Singh”, we will select the value from the dictionary by passing the key “Renuka Singh” into the square brackets of the dictionary. It will return a reference to the value field of that key-value pair. Then, we add 5 to it, which will increment the value of that key-value pair by 5.

Let’s see the complete example,

# Student data dictionary with
# names as keys and ages as values
student_data = {
    "Varun Sharma": 21,
    "Renuka Singh": 19,
    "Sachin Roy": 20
}

# Add a value to an existing value
student_data["Renuka Singh"] += 5

# Print the updated dictionary
print(student_data)

Output

{'Varun Sharma': 21, 'Renuka Singh': 24, 'Sachin Roy': 20}

However, if we try to add an integer value to the value field of a key that does not exist in the dictionary (basically passing a key into the square brackets that does not exist in the dictionary), it will cause a KeyError because we are trying to access a key that does not exist in the dictionary.

Like in the below example, we are trying to incremenet the value of a Key that does not exists in the Dictionary.

Let’s see the complete example,

# Student data dictionary with
# names as keys and ages as values
student_data = {
    "Varun Sharma": 21,
    "Renuka Singh": 19,
    "Sachin Roy": 20
}

# Add a value to an existing value
# Will Cause KeyError
student_data["Bob Singh"] += 5

# Print the updated dictionary
print(student_data)

Output

Traceback (most recent call last):
  File "./temp.py", line 11, in <module>
    student_data["Bob Singh"] += 5
KeyError: 'Bob Singh'

Summary

Today, we learned how to add to a value in a dictionary in Python.