How to handle a Python Exception in a List Comprehension?

Last Updated : 12 Feb, 2025

In Python, there are no special built-in methods to handle exceptions in the list comprehensions. However, exceptions can be managed using helper functions, try-except blocks or custom exception handling. Below are examples of how to handle common exceptions during list comprehension operations:

Handling ZeroDivisionError

When dividing elements of two lists, a ZeroDivisionError may occur if the denominator is zero. To prevent program crashes, we can handle this error gracefully.

Python

a = [2, 6, 41, 1, 58, 33, -7, 90]
b = [1, 3, 2, 0, 6, 3, 7, 0]

def fun(a, b):
    try:
        return round(a / b, 3)
    except ZeroDivisionError:
        return None  # if division by zero occurs

res = [fun(x, y) for x, y in zip(a, b)]

print(res)

Output

[2.0, 2.0, 20.5, None, 9.667, 11.0, -1.0, None]

Explanation:

Inside the try block, the function attempts to divide a by b and rounds the result to three decimal places. If b is 0, a ZeroDivisionError occurs.
except block catches the exception and returns None instead of causing an error due to division by zero.
List comprehension applies the function to each pair (x, y) from lists a and b, computing the division results while handling any zero-division errors.

Handling ValueError

When working with mixed data types in a list, a ValueError can occur if we attempt to perform mathematical operations on non-numeric values. For example, trying to convert a string like "abc" to an integer will result in an error.

Python

a= ['10', '11', 7, 'abc', 'cats', 3, '5']   

def fun(a): 
    try: 
        return int(a)*int(a) 
    except ValueError: 
        pass 

res= [fun(x) for x in a]   
print(res)

Output

[100, 121, 49, None, None, 9, 25]

Explanation:

Inside the try block, the function attempts to convert a to an integer and then squares it (int(a) * int(a)). If a is a valid integer either as a string or number , the operation succeeds.
except block catches the exception if a cannot be converted to an integer e.g., non-numeric strings like 'abc' and 'cats' . In such cases, the function returns None by default.
List comprehension applies the function to each element in list a, squaring numeric values while ignoring invalid ones.

User-defined exception handling

Sometimes, we may need to enforce specific rules, such as filtering numbers based on custom conditions e.g., checking if numbers are divisible by 2. If the condition is not met, we can raise a custom exception.

Python

a = [2, 43, 56, -78, 12, 51, -29, 17, -24]

def fun(x):
    try:
        if x % 2 != 0:
            raise Exception(f"{x} is not divisible by 2")
        return x  
    except Exception as e:
        print(e)
        return None 

b = [fun(x) for x in a if fun(x) is not None]

print(b)