How to check that a Python function throws an exception?

Question

Aryan Kumar · Answer

Sure, there are a few ways to check that a Python function throws an exception.

One way is to use the assertRaises() function from the unittest module. The assertRaises() function takes two arguments: the first argument is the exception that you want to check for, and the second argument is the function that you want to call.

For example, the following code checks that the divide_numbers() function throws a ZeroDivisionError exception if the denominator is zero:

Python

import unittest

def divide_numbers(x, y):
    return x / y

class TestDivideNumbers(unittest.TestCase):
    def test_divide_by_zero(self):
        with self.assertRaises(ZeroDivisionError):
            divide_numbers(10, 0)

if __name__ == "__main__":
    unittest.main()

Another way to check that a Python function throws an exception is to use the assert() statement. The assert() statement takes two arguments: the first argument is the expression that you want to check, and the second argument is the error message that you want to print if the expression is false.

For example, the following code checks that the divide_numbers() function throws a ZeroDivisionError exception if the denominator is zero:

Python

def divide_numbers(x, y):
    return x / y

def test_divide_by_zero():
    assert divide_numbers(10, 0) == "Error: Can't divide by zero", "The function did not throw an exception."

if __name__ == "__main__":
    test_divide_by_zero()

Finally, you can also use the try/except block to check that a Python function throws an exception. The try/except block is a control flow statement that allows you to execute a block of code and handle any exceptions that are raised.

The following code checks that the divide_numbers() function throws a ZeroDivisionError exception if the denominator is zero:

Python

def divide_numbers(x, y):
    return x / y

def test_divide_by_zero():
    try:
        divide_numbers(10, 0)
    except ZeroDivisionError:
        pass
    else:
        raise Exception("The function did not throw an exception.")

if __name__ == "__main__":
    test_divide_by_zero()

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