Testing for Exceptions

Overview

Questions

• How check that a function raises an exception?

Objectives

• Learn how to test exceptions using pytest.raises.

What to do about code that raises exceptions?

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Sometimes you will want to make sure that a function raises an exception when it should. For example, you might want to check that a function raises a ValueError when it receives an invalid input.

Take this example of the square_root function. We don’t have time to implement complex numbers yet, so we can raise a ValueError when the input is negative to crash a program that tries to compute the square root of a negative number.

PYTHON

def square_root(x):
   if x < 0:
      raise ValueError("Cannot compute square root of negative number yet!")
   return x ** 0.5

We can test that the function raises an exception using pytest.raises as follows:

PYTHON

import pytest

from advanced.advanced_calculator import square_root

def test_square_root():
    with pytest.raises(ValueError):
        square_root(-1)

Here, pytest.raises is a context manager that checks that the code inside the with block raises a ValueError exception. If it doesn’t, the test fails.

If you want to get more detailled with things, you can test what the error message says too:

PYTHON

def test_square_root():
    with pytest.raises(ValueError) as e:
        square_root(-1)
    assert str(e.value) == "Cannot compute square root of negative number yet!"

Challenge : Ensure that the divide function raises a ZeroDivisionError when the denominator is zero.

• Add a divide function to calculator.py:

PYTHON

def divide(numerator, denominator):
    if denominator == 0:
        raise ZeroDivisionError("Cannot divide by zero!")
    return numerator / denominator

• Write a test in test_calculator.py that checks that the divide function raises a ZeroDivisionError when the denominator is zero.

Show me the solution

PYTHON

import pytest

from calculator import divide

def test_divide():
    with pytest.raises(ZeroDivisionError):
        divide(1, 0)

Key Points

• Use pytest.raises to check that a function raises an exception.