How to Test Exception Throwing in Python Functions?

Testing Exception Throwing in Python Functions

In Python, testing that a function raises a specific exception is crucial for ensuring the correctness and robustness of the code. To accomplish this, the unittest module provides the TestCase.assertRaises method.

How to Use TestCase.assertRaises

The syntax of TestCase.assertRaises is as follows:

<code class="python">self.assertRaises(ExpectedException, function, *args, **kwargs)</code>

Where:

• ExpectedException is the type of exception that is expected to be thrown by the function.
• function is the function that is being tested.
• args and kwargs are the arguments and keyword arguments that are passed to the function.

Example Usage

Consider a function myfunc defined in a module named mymod, which is expected to throw a SomeCoolException when certain conditions are met. To test this behavior, we can use the following unit test:

<code class="python">import mymod
import unittest

class MyTestCase(unittest.TestCase):
    def test1(self):
        self.assertRaises(mymod.SomeCoolException, mymod.myfunc)</code>

Explanation

In this test case, the self.assertRaises method is used to assert that the mymod.myfunc function raises a mymod.SomeCoolException when it is called without any arguments. If the exception is not raised, the test will fail.

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