If your code raises exceptions it’s a good idea to test them to make sure they get raised properly.
Luckily unittest provides a mechanism for this.

Exception Assertion

• assertRaises(x) – Make sure the following code raises the x exception.

Example

with assertRaises(ValueError):
    int('a')

You can also use @unittest.expectedFailure on tests that you know will fail.

Dice Example

We’re going to test a bad description value to see if it will raise an exception as it should.

dice .py

http://ref.ghadanfar.com/2018/01/29/dice-py/

test .py

To do that, add to the bottom of the previous test.py script the following code.

import unittest
import dice

...

class DieTests(unittest.TestCase):

    ...
        
    def test_bad_description(self):
        with self.assertRaises(ValueError):
            dice.Roll('2b6')
 

Anagram Example

string_fun.py

import itertools


def is_palindrome(yarn):
    """Return whether or not a string is a palindrome.

    A palindrome is a word/phrase that's the same in
    both directions.
    """
    return yarn == yarn[::-1]


def get_anagrams(*yarn):
    """Return a list of anagrams for a string."""
    # If only one letter came in, return it
    if yarn:
        if len(yarn[0]) == 1:
            return list(yarn)
        elif len(yarn[0] == 0):
            raise ValueError("Must provide at least two letters")
    else:
        raise ValueError("Must provide at least two letters")

    # Get all of the words from the dictionary
    words = set([
        w.strip().lower() for w in open('words.txt')
    ])

    output = set()
    for thread in yarn:
        thread = thread.lower()
        # Find all possible anagrams
        for i in range(2, len(thread)):
            fibers = set(
                [''.join(w) for w in itertools.permutations(thread, i)]
            )
            output.update(fibers.intersection(words))

    # Finally, return all of the combinations that are in the dictionary
    return sorted(list(output))

test.py

import unittest

from string_fun import get_anagrams


class AnagramTestCase(unittest.TestCase):
    def test_empty_string(self):
        with self.assertRaises(ValueError):
            get_anagrams("")

    def test_no_args(self):
        with self.assertRaises(ValueError):
            get_anagrams()

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