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Python Python Testing Be Assertive assertRaises

Posted by Emily Strobl
Emily Strobl
5,429 Points

assertRaises part 1

i dont get what i am doing wrong

tests.py 
import unittest

from string_fun import get_anagrams


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

1 Answer

Alex Koumparos
seal-mask
.a{fill-rule:evenodd;}techdegree
Alex Koumparos
Python Development Techdegree Student 36,887 Points
Alex Koumparos
.a{fill-rule:evenodd;}techdegree
Alex Koumparos
Python Development Techdegree Student 36,887 Points

Hi Emily,

When you test exceptions, you are testing that the code you execute inside the with block causes the error you are asserting. In this case you want to test that some particular code triggers a ValueError. Right now, the code you are testing is just pass. pass won't cause a ValueError. Thus you need to replace pass with what you actually want to test: some particular call to get_anagrams() that should raise a ValueError.

Looking at the code of get_anagram(), we can see that a string of length 0 will trigger the exception:

if yarn:
        if len(yarn[0]) == 1:
            return list(yarn)
        elif len(yarn[0] == 0):
            raise ValueError("Must provide at least two letters")   # Here is the raise, when the length of yarn[0] is 0

Therefore, to trigger the exception in our test code, we need to call get_anagrams() with a string of zero length.

Hope that helps.

Cheers,

Alex