AssertionError when using nosetests

Question

On exercise 48 of Learn Python the Hard Way, I'm asked to create a module to be tested by this one, lexicon_tests.py:

from nose.tools import *
from ex48 import lexicon


def test_directions():
    assert_equal(lexicon.scan("north"), [('direction', 'north')])
    result = lexicon.scan("north south east")
    assert_equal(result, [('direction', 'north'),
                          ('direction', 'south'),
                          ('direction', 'east')])

def test_verbs():
    assert_equal(lexicon.scan("go"), [('verb', 'go')])
    result = lexicon.scan("go kill eat")
    assert_equal(result, [('verb', 'go'),
                          ('verb', 'kill'),
                          ('verb', 'eat')])


def test_stops():
    assert_equal(lexicon.scan("the"), [('stop', 'the')])
    result = lexicon.scan("the in of")
    assert_equal(result, [('stop', 'the'),
                          ('stop', 'in'),
                          ('stop', 'of')])


def test_nouns():
    assert_equal(lexicon.scan("bear"), [('noun', 'bear')])
    result = lexicon.scan("bear princess")
    assert_equal(result, [('noun', 'bear'),
                          ('noun', 'princess')])

def test_numbers():
    assert_equal(lexicon.scan("1234"), [('number', 1234)])
    result = lexicon.scan("3 91234")
    assert_equal(result, [('number', 3),
                          ('number', 91234)])


def test_errors():
    assert_equal(lexicon.scan("ASDFADFASDF"), [('error', 'ASDFADFASDF')])
    result = lexicon.scan("bear IAS princess")
    assert_equal(result, [('noun', 'bear'),
                          ('error', 'IAS'),
                          ('noun', 'princess')])

So I created the module, lexicon.py, to be tested here:

def scan(words):
    directions = ['north', 'south', 'east', 'west', 'down', 'up', 'left', 'right', 'back']
    verbs = ['go', 'stop', 'kill', 'eat']
    stop_words = ['the', 'in', 'of', 'from', 'at', 'it']
    nouns = ['door', 'bear', 'princess', 'cabinet']


    lex = words.split()
    list1 = []

    for i in lex:
        if i in directions:
            list1.append(('direction', i))
        elif i in verbs:
            list1.append(('verb', i))
        elif i in stop_words:
             list1.append(('stop-word', i))
        elif i in nouns:
            list1.append(('noun', i))
        elif i.isdigit():
            list1.append(('number', convert_number(i)))
        else:
            list1.append(('error', i))
    print list1 

def convert_number(s):
    try:
        return int(s)

    except ValueError:
        return None

However when I run nosetests in powershell I get this AssertionError:

Traceback (most recent call last):
  File "G:\Python27\lib\site-packages\nose\case.py", line 197, in runTest
    self.test(*self.arg)
  File        "G:\Users\Charles\dropbox\programming\lexicon_test\skeleton\tests\lexicon_tests.py", line   6, in test_directions
    assert_equal(lexicon.scan("north"), [('direction', 'north')])
AssertionError: None != [('direction', 'north')]
-------------------- >> begin captured stdout << ---------------------
[('direction', 'north')]

That's the same error message I get for each test run, six of them for the six functions in lexicon_tests.py. What does this error mean? It's been irritating be for a while now. Thanks in advance.

Answer 1

The assert_equal function takes two arguments, and throws an error if the arguments aren't equal to each other. In this case, the result of lexicon.scan("north") is None, and since this isn't equal to [('direction', 'north')], it's throwing an error.

In other words, your lexicon.scan function isn't working properly. It might have something to do with it missing a return statement.