How to test command line scripts with nose?

Question

I've created a Python library with some command-line scripts in a 'bin' directory (so that setup.py will install it into 'bin' when installing it with pip). Since this isn't a Python module, I can't work out how to test it with nose.

How can I test a command line script that's part of a library using nose/unittest?

Answer 1

Use the "if __name__ == "__main__":" idiom in your scripts and encapsulate all of the function-ality in function-s.

Then you can import your scripts into another script (such as a unit test script) without the body of it being executed. This will allow you to write unit-tests for the functionality and run them through nose.

I recommend keeping the "main" block to a line or two.

For example:

plus_one.py

#!/usr/bin/env python

import sys


def main(args):
    try:
        output(plus_one(get_number(args)))
    except (IndexError, ValueError), e:
        print e
        return 1
    return 0


def get_number(args):
    return int(args[1])


def plus_one(number):
    return number + 1


def output(some_text):
    print some_text


if __name__ == '__main__':
    sys.exit(main(sys.argv))

You can test command-line parameters, output, exceptions and return codes in your unittests...

t_plus_one.py

#!/usr/bin/env python

from StringIO import StringIO
import plus_one
import unittest


class TestPlusOne(unittest.TestCase):

    def test_main_returns_zero_on_success(self):
        self.assertEquals(plus_one.main(['test', '1']), 0)

    def test_main_returns_nonzero_on_error(self):
        self.assertNotEqual(plus_one.main(['test']), 0)

    def test_get_number_returns_second_list_element_as_integer(self):
        self.assertEquals(plus_one.get_number(['anything', 42]), 42)

    def test_get_number_raises_value_error_with_string(self):
        self.assertRaises(ValueError, plus_one.get_number, ['something',
                                                            'forty-two'])

    def test_get_number_raises_index_error_with_too_few_arguments(self):
        self.assertRaises(IndexError, plus_one.get_number, ['nothing'])

    def test_plus_one_adds_one_to_number(self):
        self.assertEquals(plus_one.plus_one(1), 2)

    def test_output_prints_input(self):
        saved_stdout, plus_one.sys.stdout = plus_one.sys.stdout, StringIO('_')
        plus_one.output('some_text')
        self.assertEquals(plus_one.sys.stdout.getvalue(), 'some_text\n')
        plus_one.sys.stdout = saved_stdout

if __name__ == '__main__':
    unittest.main()

Output

python plus_one.py 41

42

nosetests -v t_plus_one.py

test_get_number_raises_index_error_with_too_few_arguments (t_plus_one.TestPlusOne) ... ok
test_get_number_raises_value_error_with_string (t_plus_one.TestPlusOne) ... ok
test_get_number_returns_second_list_element_as_integer (t_plus_one.TestPlusOne) ... ok
test_main_returns_nonzero_on_error (t_plus_one.TestPlusOne) ... ok
test_main_returns_zero_on_success (t_plus_one.TestPlusOne) ... ok
test_output_prints_input (t_plus_one.TestPlusOne) ... ok
test_plus_one_adds_one_to_number (t_plus_one.TestPlusOne) ... ok

----------------------------------------------------------------------
Ran 7 tests in 0.002s

OK