the amazing world of python: testing

Python is my primary language of choice when I want to program something (I’m also a fan of C but this is a sort of foolness of mine). Between the things that make me choose python there is the way it manages to make easy to maintain our code, in this case testing it.

I’m an advocate of TDD (test driven development) and I think that code without tests is a useless code and writing tests in python is simple as insert the description of API in the doctest string (this is a simplification of course).


Let’s start with a simple module with the definition of a function able to calculate the Fibonacci numbers:

""" This is the Fibonacci module. Here there is only a simple function the fib(). >>> fib(10) 55 Since the implementation is a very simple one, don't try to calculate it for arguments greater than 1000 otherwise it could not return very soon. """ def fib(n): """Return the n-th number of Fibonacci. n must be an integer greater than zero. >>> fib(1) 1 >>> fib(2) 1 >>> fib(12) 144 >>> fib(-4) Traceback (most recent call last): ... ValueError: n must be > 0 >>> fib(10*3) 832040 """ if n <= 0: raise ValueError("n must be > 0") if n == 1 or n == 2: return 1 return fib(n - 1) + fib(n - 2)

If we save this in a file called we can call the module doctest on it by using the following command

$ python -m doctest

With this example nothing is printed in the terminal but it’s not bad, only means that the tests passed with success. It’s possible to add the option -v in order to obtain a summary of the running

$ python -m doctest -v Trying: fib(10) Expecting: 55 ok Trying: fib(1) Expecting: 1 ok Trying: fib(2) Expecting: 1 ok Trying: fib(12) Expecting: 144 ok Trying: fib(-4) Expecting: Traceback (most recent call last): ... ValueError: n must be > 0 ok Trying: fib(10*3) Expecting: 832040 ok 2 items passed all tests: 1 tests in fib 5 tests in fib.fib 6 tests in 2 items. 6 passed and 0 failed. Test passed.

There are also a set of options that indicate how manage the tests behaviour: for example in this case is possible to wrap the list content using two lines, ignoring whitespaces and newlines with the option NORMALIZE_WHITESPACE:

>>> a = list(xrange(20)) >>> a # doctest: +NORMALIZE_WHITESPACE [0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19]


A special mention for the assert statement that permits to insert programmatic check in your program; if you have doubts in inserting an assert or a raise you have to know that the assert s are removed when the python interpreter is launched with optimization activated (i.e. with flag -O).

Furthemore, if you think is overkilling adding an assert that stop your code, you must note that is better to stop your program early with a known cause that have a not-so-obvious-bug in your code.

Test runner

The doctests are useful, but primarly for documentation purpouse (will be a post dedicated to it in the future), so is also possible to create more elaborate stuff using the unittest library; let’s start with a simple example: suppose we have a piece of our web application that will build a select menu from some entries in a database and we want to know that this is done correctly

import sqlite3 import tempfile import os import unittest def build_select(dbname): """Build a HTML select with the entries in the database. Suppose that this method belongs to another module that you want to test """ conn = sqlite3.connect(dbname) cursor = conn.cursor() options = "\n".join([ u'<option value="%s">%s</option>' % (row[0], row[0],) for row in cursor.execute("SELECT nation from nations")]) cursor.close() return "<select>%s</select>" % options class SelectTests(unittest.TestCase): def setUp(self): fd, self.db_path = tempfile.mkstemp() connection = sqlite3.connect(self.db_path) cursor = connection.cursor() cursor.execute("CREATE TABLE nations (nation)") nations = [ ('Italy',), ('Spain',), ('Greece',), ] cursor.executemany('INSERT INTO nations VALUES (?)', nations) connection.commit() connection.close() def tearDown(self): os.remove(self.db_path) def test_nations(self): select = build_select(self.db_path) self.assertEqual(u'''<select><option value="Italy">Italy</option> <option value="Spain">Spain</option> <option value="Greece">Greece</option></select>''', select)

If we save the code above in the file named we can launch the test with the following command

$ python -m unittest -v select_creation test_nations (select_creation.SelectTests) ... ok ---------------------------------------------------------------------- Ran 1 test in 0.228s OK

All the tests are searched in the methods which name starts with test that are subclasses of unittest.TestCase; first of launch any test the method setUp is called in order to prepare the environment for the tests (in the previous case we create a database with some entries in it). Obviously there is a tearDown method used to clean up the environment.

It’s possible to do more but I don’t want to wrote a complete guide, refere to the documentation.


In some cases is useful to fake objects in order to test our code against external library or not static data (think about external web service not under your direct control).

A possibility is offered by the mock library; if we want to test a function using the output from the twitter API, would be great to avoid the network creating a fake response as in the following example

import requests import unittest import simplejson def twitter_shit(tweetid): """Retrieve and elaborate a tweet """ url = "" % tweetid response = requests.get(url) if response.status_code != 200: raise Exception("obtained %d from server" % response.status_code) json = simplejson.loads(response.text) return "%s: %s" % (json["user"]["screen_name"], json["text"],) class TwitterTests(unittest.TestCase): def test_twitter_shit(self): fake_tweet = """{"created_at":"Mon Oct 08 08:34:49 +0000 2012","id":255224718150492162,"id_str":"255224718150492162","text":"hello world","user":{"id":378779203,"screen_name":"user1"}}""" with mock.patch("requests.get") as mocked_get: mocked_get.return_value.status_code = 200 mocked_get.return_value.text = fake_tweet response = twitter_shit("whatever") self.assertEqual(response, "user1: hello world")

In this case we simply patch with a context manager the get() method to return a status code equal to 200 with a predetermined text. At this point is easy to check that the response is parsed correctly.

This is not the only available library, use google to find them and report in the comments if you think there are alternatives :P.


Not only the tests are important, but also how they are written and which lines of code will be executed: a test missing some branching point (roughly speaking one of an if condition) or some condition, is a test that gives false sense of security. For code more elaborated and complex that the fibonacci example (think for example to a Django project) we need to know exactly how our code is tested.

This is possible using the coverage module that will print a report indicating which lines of code are executed and which branch of execution are missing. Let’s try to use this module: first of all install it

$ pip install coverage

Since with coverage we need to launch directly the tests (i.e. without the -m doctest command line option) we add the following lines at the bottom of

if __name__ == "__main__": import doctest doctest.testmod()

(they simply launch the tests when we do ./ and execute the following command

$ coverage run ./

This will create a file named .coverage in the working directory. Now is possible to have a report relative to our test

$ coverage report Name Stmts Miss Cover --------------------------- fib 11 0 100%

If we want a real report with each line annotated is possible to create an HTML one using

$ coverage html

It worth to know that is possible to use it with your django project following this instructions.