Simple Example

This section shows a simple, contrived example of how Fakeable can be used to test otherwise difficult to create scenarios.

Suppose you have a class named HttpDownloader that downloads files from the Internet using the HTTP protocol:

import urllib2
class HttpDownloader(object):
    # download the contents of the given URL and return it;
    # return None if the given URL is invalid
    def download(self, url):
        try:
            f = urllib2.urlopen(url)
        except urllib2.URLError:
            return None
        else:
            data = f.read()
            f.close()
            return data

Now suppose that you want to test another function called download_urls() that uses HttpDownloader in its implementation:

def download_urls(urls):
    downloader = HttpDownloader()
    datas = []
    for url in urls:
        data = downloader.download(url)
        datas.append(data)
    return datas

In order to test this function properly you would have to start a real HTTP server, presumaby on localhost, and have it return some pre-canned responses. This seems reasonable.

But what about testing the error conditions? What would you specify for an invalid URL? A bit of experimentation would give some good sample data but there is no guarantee that the same failures would occur on another computer, another operating system, or another Python implementation. This is where faking comes in handy.

Let’s create a fake version of HttpDownloader that simply returns None, simulating the URLError being handled:

class FakeHttpDownloader(object):
    def download(self, url):
        return None

Next, make the real HttpDownloader class fakeable:

import fakeable
import urllib2
class HttpDownloader(object):
    __metaclass__ = fakeable.Fakeable
    def download(self, url):
        ...

Finally, write the unit test that uses the fake version of HttpDownloader:

import unittest
class TestHttpDownloader(unittest.TestCase):
    def test_InvalidUrl(self):
        with fakeable.set_fake_class("HttpDownloader", FakeHttpDownloader):
            retval = download_urls(["foo", "bar"])
        self.assertListEqual(retval, [None, None])

This unit test can be further simplified by using the FakeableCleanupMixin to automatically unregister the fakes:

import unittest
class TestHttpDownloader(fakeable.FakeableCleanupMixin, unittest.TestCase):
    def test_InvalidUrl(self):
        fakeable.set_fake_class("HttpDownloader", FakeHttpDownloader)
        retval = download_urls(["foo", "bar"])
        self.assertListEqual(retval, [None, None])