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Zope

pycon, gheorghiu, unit tests, automated testing, mocking, monkey, monkey-patching, unit/integration, pop3

14^th of March 2008

Here's one of many things I've learnt today at PyCon. Inspired by code that Grig Gheorghiu showed in his slides on automated testing, you can monkey patch a standard library that your application is using in your unit tests to, in my case, mock a remote service without having to run a server. I've done lots of monkey-patching in Zope but then I've only been monkey patching individual methods or attributes of imported classes. This is very similar to that. Here's what my application does:

from poplib import POP3
class MyZopeApp(...):
    def check4mail(self, hostname, port, user, pwd):
        connection = POP3(hostname, port=port)
        ...download emails and process them...

Adjacent to this I have a unit/integration test that looks like this:

class TestCase(ZopeTestCase):
    def test_check4mail(self):
        # monkey patch!
        # note that this imports a module, not a class
        from Products.IssueTrackerProduct import IssueTracker
        FakePOP3.files = ('test1.email',)
        IssueTracker.POP3 = FakePOP3

        # now check what happens when check4mail() is run
        result = self.folder.tracker.check4mail()
        assert ...

Now for the mock. The mock is a fake POP3 class that instead of getting its data from the network reads local filesystem files. Here's what the code for FakePOP3 is:

from poplib import POP3, error_proto

class FakePOP3(POP3):

    username = 'test'
    password = 'test'
    files = []

    def __init__(self, hostname, port=110):
        self.hostname = hostname
        self.port = port

    def getwelcome(self):
        return "Welcome to fake account"

    def user(self, user):
        if user != self.username:
            raise error_proto("Wrong username.")

    def pass_(self, pswd):
        if pswd != self.password:
            raise error_proto("Wrong password.")

    def list(self, which=None):
        # eg. ('+OK 4 messages:', ['1 71017', '2 2201', '3 7723', '4 44152'], 34)
        files = self.files
        responses = []
        for i, f in enumerate(files):
            responses.append('%s %s' % (i+1, os.stat(f)[stat.ST_SIZE]))
        return ('+OK %s messages:' % len(files), responses, None)

    def retr(self, which):
        # ['response', ['line', ...], octets]
        filename = self.files[which-1]
        return ('response', open(filename, 'r').xreadlines(), None)

    def quit(self):
        pass

That's it! That's how you fake a POP3 server without having to run an actual mock server which could have been a solution.

Comment

Peter Bengtsson - 16th March 2008 [«« Reply to this]
A less advanced alternative is minimock by Ian Bicking:
http://blog.ianbicking.org/minimock.html

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