To Mock or Not to Mock when testing code that relies on third party libraries

Question

I have written a function which basically hides the complication of using the pySnmp library to send an SNMP message and it looks like:

snmpWalk(host,oid):
    from pysnmp.entity.rfc3413.oneliner import cmdgen
    cg = cmdgen.CommandGenerator()
    comm_data = cmdgen.CommunityData('my-manager','public')
    transport = cmdgen.UdpTransportTarget((host,161))
    variables = oid
    return cg.nextCmd(comm_data,transport,variables)

I am new to unit testing, and I was trying to write a unit test for this code as a doctest using minimock and this is what I came up with:

#Setup the mocks
>>> from minimock import mock, Mock
>>> cmdgen.CommandGenerator = Mock('cmdgen.CommandGenerator')
>>> cmdgen.CommunityData = Mock('cmdgen.CommunityData')
>>> cmdgen.UdpTransportTarget = Mock('cmdgen.UdpTransportTarget')
>>> cgMock = Mock('cgMock')
>>> cmdgen.CommandGenerator.mock_returns = cgMock
>>> cmdgen.CommunityData.mock_returns = 1
>>> cmdgen.UdpTransportTarget.mock_returns = 2
>>> cgMock.nextCmd.mock_returns = (1,2,3,4)
#run the test
>>> PrinterMonitor.SnmpWalk('192.0.0.1','1.1.1.1.1.1.1') 
Called cmdgen.CommandGenerator()
Called cmdgen.CommunityData('my-manager', 'public')
Called cmdgen.UdpTransportTarget(('192.0.0.1', 161))
Called cgMock.nextCmd(1, 2, (1, 1, 1, 1, 1, 1, 1))
(1, 2, 3, 4)

So I run this test and it works out fine, but my question is, do I really need to go to all this effort in mocking every aspect of the 3rd party library? In my mind the minimum I would need to mock is the final line cg.nextCmd(comm_data,transport,variables) because I don't actually want to start firing off messages to printers in my unit tests, but the other calls to the library code should give me consistent results, depending on what I pass to them. I can make sure that my code interacts appropriately with the library (i.e. make sure that it passes the host argument to the transport variable correctly, by asserting this on the object passed to the mocked cg.nextCmd(...) call?

Answer 1

Do you need to go through all that effort? No.

To test code that uses your snmpWalk() function, you would be better to mock out the class that contains the function. I don't know the Python terms, but in Java terms, I would put snmpWalk() in an interface, then have a pySnmp implementation and any number of mock/test implementations. The code calling snmpWalk() shouldn't care about any internal details, it just cares about the return value, so that's the level you want to mock at.

To test the the pySnmp implementation class itself, you could mock out the library calls, but that's a lot of work and is risky, given your mocks have to emulate the library very closely. I would just stick with the real calls. If that means firing off real printer requests, don't run that test very often -- or see if there's a way to configure the library to print to virtual/fake printers.