Art of Unit Test :: Stubs

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Glossary

SUT SYSTEM UNDER TEST

CUT CLASS UNDER TEST

MUT METHOD UNDER TEST

Tests without Use of Stub

You do not have to use stub when there are no external dependencies involved in the system under test (SUT). The point is only take it into consideration when you really need it

'''CUT'''
class LogAnalyzer_NoUseStub(object):
def IsValidLogFileName(self, fileName):
return str(fileName).endswith('.sln')

'''TEST CASE'''
class LogAnalyzerTestCase(unittest.TestCase):
# No stub used just simply perform the test
def test_IsValidLogFileName_BadExtension_ReturnFalse_NoStub(self):
logAnalyzer0 = LogAnalyzer_0()
ret = logAnalyzer0.IsValidLogFileName('fn1.sl')
self.assertFalse(ret)

Tests Must Use Stub

However somethimes we have to rely on the extenal class or method that we cannot control on it or it has not been finished yet. This is when we need stub to help us. eg, draw_from_weighted_range() and randrange(), interacting with filesystem. Fakes here include stub (assert on CUT) and mock (assert on Fake) we talk about stub and mock in later posts. Say our IsValidLogFileName() method needs read through the config file and return true if extension is supported in config file.

There two big types to inject fakes into MUT(Method Under Test)

Test are performed on the MUT itself (eg. assert(mut.dosomething(),true)

Step 1 - Abstracting concrete objects into interfaces or delegates

How: Extract an interface to allow replacing or extending underlying impl

Example codes

'''
Refered to 'Abstracting concrete objects into interfaces or delegates'
'''
class ExtensionMgr_AbstractedInterface(object):
def IsValid(self, filename):  # should be overwriten by child
pass

class FileExtensionMgr_ConcreteImpl(ExtensionMgr_AbstractedInterface):
def IsValid(self, filename):
return str(filename).endswith('.sln')

'''
Stubs
'''
class ExtendMgrStub(ExtensionMgr_AbstractedInterface):
def __init__(self):
self.mWillBeValid = False
return ExtensionMgr_AbstractedInterface.__init__(self)

def IsValid(self, filename):
return self.mWillBeValid

d1eb
class ExtendMgrStub_WithoutIngeritingFrom_ExtensionMgr_AbstractedInterface(object):
def __init__(self):
self.mWillBeValid = False

def IsValid(self, filename):
return self.mWillBeValid

Step 2 - Refactoring to allow injection of faked implementations of those delegates or interface

How:

Inject stub in code under test using factory design (layer of indirection 2 faking a member in factory class)

the difference is that the object initiating the stub request is the code under test. the fake instances was set by code external to the code under test before the test started in the below.

A test configures the factory class to re turn a stub object. The class usess the factory class to get the stub instance, which in production code would return an object that is not a stub Preferred to using this layer

Example code

class ExtensionMgrFactory(object):
iExtMgr = None

@staticmethod
def Create():
# define factory that can use and return custom manager instance
if ExtensionMgrFactory.iExtMgr is None:
ExtensionMgrFactory.iExtMgr = FileExtensionMgr_ConcreteImpl()
else:
return ExtensionMgrFactory.iExtMgr

@staticmethod
def SetExtMgr(extmgr):
ExtensionMgrFactory.iExtMgr = extmgr

class LogAnalyzer_StubInjectedViaFactory(object):
def __init__(self):
self.mIExtensionMgr = ExtensionMgrFactory.Create()

def IsValidLogFileName(self, fileName):
self.mIExtensionMgr.IsValid(fileName)

class LogAnalyzerTestCase(unittest.TestCase):
# StubIjectedViaFactory
def test_IsValidLogFileName_BadExtension_ReturnFalse_ \
StubIjectedViaFactory(self):
ext = ExtendMgrStub()
ext.mWillBeValid = False
ExtensionMgrFactory.SetExtMgr(ext)

logAnalyzer = LogAnalyzer_StubInjectedViaFactory()
ret = logAnalyzer.IsValidLogFileName('fn1.sl')

self.assertFalse(ret)

Injection of a stub in test code (layer of indiretion 1 faking a member in class under test)

Inject stub via ctor (cumbersome when you have many dependencies)

Example code

'''
Refered to Inject stub impl via ctor but will be cumbersome when you have many dependencies
'''
class LogAnalyzer_StubInjectedViaCtor(object):
def __init__(self, iExtensionMgr):
self.mIExtensionMgr = iExtensionMgr
def IsValidLogFileName(self, fileName):
self.mIExtensionMgr.IsValid(fileName)

class LogAnalyzerTestCase(unittest.TestCase):
# No stub used just simply perform the test
def test_IsValidLogFileName_BadExtension_ReturnFalse_NoStub(self):
logAnalyzer0 = LogAnalyzer_0()
ret = logAnalyzer0.IsValidLogFileName('fn1.sl')
self.assertFalse(ret)

#StubIjectedViaCtor
def test_IsValidLogFileName_BadExtension_ReturnFalse_StubIjectedViaCtor(self):
ext = ExtendMgrStub()
ext.mWillBeValid = False

logAnalyzer = LogAnalyzer_StubInjectedViaCtor(ext)
ret = logAnalyzer.IsValidLogFileName('fn1.sl')

self.assertFalse(ret)

#StubIjectedViaCtor This is what I wrote because python is weak-type language

def test_IsValidLogFileName_BadExtension_ReturnFalse_StubIjectedViaCtor_WithoutInhertingFrom_ExtensionMgr_AbstractedInterface(self):
ext = ExtendMgrStub_WithoutIngeritingFrom_ExtensionMgr_AbstractedInterface()
ext.mWillBeValid = False

logAnalyzer = LogAnalyzer_StubInjectedViaCtor(ext)
ret = logAnalyzer.IsValidLogFileName('fn1.sl')

self.assertFalse(ret)

Inject stub via setter/getter

This is much simpler than ctor injection as each test can set only the dependencies that it needs to get the test underway.

Use this when you want to signify that the dependency is optional or the dependency has a default instance created that does not create any problems;

Example code

'''
Refered to Inject stub impl via a setter and getter
'''
class LogAnalyzer_StubInjectedViaPropertySetter(object):
def __init__(self):
self.mIExtensionMgr = FileExtensionMgr_ConcreteImpl()

def IsValidLogFileName(self, fileName):
self.mIExtensionMgr.IsValid(fileName)

def SetIExtensionMgr(self, ext):
self.mIExtensionMgr = ext

def GetIExtensionMgr(self):
return self.mIExtensionMgr

class LogAnalyzerTestCase(unittest.TestCase):
def test_IsValidLogFileName_BadExtension_ReturnFalse_
StubInjectedViaPropertySetter(self):
ext = ExtendMgrStub()
ext.mWillBeValid = False

logAnalyzer = LogAnalyzer_StubInjectedViaPropertySetter()
logAnalyzer.SetIExtensionMgr(ext)
ret = logAnalyzer.IsValidLogFileName('fn1.sl')

self.assertFalse(ret)

Inject stub impl via parameter

Test are performed on the class that inhetites from MUT (eg. assert(mut_child.dosomething(),true)

It is also known as Extract and override, which is is good to for sumulating inputs into your code under test(in other words, return values from dependency). It is cumbersome when you want t verify and check interactions that are coming out of the code under test int othe dependency (in other words, it is good to play stub but very bad to play mock)

Use local virtual factory method to get instance of stub

The time not to use this is there is an interface ready to fake or there is already a place that seam can be injected.

Example code

class LogAnalyzer_StubInjectedViaLocalFactoryMethod(object):
def IsValidLogFileName(self, fileName):
self.GetMgr().IsValid(fileName)

def GetMgr(self):
return FileExtensionMgr_ConcreteImpl()

class TestableLogAnalyzer_ReturnStub(LogAnalyzer_StubInjectedViaLocalFactoryMethod):
def __init__(self, iExtensionMgr):
self.mIExtensionMgr = iExtensionMgr
def GetMgr(self):
return self.mIExtensionMgr

class LogAnalyzerTestCase(unittest.TestCase):
def test_IsValidLogFileName_BadExtension_ReturnFalse_ \
StubIjectedViaLocalVirtulFactory(self):
ext = ExtendMgrStub()
ext.mWillBeValid = False

logAnalyzer = TestableLogAnalyzer_ReturnStub(ext)
ret = logAnalyzer.IsValidLogFileName('fn1.sl')

self.assertFalse(ret)

Use extract and override to return a logical result instead of calling an actual denpendency

This uses a simple faked result instead of a stub and is much easier than 2.1 preferred to use

Example code

class LogAnalyzer_OverrideMethodReturnsResult(object):
def __init__(self):
self.mIExtensionMgr =  FileExtensionMgr_ConcreteImpl()
def IsValidLogFileName(self, fileName):
self.IsValidExtension(fileName)
def IsValidExtension(self,filename):
return self.mIExtensionMgr.IsValid(filename)

class TestableLogAnalyzer_OverrideMethodReturnsResult(LogAnalyzer_OverrideMethodReturnsResult):
def __init__(self, is_valid_entension):
self.is_valid_entension = is_valid_entension
def IsValidExtension(self,filename):
return self.is_valid_entension

class LogAnalyzerTestCase(unittest.TestCase):
# OverrideMethodReturnsResult
def test_IsValidLogFileName_BadExtension_ReturnFalse_ \
OverrideMethodReturnsResult(self):
is_valid_extension = False

testableLogAnalyzer = TestableLogAnalyzer_ \
OverrideMethodReturnsResult(is_valid_extension)
ret = testableLogAnalyzer.IsValidLogFileName('fnl.sl')

self.assertFalse(ret)