Python: Object-Oriented Testing (Unit Testing)

Object Oriented Testing
(Unit Testing)
Damian Gordon

• Unit Testing is concerned with testing small chunks of a
program, for example, testing a single class or a single method.
• Python has a library for unit testing called unittest. It
provides several tools for creating and running unit tests.

• One of the most important classes in unittest is called
TestCase which provides a set of methods to compare
values, set up tests and clean up after tests are finished.
• To write unit tests, we create a subclass of TestCase and
write individual methods to do the actual testing. Typically we
start all of these methods with the name test.

• Let’s look at a simple example:

import unittest
class CheckNumbers(unittest.TestCase):
def test_int_float(self):
self.assertEqual(1, 1.0)
# END test_int_float
# END CheckNumbers
if __name__ == "__main__":
unittest.main()
# ENDIF

import unittest
# END CheckNumbers
if __name__ == "__main__":
unittest.main()
# ENDIF
Create a subclass of
the TestCase class

import unittest
# END CheckNumbers
if __name__ == "__main__":
unittest.main()
# ENDIF
the TestCase class
This test checks if the
integer and real
value 1 are equal.

import unittest
# END CheckNumbers
if __name__ == "__main__":
unittest.main()
# ENDIF
Make sure this is
being run as a script
the TestCase class
integer and real
value 1 are equal.

• And if we run this, we get:

.
-------------------------------------------------
Ran 1 test in 0.020s
OK

.
-------------------------------------------------
OK
Dot means the test has passed

• Let’s try another example:

import unittest
def test_string_float(self):
self.assertEqual(“1”, 1.0)
# END test_string_float
# END CheckNumbers
if __name__ == "__main__":
unittest.main()
# ENDIF

import unittest
# END CheckNumbers
if __name__ == "__main__":
unittest.main()
# ENDIF
string and real value
1 are equal.

F
=========================================================
FAIL: test_string_float (__main__.CheckNumbers)
This test checks if the string and real value 1 are equal.
--------------------------------------------------------
Traceback (most recent call last):
File "C:/Users/damian/AppData/Local/Programs/Python/Python35-
32/CheckNumbers-string-float.py", line 9, in test_string_float
self.assertEqual("1", 1.0)
AssertionError: '1' != 1.0
--------------------------------------------------------
FAILED (failures=1)

F
=========================================================
This test checks if the string and real value 1 are equal.
--------------------------------------------------------
File "C:/Users/damian/AppData/Local/Programs/Python/Python35-
32/CheckNumbers-string-float.py", line 9, in test_string_float
--------------------------------------------------------
FAILED (failures=1)
“F” means the test has failed

• Let’s combine the two tests together:

• Let’s combine the two tests together:
import unittest
if __name__ == "__main__":
unittest.main()
# ENDIF

.F
=================================================================
---------------------------=-------------------------------------
File "C:UsersdamianAppDataLocalProgramsPythonPython35-
32CheckNumbers.py", line 10, in test_string_float
-----------------------------------------------------------------
Ran 2 tests in 0.010s
FAILED (failures=1)

.F
=================================================================
---------------------------=-------------------------------------
File "C:UsersdamianAppDataLocalProgramsPythonPython35-
32CheckNumbers.py", line 10, in test_string_float
-----------------------------------------------------------------
FAILED (failures=1)
“.F” means the first test passed and the
second test has failed

• A test case typically sets certain variables to known values, runs
one or more methods or processes, and then show that correct
expected results were returned by using TestCase assertion
methods.
• There are a few different assertion methods available to confirm
that specific results have been achieved. We already saw
assertEqual, which will cause a test failure if the two
parameters do not pass an equality check. The inverse,
assertNotEqual, will fail if the two parameters do compare as
equal.

• The assertTrue and assertFalse methods each accept
a single expression, and fail if the expression does not pass an
IF test. These tests are not checking for the Boolean values
True or False, but instead:
– To pass the assertFalse method the test should return False,
None, 0, or an empty list, dictionary, string, set, or tuple.
– To pass the assertFalse method the test should return True,
non-zero numbers, containers with values in.

Methods Description
assertEqual
assertNotEqual
Accept two comparable objects and ensure the named equality
holds.
assertTrue
assertFalse
Accept a single expression, and fail if the expression does not
pass an IF test.
assertGreater
assertGreaterEqual
assertLess
assertLessEqual
Accept two comparable objects and ensure the named
inequality holds.
asssertIn
assertNotIn
Ensure an element is (or is not) an element in a container
object.
assertIsNone
assertIsNotNone
Ensure an element is (or is not) the exact value None (but not
another false value).
assertSameElements Ensure two container objects have the same elements, ignoring
the order.
assertSequenceEqualassertDictEqual
assertSetEqual
assertListEqual
assertTupleEqual
Ensure two containers have the same elements in the same
order. If there's a failure, show a code diff comparing the two
lists to see where they differ. The last four methods also test the
type of the list.
assertRaises Ensure that a specific function call raises a specific exception.

• Let’s look at the assertRaises method in a bit more detail.
• This method can be used to ensure a specific function call
raises a specific exception. The test passes if the code inside
the with statement raises the proper exception; otherwise, it
fails.

• Let’s look at an example:

import unittest
def MyAverage(seq):
return sum(seq) / len(seq)
# END average
Continued 

class TestAverage(unittest.TestCase):
def test_zero(self):
self.assertRaises(ZeroDivisionError, MyAverage, [])
# END test_zero
def test_with_zero(self):
with self.assertRaises(ZeroDivisionError):
MyAverage([])
# END test_with_zero
# END CLASS TestAverage
Continued 
 Continued

# END test_zero
MyAverage([])
Continued 
 Continued
We can test if a call to
MyAverage gives an error is
a blank list is passed in

# END test_zero
MyAverage([])
Continued 
 Continued
We can test if a call to
MyAverage gives an error is
a blank list is passed in
The same test, but calling the
method directly, which will
return a divide-by-zero error,
so we need to use the with
statement to tidy up.

if __name__ == "__main__":
unittest.main()
# ENDIF
 Continued

• Now let’s look at a more detailed example.
• Let’s look at a program, and it’s test program:
Stats.py Stats-test.py
class StatsList class TestValidInputs
def mean
def median
def mode
def test_mean
def test_median
def test_mode

• Here’s stats.py:
from collections import defaultdict
class StatsList(list):
def mean(self):
return sum(self) / len(self)
# END mean
Continued 

Continued 
 Continued
def median(self):
if len(self) % 2:
return self[int(len(self) / 2)]
else:
idx = int(len(self) / 2)
return (self[idx] + self[idx-1]) / 2
# ENDIF
# END median

 Continued
def mode(self):
freqs = defaultdict(int)
for item in self:
freqs[item] += 1
mode_freq = max(freqs.values())
modes = []
for item, value in freqs.items():
if value == mode_freq:
modes.append(item)
# ENDIF
# ENDFOR
return modes
# END mode

• We are going to test this program by creating a new file with
our testing code in it.
• So we’ll import unittest, and use the TestCase class
from it, to create a setUp method to do initialization for each
test.
• The setUp method accepts no arguments, and allows us to do
arbitrary setup before each test is run.

• Here’s stats-test.py:
from stats import StatsList
import unittest
class TestValidInputs(unittest.TestCase):
def setUp(self):
self.stats = StatsList([1,2,2,3,3,4])
# END setUp
Continued 

import unittest
def setUp(self):
# END setUp
Continued 
Import the program we’ve just
created, and it’s main class.

import unittest
def setUp(self):
# END setUp
Continued 
Import unittest.

import unittest
def setUp(self):
# END setUp
Continued 
Import unittest.
Create the setup method, to
set up values to be tested.

Continued 
 Continued
def test_mean(self):
self.assertEqual(self.stats.mean(), 2.5)
# END test_mean

Continued 
 Continued
def test_median(self):
self.assertEqual(self.stats.median(), 2.5)
self.stats.append(4)
self.assertEqual(self.stats.median(), 3)
# END test_median

 Continued
def test_mode(self):
self.assertEqual(self.stats.mode(), [2,3])
self.stats.remove(2)
self.assertEqual(self.stats.mode(), [3])
# END test_mode

...
----------------------------------------------------------
OK

...
----------------------------------------------------------
OK
“…” means all three tests have passed

• The setUp method is never explicitly called inside any of the
three test_* methods, the test suite does the call.
• Also note that test_median alters the list, by adding a “4”
to it, yet when test_mode is called, the list has returned to
the values specified in setUp. This shows that setUp is called
individually before each test, to ensure the test class starts
with a clean slate. Tests can be executed in any order, and the
results of one test should not depend on any other tests.

• TestCase also offers a no-argument tearDown method, which
can be used for cleaning up after each and every test on the class
has run.
• This is useful, for example, if we are testing code that does file I/O,
our tests may create new files as a side effect of testing; the
tearDown method can remove these files and ensure the system
is in the same state it was before the tests ran.
• Test cases should never have side effects.

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