Why Learn Assert Statements For Unit Tests?
Knowing how to write assert statements in Python allows you to easily write mini-tests for your code.
Additionally testing frameworks such as PyTest can work directly with assert statements to form fully functioning UnitTests.
Table of Contents
You can also find a list of all of the different standard UnitTest Module examples here.
Firstly let’s review all of the different types of assert statements that we can make for PyTest.
PyTest Python Assert Statements List
# Module Imports
from types import *
import pandas as pd
import numpy as np
from collections.abc import IterableNB: Whenever you see # Success Example, this means that the assert test will succeed. However when you see # Fail Example, this means that the assert test will fail.
1. Equal to or not equal to [value]
assert 5 == 5 # Success Example
assert 5 == 3 # Fail Example
---------------------------------------------------------------------------
AssertionError Traceback (most recent call last)
<ipython-input-106-db6ee5a4bb16> in <module>
----> 1 assert 5 == 3 # Fail Example
AssertionError: assert 5 != 3 # Success Example
assert 5 != 5 # Fail Example
---------------------------------------------------------------------------
AssertionError Traceback (most recent call last)
<ipython-input-108-de24f583bfdf> in <module>
----> 1 assert 5 != 5 # Fail Example
AssertionError: 2. type() is [value]
assert type(5) is int # Success Example
assert type(5) is not int # Fail Example
---------------------------------------------------------------------------
AssertionError Traceback (most recent call last)
<ipython-input-110-e4cc0467bcd9> in <module>
----> 1 assert type(5) is not int # Fail Example
AssertionError: 3. isinstance
assert isinstance('5', str) # Success Example
assert isinstance('5', int) # Fail Example
---------------------------------------------------------------------------
AssertionError Traceback (most recent call last)
<ipython-input-112-28175282ec92> in <module>
----> 1 assert isinstance('5', int) # Fail Example
AssertionError: assert not isinstance('5', int) # Success Example
assert not isinstance('5', str) # Fail Example
---------------------------------------------------------------------------
AssertionError Traceback (most recent call last)
<ipython-input-116-1c215fe3a6a5> in <module>
----> 1 assert not isinstance('5', str) # Fail Example
AssertionError: 4. Boolean is [Boolean Type]
true = 5==5
assert true is True # Success Example
assert true is False # Fail Example
---------------------------------------------------------------------------
AssertionError Traceback (most recent call last)
<ipython-input-118-45032f0eff77> in <module>
----> 1 assert true is False # Fail Example
AssertionError: 5. in and not in [iterable]
list_one=[1,3,5,6]
assert 5 in list_one # Success Example
assert 5 not in list_one # Fail Example
---------------------------------------------------------------------------
AssertionError Traceback (most recent call last)
<ipython-input-121-90e8a0f2ef02> in <module>
----> 1 assert 5 not in list_one # Fail Example
AssertionError: 6. Greater than or less than [value]
assert 5 > 4 # Success Example
assert 5 > 7 # Fail Example
---------------------------------------------------------------------------
AssertionError Traceback (most recent call last)
<ipython-input-123-3068a8105e75> in <module>
----> 1 assert 5 > 7 # Fail Example
AssertionError:
assert 2 < 4 # Success Example
assert 4 < 2 # Fail Example
---------------------------------------------------------------------------
AssertionError Traceback (most recent call last)
<ipython-input-125-089b0fa99ac6> in <module>
----> 1 assert 4 < 2 # Fail Example
AssertionError: 7. Modulus % is equal to [value]
assert 2 % 2 == 0 # Success Example
assert 2 % 2 == 1 # Fail Example
---------------------------------------------------------------------------
AssertionError Traceback (most recent call last)
<ipython-input-127-2c429e622b13> in <module>
----> 1 assert 2 % 2 == 1 # Fail Example
AssertionError: 8. any() assert statements
example = [5,3,1,6,6]
booleans = [False, False,True, False]>>> any(example)
True
>>> any(booleans)
TrueNotice that the example list python list is True because at least one of the numbers is not a 0, all numbers above 0 are ‘Truthy’.
assert any(example) == True # Success Example
assert any(booleans) == True # Success Example9. all() assert statements
>>> all(example)
True
>>> all(booleans)
Falseassert all(example) # Success Example
assert all(booleans) # Fail Example
---------------------------------------------------------------------------
AssertionError Traceback (most recent call last)
<ipython-input-135-c422689f500e> in <module>
----> 1 assert all(booleans) # Fail Example
AssertionError: 10. Custom Python Objects
It’s possible to identify if a class is a specific type of object. We can do this by using:
type(object).__name__df = pd.DataFrame()
>>> type(df).__name__
'DataFrame'
type(df).__name__ == 'DataFrame' # True Boolean
type(df).__name__ is 'DataFrame' # True Boolean
type(df).__name__ == type([]).__name__ # False Boolean
type(df).__name__ is type([]).__name__ # False Booleanassert(type(df).__name__ == 'DataFrame') # Success Example
assert(type(df).__name__ == type([]).__name__) # Fail Example
---------------------------------------------------------------------------
AssertionError Traceback (most recent call last)
<ipython-input-147-2332f54f50a3> in <module>
----> 1 assert(type(df).__name__ == type([]).__name__) # Fail Example
AssertionError: 11. Iterables
It’s also possible to determine whether a variable is an iterable with:
from collections.abc import Iterable
iterable_item = [3,6,4,2,1]
>>> isinstance(iterable_item, Iterable)
True
>>> isinstance(5, Iterable)
Falseassert isinstance(iterable_item, Iterable) # Success Example
assert isinstance(3, Iterable) # Fail Example
---------------------------------------------------------------------------
AssertionError Traceback (most recent call last)
<ipython-input-153-e96805891245> in <module>
----> 1 assert isinstance(3, Iterable) # Fail Example
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Combining Multiple And / Or Statements With Assert Statements:
It’s also possible to combine multiple conditions with either OR or AND and to test the chained commands with the assert statement:
true_statement = 5 == 5 and 10 == 10
false_statement = 5 == 3 and 10 == 2
>>> print(true_statement, false_statement)
True Falseassert true_statement # Success Example
assert false_statement # Fail Example
---------------------------------------------------------------------------
AssertionError Traceback (most recent call last)
<ipython-input-157-452ef20f327f> in <module>
----> 1 assert false_statement # Fail Example
AssertionError: true_or_statement = 5 == 5 or 3 == 3
false_or_statement = 7 == 3 or 10 == 1
>>> print(true_or_statement, false_or_statement)
True Falseassert true_or_statement # Success Example
assert false_or_statement # Fail Example
---------------------------------------------------------------------------
AssertionError Traceback (most recent call last)
<ipython-input-161-38343a099bdc> in <module>
----> 1 assert false_or_statement # Fail Example
AssertionError: Testing Multiple Commands
Also we can test more than one thing at a time by having multiple assert statements inside of the same Python method:
class Test(object):
def __init__(self, first_name, last_name ):
self.first_name = first_name
self.last_name = last_name
def test_all_class_arguments(self):
print('Testing both of the class variables to see whether they are both strings!')
for _ in [self.first_name, self.last_name]:
assert(type(_) is str)
print('------')
print('Passed all of the tests')yay = Test('James' , 'Phoenix') # Success Example
yay.test_all_class_arguments()
Testing both of the class variables to see whether they are both strings!
------
Passed all of the testsyay = Test(5 , 'Phoenix') # Fail Example
yay.test_all_class_arguments()
Testing both of the class variables to see whether they are both strings!
---------------------------------------------------------------------------
AssertionError Traceback (most recent call last)
<ipython-input-164-64cb2bee07e3> in <module>
1 yay = Test(5 , 'Phoenix') # Fail Example
----> 2 yay.test_all_class_arguments()
<ipython-input-162-3ae9548ef4b7> in test_all_class_arguments(self)
8
9 for _ in [self.first_name, self.last_name]:
---> 10 assert(type(_) is str)
11 print('------')
12 print('Passed all of the tests')
AssertionError: Python 3.x UnitTest Assert Methods
Below you’ll find a list of all of the UnitTest Assert Methods:
| Method | Checks | Version |
|---|---|---|
| assertEqual | a == b | 3.x |
| assertNotEqual | a != b | 3.x |
| assertTrue | bool(x) is True | 3.x |
| assertFalse | bool(x) is False | 3.x |
| assertIs | a is b | 3.x |
| assertIsNot | a is not b | 3.x |
| assertIsNone | x is None | 3.x |
| assertIsNotNone | x is not None | 3.x |
| assertIn | a in b | 3.x |
| assertNotIn | a not in b | 3.x |
| assertIsInstance | is instance(a,b) | 3.x |
| assertNotIsInstance | not is instance(a,b) | 3.x |
| assertRaises | fun(*args,**kwds) raises exc | 3.x |
| assertRaisesRegexp | fun(*args,**kwds) raises exc(regex) | 3.x |
| assertAlmostEqual | round(a-b,7) == 0 | 3.x |
| assertNotAlmostEqual | round(a-b,7) != 0 | 3.x |
| assertGreater | a > b | 3.x |
| assertGreaterEqual | a >= b | 3.x |
| assertLess | a < b | 3.x |
| assertLessEqual | a <= b | 3.x |
| assertRegexpMatches | r.search(s) | 3.x |
| assertNotRegexpMatches | not r.search(s) | 3.x |
| assertItemsEqual | sorted(a) == sorted(b) | 3.x |
| assertDictContainsSubset | all the key/value pairs in a exist in b | 3.x |
| assertMultiLineEqual | strings | 3.x |
| assertSequenceEqual | sequences | 3.x |
| assertListEqual | lists | 3.x |
| assertTupleEqual | tuples | 3.x |
| assertSetEqual | sets or frozensets | 3.x |
| assertDictEqual | dicts | 3.x |
Typing Assert Statements
As well as using simple assert statements, by importing the types module of python we can make more abstract assert statements on specific Types:
class Example():
def __init__(self, id_, name):
self._id = id_
self.name = name
def subtract(self):
answer = 5 + 5
return answer
def test_lambda_function(self):
assert(lambda x: x is LambdaType)
def test_subtract_function(self):
assert(self.subtract is LambdaType)example_class = Example("123", 'James Phoenix')
>>> print(example_class._id, example_class.name)
123 James Phoenixexample_class.test_lambda_function() # Success Example
example_class.test_subtract_function() # Fail Example
---------------------------------------------------------------------------
AssertionError Traceback (most recent call last)
<ipython-input-169-e96c76763824> in <module>
----> 1 example_class.test_subtract_function() # Successs
<ipython-input-165-28a62a6c7adf> in test_subtract_function(self)
12
13 def test_subtract_function(self):
---> 14 assert(self.subtract is LambdaType)
AssertionError: Above we’ve tested two class instance methods to see if either of them is a lambda: x style function!
You can find all of the different types here.
Hopefully this provides you with an extensive list of assert statements and methods for your Unit Tests.
Please let me know if you have any more examples that you’d like me to add them to this post 😉
