Assert statement

Testing exists in a great variety of programming languages, its aim is to check whether your code contains errors and if so, what their causes are. It does not have to be hard, you can start with easy steps. In this topic, we are going to deal with the assert statement. It is a useful debugging tool that uses Boolean logic and checks whether a given expression is true or false. If the condition is True, the program will keep running. Otherwise, it will return the AssertionError.

Syntax of assert statements

The assert keyword can be used in two ways.

# 1
assert <condition>

# 2
assert <condition>, <message>

The <condition> attribute shows what you want to test, so it is always required. When the condition is false, the AssertionError is raised. The optional <message> attribute may be used to specify the message displayed with the error. Now let's illustrate assert statements in action.

# 1
word = input("Enter a word: ")
assert word != "cat"
print("Your word is", word)

# 2
word = input("Enter a word: ")
message = "'cat' is the wrong word!"
assert word != "cat", message
print("Your word is", word)

For instance, if you enter the word dog in both examples, the condition word != 'cat' will be True, and you will have the following output:

# Your word is dog

However, if you happen to enter cat, the AssertionError will be raised. In the second case, an error message will also appear.

# 1
# Traceback (most recent call last):
#   File "main.py", line 2, in <module>
#     assert word != "cat"
# AssertionError

# 2
# Traceback (most recent call last):
#   File "main.py", line 3, in <module>
#     assert word != "cat", message
# AssertionError: 'cat' is the wrong word!

As you can see, the AssertionError is a built-in error, so you can handle it with the try-except block, i.e. print the message without stopping the program.

try:
    word = input("Enter a word: ")
    message = "'cat' is a wrong word!"
    assert word != "cat", message
    print("Your word is", word)
except AssertionError as err:
    print(err)  # if the word is 'cat', "'cat' is a wrong word!" is printed

We can also check several variables or use more sophisticated logical expressions with the help of the assert keyword:

x = 4
y = 2
assert (x ** 2 / y ** 2) - y != 2, "There are wrong values!"
print(x, y)
# Traceback (most recent call last):
#   File "main.py", line 3, in 
#     assert (x ** 2 / y ** 2) - y != 2, "There are wrong values!"
# AssertionError: There are wrong values!

Finally, the assert keyword can be used with functions. In the example below the AssertionError is raised when the parameter 2 given to the function does not fulfill the condition in the test_mark(i).

def test_mark(i):
    message = "This student got a bad mark!"
    assert i > 4, message
    return i


print(test_mark(5))  # 5
print(test_mark(2))
# Traceback (most recent call last):
#   File "main.py", line 7, in 
#     print(test_mark(i))
#   File "main.py", line 3, in test_mark
#     assert i > 4, message
# AssertionError: This student got a bad mark!

Assert vs Raise

You may have noticed that raise and assert are similar to each other. What is the actual difference between them?

• raise is used for raising an exception;

• assert is used for raising an exception if the given condition is False.

Let's analyze some examples.

word = input("Enter a word: ")
if word != "cat":
    print(word)
else:
    raise Exception("There is a wrong word!")

As you can see, the raise keyword together with the if-else statement is very similar to the assert keyword, but their purposes are different. In the first case, raise is used mainly for data validation, it allows you to customize exceptions and raise errors of your choice, while assert is mainly used for debugging, e. g., to stop the execution based on a certain condition and return a message that will help debug faster.

The Python documentation (the Assert Statement paragraph) also provides an explanation of how the assert keyword works. The first example shows raising the error without a message and the second one — with the message.

# 1
if __debug__:
   if not condition: 
       raise AssertionError

# 2
if __debug__:
   if not condition: 
       raise AssertionError(message)

The __debug__ is a built-in variable that is True by default and is only set to False when Python is started in the optimization mode, so the first lines can be interpreted as if True.

Mind the difference between the raise and the assert keywords and use them wisely.

Summary

In this topic, you have learned some basics of the assert keyword, a tool for program testing:

• the purpose of the assert keyword is to check whether a condition is False;

• to use the assert keyword, we should specify the mandatory attribute <condition> and an optional <message>;

• as opposed to raise, the assert keyword is used for debugging.

Now, let's proceed to the tasks.