Python Assert Statement and AssertionError Explained with Example

Have you heard about ‘assert’ in Python? Or you may have got ‘AssertionError’ while executing any project code.

In this Python tutorial, you will learn Python assert statement with examples and how to use it in your code.

Let’s start with the basic first.

What is Assert Statement in Python?

The assert statement is used in Python to verify the condition.

• If the condition is False, it terminates the program by throwing an exception.
• If the condition is True, it does not make any changes to the execution of your program.

It has a very simple syntax.

assert <condition>

You can use any condition statement which returns either True or False.

Condition in the Python code is executed first. Based on the result of condition, assert statement either don’t make any changes (just like nothing happened) or terminates the program.

Python Assert Statement Example

Let’s understand this concept with simple examples…

When the condition is True…

>>> assert True
>>>
>>>

If you run above the line of code on your Python console, nothing happens as the condition is True.

When the condition is False…

>>> assert False
Traceback (most recent call last):
File "<stdin>", line 1, in <module>
AssertionError
>>>

Try running the above statement on your Python console. For any False condition, it throws an AseertionError error.

You can capture AssertionError error as a part of exception handling in Python.

Python Assert Exception Message

To distinguish multiple assert failure messages, you can also customize the message.

Customizing the message to display for the False condition:

>>> assert False, "Assertion failed!"
Traceback (most recent call last):
File "<stdin>", line 1, in <module>
AssertionError: Assertion failed!
>>>

Many times this assert statement is very useful when you don’t want to execute your program for invalid conditions or parameter values.

Another Example:

Let’s take a real and practical programming example. You are asking the users to enter their birthday month, so the accepted values are 1 to 12.

Here you don’t want your program to continue running for an invalid (erroneous) month.

When the user provides a valid month number:

>>> month=int(input('birthday month: '))
birthday month: 11
>>> assert month in range(1,13)
>>>

When the user provides an invalid month number:

>>> month=int(input('birthday month: '))
birthday month: 14
>>> assert month in range(1,13)
Traceback (most recent call last):
File "<stdin>", line 1, in <module>
AssertionError
>>>

In the above code, we have used:

• taking user input from the keyboard
• range() function in Python

You can also disable assert statement globally.

How to disable Assert Statement Globally?

Assert statements can be used for debugging code and for the unit testing.

If your script already have assert statement and you don’t want to execute it, you can use -O command-line switch while starting Python.

Use the following command to open Python console.

python -O

This will disable the debugging mode of the Python.

Now, even if you provide False condition to the assert, it will not throw an error.

>python -O
Python 2.7.13 (v2.7.13:a06454b1afa1, Dec 17 2016, 20:42:59)
Type "help", "copyright", "credits" or "license" for more information.
>>>
>>> assert False
>>>
>>>

This way it will disable assert statements globally.

Use of Assert to Write Unit Test Cases in Python

The keyword assert is used for writing the unit test cases. When you call the actual function from the test case, it returns certain values.

To test if the actual function is working as expected, we usually compare the return value with the expected value.

Something like this…

assert <return_value_of_function> == <expected_return_value>

If the return value of the function does not match with the expected return value, the test case throws an exception. This means the test case has failed.

If the return value of the function is the same as the expected value, the test case passes without throwing any exception.

One test case can have multiple assert statements.

Great! With this Python assert tutorial, you have learned some of the interesting concepts in Python. If you have any doubts about Python assert statement, share it with me in the comment.

Happy Pythoning!