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Python Assert Example | Assert Statement In Python


Python assert statements are boolean expressions to check if the condition is True.  Assertions are the statements that state the fact confidently in your program. An assertion is the sanity-check that you can turn on or turn off when you are done with your testing of the program.

Let’s take a simple example of writing the division function; you’re confident that the divisor shouldn’t be zero, you assert divisor is not equal to zero. We will see this example later in this post.

What is Assertion

Content Overview

Assertions are statements that assert or state a case confidently in your program.

Assertions in Python are boolean expressions check if the conditions return true or not. 

If it is true, the program does nothing and continues to execute the next line of code. 

However, if it’s False, then the program stops and throws an error.

Python’s Assert Syntax

The following syntax is the basic structure of python assert statements.

assert condition

If you wish to write a message when the condition is false, then its syntax is following.

assert condition, message

You can also send information like a message with the assert statement for a better understanding of the fault of the code.

Python Assert Example

If you want to simulate or debug your code like what is happening in which stage, then you can use the python assert statements in your code.

It is the debugging tool as it brings a program on halt as soon as any error is occurred and shows on which point of the program that error has occurred.

Assertions are the conditions or boolean expressions that are always supposed to be true in the code.

The assert statement takes an expression and an optional message.

An assert statement is used to check types, values of the argument, and the output of the function.

The assert statement is used as the debugging tool as it halts the program at the point where an error occurs.

See the following program of assertions in Python.


def avg(ranks):
    assert len(ranks) != 0
    return round(sum(ranks)/len(ranks), 2)

ranks = [62, 65, 75]
print("Average of mark1:",avg(ranks))

In the above example, we do not want to pass the empty ranks, and if it is empty, then it will throw an Assertion Error. See the below output.

Python Assert Statement Tutorial

In the above example, we will get an average because we have passed the list of items.

Now, let’s pass the empty list and see the output.


def avg(ranks):
    assert len(ranks) != 0
    return round(sum(ranks)/len(ranks), 2)

ranks = []
print("Average of mark1:",avg(ranks))

Now, in the above example, we have not passed the ranks items. So the length will be 0, and we will get an Assertion Error.

Python Assert Statement Tutorial With Example

If the assertion fails, Python uses ArgumentExpression as the argument for the AssertionError.

AssertionError exceptions can be caught and handled like any other exception using the try-except statement, but if not handled properly, they will terminate the program and produce the traceback.

But, in the above example, it does not throw an error with a proper message. You can write your custom error message as well. Let’s do that.


def avg(ranks):
    assert len(ranks) != 0, 'Ranks list should not be empty'
    return round(sum(ranks)/len(ranks), 2)

ranks = []
print("Average of ranks:",avg(ranks))

In the above example, we have passed the second argument to the assert statement, which is the message. So in the output, we will see the message as well.

Assert Statement In Python

Python Assert Keyword with assert with error message

Let’s see another example which is divide by 0.


def divide(x, y):
    assert y != 0 , 'Divisor cannot be zero'
    return round(x/y, 2)

z = divide(21,3)

a = divide(21,0)

In the above example, if the divisor is 0, then assert will throw an error with a message. Let’s see the output.

More Examples on Python Assert Statement

In the third line of the above code, we can see the assert statement.

In this line, it is checked whether the variable y value is equal to 0 or not. 

If it is greater than zero, i.e., the condition is true, then no problem occurs, and we will get the output accordingly.

But when we called the method division() with the 2nd argument 0, then the assert condition is false. 

That is why an AssertionError occurs, and it throws an error with a message ‘Divisor cannot be zero,’ that we wrote in the message part of the python assert statement.

Python Assert Methods

Method Checks that New in
assertEqual(x, y) x == y 
assertNotEqual(x, y) x != y 
assertTrue(x) bool(x) is True
assertFalse(x) bool(x) is False
assertIs(x, y) x isy  3.1
assertIsNot(x, y) x is noty  3.1
assertIsNone(x) x is None 3.1
assertIsNotNone(x) x is not None 3.1
assertIn(x, y) x iny  3.1
assertNotIn(x, y) x not 3.1
assertIsInstance(x, y) isinstance(x, y) 3.2
assertNotIsInstance(x,y) not isinstance(x, y) 3.2


How do you test that a Python function throws an exception

We can use TestCase.assertRaises (or TestCase.failUnlessRaises) from the unit test module, for example.


import unittest

def broken_function():
    raise Exception('This is broken')

class MyTestCase(unittest.TestCase):
    def test(self):
        with self.assertRaises(Exception) as context:

        self.assertTrue('This is broken' in str(context.exception))

if __name__ == '__main__':

See the output.

➜  pyt python3
Ran 1 test in 0.000s

➜  pyt

Common Pitfalls With Using Asserts in Python

There are two important cautions with using assertions in Python that I’d like to shout out.

  1. The first one has to do with introducing security risks and bugs into your applications. Don’t Use Asserts for Data Validation
  2. The second one is about a syntax quirk that makes it easy to write useless assertions. Asserts That Never Fail.

Key Points of Assert In Python

  1. Assertions are the condition or boolean expression which are always supposed to be true in the code.
  2. The assert statement takes an expression and an optional message.
  3. The assert statement is used to check types, values of the argument, and the output of the function.
  4. The assert statement is used as a debugging tool as it halts the program at the point where an error occurs.
  5. The proper use of assertions is to inform developers about unrecoverable errors in a program. They’re not intended to signal expected the error conditions, like “file not found”, where the user can take corrective action or just try again.
  6. Assertions are internal self-checks for your program. They work by declaring some conditions as impossible in your code. If one of these conditions doesn’t meet that means there’s a bug in your program.

Finally, Python Assert Statement Tutorial With Example is over.

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