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Ensuring Code Reliability with Python Assertions

Assertions in Python: Ensuring the Reliability of Your Code

Have you ever encountered situations wherein your code is behaving abnormally? Maybe it is because of bugs and errors?

Such scenarios are ubiquitous in programming. The real challenge for a programmer is to identify and eliminate these bugs efficiently.

This is where assertions come in handy. What are Assertions in Python?

In Python, assertions are statements that check for the fulfilment of specific conditions during the runtime. Essentially, when we implement assertions in our code, we are choosing to verify that our assumptions about the code are correct, and it is functioning correctly based on these assumptions.

These verification statements help us test our code and identify bugs.

Purpose of Using Assertions

Assertions are an excellent tool for testing and debugging your code. While coding, you make assumptions about the program’s behavior.

Using assertions, you can quickly check that these assumptions hold during runtime. A failed assertion immediately lets the programmer know that a particular assumption has not held up, becoming aware of the problem and making changes accordingly.

When Not to Use Assertions

While assertions are crucial in many situations, there are cases in which they ought not to be used. For example, if the program contains code in the else-clause, the assertion can’t check which of the condition(s) is false.

Therefore, it would be better to use the except-clause. Additionally, when developers switch to optimised mode, assertions skip the compilation and execution step.

This action may cause performance problems as well. Finally, they may expose security loopholes.

Not all errors are just problems to be solved. Sometimes mistakes can lead to security breaches through intentionally malicious code.

How to Implement Assertions in Python

Implementing assertions in Python is quite simple. First, you write an expression that should be true under normal circumstances, following it up with the assert keyword.

This keyword is then followed by the expression returned as either True or False. If the returned value is True, the code proceeds to execute unaffectedly.

But if the expression returns False, the code raises an AssertionError runtime exception. An example of implementing assertions is demonstrated below:

def calc_antilogarithm(num):
assert num >= 0, "The input must be zero or positive."
# Ensures positive input for the calculation of antilogarithm.
return 10**num

The AssertionError is triggered when you wrongly supply a negative number.

Usage of Assertions

Assertions are useful in various application scenarios, ranging from implementing user input validation to external factor validation.

Example 1: Calculation of Anti-Logarithm

Consider the below Python function to calculate anti-logarithm:

def calc_antilogarithm(num):
assert num >= 0, "The input must be zero or positive."
# Ensures positive input for the calculation of antilogarithm.
return 10**num

The assertion checks whether an input number is positive or zero. It further ensures the correct output by verifying the number’s positivity, as a negative number invalidates the calculation of the antilogarithm.

The assertion helps reduce bugs in this instance and ensure reliable function execution.

Example 2: Input Validation

For system administrators or developers who are developing login modules or user interfaces, input validation is one of the critical components.

It reduces security breaches that might occur due to input items having incorrect data types. For example, in the case of age validation, the code may contain a part like this:

if (age < 18): print("Hello {}, you're too young to be an administrator!".format(user)) else: print("Welcome, {}, you're an admin".format(user))

To ensure the right type of data is entered, add the assertion statement:

assert (type(age) == int or age.isnumeric()), "Age must be a number."

Keep in mind that the assertion only returns true or false.

Example 3: External Factor Validation

Lastly, assertions are useful in validating external factors. When running a program, it is essential to know the condition of the computer system you are running it on.

For example, your code might attempt to read a file, which might not be present. The code that checks that file could check for the existence of the file with the following assertion:

assert (os.path.isfile(file_name)), "File {} not found.".format(file_name)

This assertion checks whether a file exists when the given line of code is executed.

It terminates execution due to an AssertionError if the file is missing.


Assertions in Python are essential to ensure the reliability of your code, making them useful during the debugging and testing phases of programming. These statements ensure that the conditions set in the code runtime directly match the developer's assumptions and help to efficiently eliminate bugs within the code.

The application possibilities of assertions are diverse, ranging from input validation to external factor validation. However, they too can have some downsides due to harmful, intentional code or unrealistic requirements.

In sum, with a little skill, a programmer can take advantage of assertions to better their code's resiliency and minimize errors.

Writing Assertions in Python: Ensuring Code Quality with Basic Syntax and Examples

Assertions in Python are used to ensure code quality by verifying that certain conditions are fulfilled during runtime.

They serve as a powerful tool for testing and debugging code and can help to identify and eliminate bugs efficiently. In this section, we will explore writing assertions in Python in detail and discuss their importance in code development.

Basic Syntax

Writing assertions in Python is a straightforward process. We use the assert keyword followed by the expression that needs to be checked for True/False.

The general syntax of writing assertions is as follows:

assert ,

The "condition" is the expression that we are evaluating, and the "message" is the optional explanation that specifies the assertion's reason. If the condition is False, an AssertionError is raised with the specified message.

Example 1: Assertion with Input

Let us consider an example of writing an assertion with input. We will write a Python function to check whether an input number is odd or even using an assertion statement.

def odd_or_even(num):
assert (num % 2 == 0), "The input number is odd."
print(f"{num} is an even number.")

In this example, we apply the assertion statement to verify whether the input number is even or odd. If the input number is odd, the assertion gets triggered, and an assertion error is raised.

If your code passes an even number, the code will continue to execute. For instance, let's say we call the function with:


The output will be:

20 is an even number.

Example 2: Assertion with Input and Message

In some cases, an assertion's message can provide more information about the assertion than just a true or false value.

The message will help identify which specific assertion failed, the reason for the failure, and how to fix it. Consider the following example:

def divide(a, b):
assert (b != 0), "Cannot divide by zero."
return a / b

We introduce a second argument 'b' to this function.

The assertion statement checks if this argument equals 0 and raises an assertion error if it does, with the message - "Cannot divide by zero."

For instance, let's say we call the function with:


The output will be:

AssertionError: Cannot divide by zero.

Importance of Adding Message to Assertion

The message specified while writing an assertion is essential as it helps to pinpoint the issue when multiple expressions lead to assertion failure. Consider the following example:

def test(a,b):
assert (a > b), "The first argument should be greater than the second."
assert (a % 3 == 0), "The first argument should be divisible by 3."
assert (b % 2 == 0), "The second argument should be even."

In this example, we have multiple assertions for different expressions.

Suppose any of these assertions fail, we won't be able to identify the cause from an AssertionError message alone. Therefore, it is crucial to specify the reason in the message.

For instance, let's say we call the function with:


The output will be:

AssertionError: The first argument should be greater than the second.


In conclusion, writing assertions in Python is a powerful technique to ensure code reliability.

They can help identify and eliminate errors, debugging code efficiently, and notifying the programmer about any bugs in the code. The basic syntax of writing assertions is simple but can be applied in numerous cases based on coding requirements.

Assertions help to test code by making some assumptions, checking to see if those assumptions are correct, and automatically raising an error message if they are not. As we have seen in the examples above, providing explicit messages with the assertion statements helps to improve debugging during problematic assertions.

Overall, it is essential to write assertions with precision and accuracy to ensure optimal results. When used correctly, assert statements can be a powerful tool for catching bugs during the development phase, leading to high-quality, reliable code.

In conclusion, assertions in Python are essential for debugging and testing code and ensuring its reliability. Writing assert statements is a straightforward process that involves using the assert keyword, followed by the condition to be tested and an optional message for clarification.

The message is crucial, especially for multiple assertion statements, as it helps to pinpoint the problem when multiple assertions fail. By using assertions, developers can detect bugs in their code with ease, making it easier to fix them.

The importance of assertions cannot be overstated, and after reading this article, readers should have gained new knowledge on how to write assertions, and their significance in creating high-quality, dependable code.

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