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What is assert in Python

Understanding Assert in Python

When you're learning to program, you'll quickly find that it's not just about writing code that does something. It's also about making sure that your code is doing the right thing. This is where the concept of testing comes in. In Python, one of the simplest forms of testing is using the assert statement. Let's dive into what assert is and how you can use it to make your code more robust.

The Role of Assert

Imagine you're building a tower with blocks. You have a picture of how the tower should look when you're finished. As you place each block, you check it against the picture. If a block doesn't fit the way it should, you know something is wrong. The assert statement in Python works in a similar way. It's like a checkpoint for your code. You tell Python what you expect at a certain point in your program. If what you expect is true, the program keeps running. If it's not, the program stops and tells you something went wrong.

How Assert Works

An assert statement has two parts: a condition and an optional message. The condition is something you expect to be true if your program is working correctly. The message is a string that can be displayed if the condition is not true. Here's a simple example:

x = 10
assert x == 10, "x should be 10"

In this case, x is indeed 10, so the assert passes and nothing happens — the program continues to run. But what if x was 9?

x = 9
assert x == 10, "x should be 10"

This would cause the assert to fail, stopping the program with an AssertionError and printing the message "x should be 10".

Using Assert for Debugging

As a beginner, you might wonder why stopping the program is a good thing. The answer lies in the process of debugging, which is like detective work for programmers. You want to catch mistakes as early as possible. By using assert, you can catch when a variable isn't what you expect or when a function doesn't return the right value.

Here's an example using a function:

def add_positive_numbers(a, b):
    assert a > 0 and b > 0, "Both numbers must be positive"
    return a + b

result = add_positive_numbers(5, -3)

In this example, we're trying to add two numbers, but our function expects both numbers to be positive. The second number is negative, so the assert will fail, and the program will raise an AssertionError with the message "Both numbers must be positive".

Assert Is Not a Substitute for Actual Testing

It's important to note that assert is not a complete testing solution. It's a tool that can help you catch errors and inconsistencies while you're writing and debugging your code. For comprehensive testing, you would use Python's built-in unittest framework or third-party tools like pytest, which allow for more extensive and complex tests.

When to Use Assert

You should use assert when you want to:

  • Ensure that internal conditions in your code are as expected.
  • Validate that input values are within an acceptable range.
  • Check that the output of a function is correct for given inputs during development.

Remember, assert statements can be removed from a compiled version of a Python program if optimization is requested. This means they are not a mechanism for handling runtime errors or validating data in a production environment.

Common Misconceptions About Assert

Some beginners confuse assert with exceptions. While both can stop your program and both are related to handling unexpected situations, they serve different purposes. Exceptions are meant for handling errors that can occur during normal operation of a program, while assert is meant for catching programmer errors during development.

Analogies to Help Understand Assert

Think of assert as the buddy system when swimming. Before you dive into the water, your buddy checks that you're ready. If you're not ready, you don't go in. This prevents problems before they happen. In programming, assert does the same by checking if your code is ready to move on to the next step.

Practical Examples

Let's look at some practical examples of using assert in Python:

# Example 1: Checking the length of a list
animals = ['dog', 'cat', 'rabbit']
assert len(animals) == 3, "There should be 3 animals in the list"

# Example 2: Verifying a function's output
def square(number):
    return number * number

assert square(4) == 16, "The square of 4 should be 16"

In both examples, the assert statements help ensure that the program's state is as expected. If the list had a different number of animals or if the square function didn't return the correct value, the assert would catch these issues.


In your journey as a budding programmer, assert is like a trusty flashlight in a dark forest. It helps illuminate the path, ensuring you don't stumble over hidden roots — or in programming terms, unexpected bugs. By incorporating assert statements into your code, you're taking proactive steps to verify that your code behaves as intended. Remember, while assert is a great tool for early-stage testing and debugging, it's not a one-stop-shop for all your testing needs. As you grow more comfortable with programming, you'll learn to complement assert with more comprehensive testing strategies, shining even more light on the path to becoming a proficient Python programmer.