Python’s assert: Debug and Test Your Code Like a Pro


Python’s assert statement allows you to write sanity checks in your code. These checks are known as assertions, and you can use them to test if certain assumptions remain true while you’re developing your code. If any of your assertions turn false, then you have a bug in your code.

Assertions are a convenient tool for documenting, debugging, and testing code during development. Once you’ve debugged and tested your code with the help of assertions, then you can turn them off to optimize the code for production. Assertions will help you make your code more efficient, robust, and reliable.

In this tutorial, you’ll learn:

What assertions are and when to use them
How Python’s assert statement works
How assert can help you document, debug, and test your code
How assertions can be disabled to improve performance in production
What common pitfalls you might face when using assert statements

To get the most out of this tutorial, you should have previous knowledge of expressions and operators, functions, conditional statements, and exceptions. Having a basic understanding of documenting, debugging, and testing Python code is also a plus.

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Getting to Know Assertions in Python

Python implements a feature called assertions that’s pretty useful during the development of your applications and projects. You’ll find this feature in several other languages too, such as C and Java, and it comes in handy for documenting, debugging, and testing your code.

If you’re looking for a tool to strengthen your debugging and testing process, then assertions are for you. In this section, you’ll learn the basics of assertions, including what they are, what they’re good for, and when you shouldn’t use them in your code.

What Are Assertions?

In Python, assertions are statements that you can use to set sanity checks during the development process. Assertions allow you to test the correctness of your code by checking if some specific conditions remain true, which can come in handy while you’re debugging code.

The assertion condition should always be true unless you have a bug in your program. If the condition turns out to be false, then the assertion raises an exception and terminates the execution of your program.

With assertions, you can set checks to make sure that invariants within your code stay invariant. By doing so, you can check assumptions like preconditions and postconditions. For example, you can test conditions along the lines of This argument is not None or This return value is a string. These kinds of checks can help you catch errors as soon as possible when you’re developing a program.

What Are Assertions Good For?

Assertions are mainly for debugging. They’ll help you ensure that you don’t introduce new bugs while adding features and fixing other bugs in your code. However, they can have other interesting use cases within your development process. These use cases include documenting and testing your code.

The primary role of assertions is to trigger the alarms when a bug appears in a program. In this context, assertions mean Make sure that this condition remains true. Otherwise, throw an error.

In practice, you can use assertions to check preconditions and postconditions in your programs at development time. For example, programmers often place assertions at the beginning of functions to check if the input is valid (preconditions). Programmers also place assertions before functions’ return values to check if the output is valid (postconditions).

Assertions make it clear that you want to check if a given condition is and remains true. In Python, they can also include an optional message to unambiguously describe the error or problem at hand. That’s why they’re also an efficient tool for documenting code. In this context, their main advantage is their ability to take concrete action instead of being passive, as comments and docstrings are.

Finally, assertions are also ideal for writing test cases in your code. You can write concise and to-the-point test cases because assertions provide a quick way to check if a given condition is met or not, which defines if the test passes or not.

You’ll learn more about these common use cases of assertions later in this tutorial. Now you’ll learn the basics of when you shouldn’t use assertions.

When Not to Use Assertions?

In general, you shouldn’t use assertions for data processing or data validation, because you can disable assertions in your production code, which ends up removing all your assertion-based processing and validation code. Using assertions for data processing and validation is a common pitfall, as you’ll learn in Understanding Common Pitfalls of assert later in this tutorial.

Additionally, assertions aren’t an error-handling tool. The ultimate purpose of assertions isn’t to handle errors in production but to notify you during development so that you can fix them. In this regard, you shouldn’t write code that catches assertion errors using a regular try … except statement.

Understanding Python’s assert Statements

Now you know what assertions are, what they’re good for, and when you shouldn’t use them in your code. It’s time to learn the basics of writing your own assertions. First, note that Python implements assertions as a statement with the assert keyword rather than as a function. This behavior can be a common source of confusion and issues, as you’ll learn later in this tutorial.

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