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Table of Contents
What is assert?
When to use assert?
Notes and usage suggestions
Let's summarize
Home Backend Development Python Tutorial Explain Python assertions.

Explain Python assertions.

Jul 07, 2025 am 12:14 AM
python

Assert is an assertion tool used in Python for debugging, and throws an AssertionError when the condition is not met. Its syntax is assert condition plus optional error information, which is suitable for internal logic verification such as parameter checking, status confirmation, etc., but cannot be used for security or user input checking, and should be used in conjunction with clear prompt information. It is only available for auxiliary debugging in the development stage rather than substituting exception handling.

Explain Python assertions.

Python's assert statement is actually a very practical but often overlooked gadget. It is mainly used in the debugging phase to help developers quickly discover situations that "should not happen" in the program. Simply put, when a certain condition is not met, the program directly reports an error instead of continuing to run, causing more difficult problems to check.

Explain Python assertions.

What is assert?

assert is a built-in keyword in Python to check whether an expression is true. If it is false (False), an AssertionError exception is thrown. The basic syntax is:

Explain Python assertions.
 assert condition, error message (optional)

For example, you want to make sure that a variable is a positive number:

 x = -5
assert x > 0, "x must be greater than 0"

At this time, the program will report an error and display the prompt message you wrote. This is especially useful in the development and testing phases.

Explain Python assertions.

When to use assert?

assert is not used to handle user input errors or external exceptions, it is more suitable for internal logical verification . for example:

  • Function parameters should meet certain prerequisites
  • After a certain piece of code is executed, the intermediate result should be a certain state
  • Some branches in the program flow will not be executed theoretically

For example: If you write a function that only accepts string-type parameters, you can add a sentence at the beginning:

 def process_name(name):
    assert isinstance(name, str), "name must be a string"
    # Follow-up operations

In this way, when others pass on the wrong type, they can immediately discover the problem.


Notes and usage suggestions

  • Don't use assert to do safety checks
    Because Python can disable all assert statements with the -O parameter at runtime, you should not put sensitive logic (such as permission judgment) in assert.

  • The error message should be as clear as possible
    Adding appropriate prompts can help you locate problems faster. For example:

     assert len(data) == expected_length, f "The data length should be {expected_length}, and it is actually {len(data)}"
  • Used to assist in development, not to replace exception handling
    Formal error handling mechanisms (such as try-except) are the standard way to deal with runtime errors. assert is more like a "self-check" and is suitable for use during the development stage.


  • Let's summarize

    Assert is essentially an assertion tool that helps you confirm that "it must be right here." Although it seems simple, it is very useful when debugging complex logic or multi-person collaboration. Just remember not to treat it as a formal error handling method.

    Basically that's all, it's not complicated to use, but it's easy to ignore its value.

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