Python Assert Statement

Assert statement in Python is used for debugging purposes. It is used to check whether a given expression is true or false.

Syntax

The syntax for assert statement in Python is:

assert Expression[, Arguments]

Here, Expression is the condition which we want to check. It can be any expression or variable.

Arguments represent the optional value that is passed along with the expression.

Example

x = 2
assert x == 2, "Value of x is not 2"

Output

If the above assertion is true, the program will run without any output. However, if the assertion is false, the interpreter will raise an AssertionError and the following output will be displayed:

Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
AssertionError: Value of x is not 2

Explanation

In the above example, we have defined a variable x and used assert statement to check its value. The assertion is that the value of x should be equal to 2. If the assertion is true, the program will run without any output. However, if the assertion is false, an error message will be displayed.

Assert statements are usually used during testing to make sure that the program is running correctly. They can be used to check the input/output values, correct function behavior and program logic.

Use

The main use cases for assert statements in Python are:

• Debugging: Assert statements are used as debugging tools for checking the program logic, input/output values, etc. They help in identifying errors and fixing them easily.
• Testing: Assert statements are used extensively in testing frameworks for writing test cases. They help in automating the process of checking the expected behavior of the program.

Important Points

• Assert statements are not meant to replace error handling. They are used for debugging and testing purposes only.
• If the assertion is false, the interpreter will raise an AssertionError exception.
• The assertion message is optional. If no message is provided, the default message will be used.
• Assertions can be disabled globally with the -O (optimize) switch when running Python programs.

Summary

The assert statement in Python is used for debugging and testing purposes. It is used to check if a given expression is true or false and raise an AssertionError if the expression is false. It is mainly used as a debugging tool for identifying errors and fixing them easily.