Python assert Keyword

Python `assert` Keyword

The assert keyword in Python is used for debugging purposes. It allows programmers to check if a condition is True and raise an AssertionError if it is not. This helps identify logical errors in the program during development.

Syntax

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assert condition, optional_message

Parameters

Parameter	Type	Description
`condition`	Boolean expression	A condition that should evaluate to `True`. If `False`, an `AssertionError` is raised.
`optional_message`	str, optional	An optional error message displayed if the assertion fails.

Return Value

The assert statement does not return anything. If the condition is True, the program continues execution normally. If the condition is False, an AssertionError is raised, which stops the program unless handled.

Examples

1. Basic Assertion Example

In this example, we use an assert statement to check if a number is positive.

We define a variable num and use assert to ensure that num is greater than zero. If the condition is met, the program continues without interruption. If num is zero or negative, an AssertionError is raised.

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# Define a number
num = 5

# Assert that num is positive
assert num > 0

# If assertion passes, this will be executed
print("The number is positive.")

Output:

The number is positive.

Since num is 5, the assertion passes, and the program executes normally.

2. Assertion with a Custom Error Message

We can add an error message to make debugging easier.

Here, we assert that age is greater than or equal to 18. If the condition fails, the message “Age must be 18 or above” is displayed along with the error.

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# Define an age
age = 16

# Assert that age is at least 18
assert age >= 18, "Age must be 18 or above"

print("Access granted.")

Output:

Traceback (most recent call last):
  File "/Users/tutorialkart/main.py", line 5, in <module>
    assert age >= 18, "Age must be 18 or above"
           ^^^^^^^^^
AssertionError: Age must be 18 or above

Since age is 16, which is less than 18, the assertion fails, and the error message is printed.

3. Using Assertions in a Function

Assertions are useful in functions to validate inputs.

Here, we define a function calculate_square_root that computes the square root of a number. We use assert to ensure the input is non-negative before performing the calculation.

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import math

def calculate_square_root(num):
    # Ensure the number is not negative
    assert num >= 0, "Number must be non-negative"
    
    return math.sqrt(num)

# Test with valid input
print(calculate_square_root(25))

# Test with invalid input
print(calculate_square_root(-4))

Output:

5.0
Traceback (most recent call last):
  File "/Users/tutorialkart/main.py", line 13, in <module>
    print(calculate_square_root(-4))
          ~~~~~~~~~~~~~~~~~~~~~^^^^
  File "/Users/tutorialkart/main.py", line 5, in calculate_square_root
    assert num >= 0, "Number must be non-negative"
           ^^^^^^^^
AssertionError: Number must be non-negative

When passing 25, the function returns 5.0. However, when passing -4, the assertion fails, raising an AssertionError.

4. Disabling Assertions in Production Code

In Python, assertions can be disabled in optimized mode by running the script with the -O flag.

Here, we define an assertion to check if value is positive. However, when running Python in optimized mode, assertions are ignored, and the program executes normally.

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# Define a value
value = -5

# Assert that value is positive
assert value > 0, "Value must be positive"

print("Program completed successfully.")

Running Normally:

Traceback (most recent call last):
  File "/Users/tutorialkart/main.py", line 5, in <module>
    assert value > 0, "Value must be positive"
           ^^^^^^^^^
AssertionError: Value must be positive

Running with `-O` (Optimized Mode):

python -O script.py
Program completed successfully.

When running in optimized mode using python -O script.py, the assertion is skipped, and no error is raised.

TutorialKart

Python assert Keyword