assert.md

Python assert keyword

The assert keyword in Python is used to set a checkpoint in your code that tests
a condition. If the condition evaluates to True, the program continues to
execute normally. If the condition evaluates to False, an AssertionError
exception is raised, halting the program and typically indicating a bug in the
code.

assert condition, message

• condition: This is the condition you want to test. If it evaluates to True,
  the program continues. If it evaluates to False, an AssertionError is
  raised.
• message (optional): An optional message to provide more information about the
  assertion error.

$ py -O main.py

Assertions can be turned off with the -O flag.

Example Usage

Basic Example

x = 10
assert x > 5, "x should be greater than 5"
print("Assertion passed")

In this example, since x is indeed greater than 5, the assertion passes and
"Assertion passed" is printed.

Example with Failing Condition

y = 2
assert y > 5, "y should be greater than 5"
print("This will not be printed")

Here, since y is not greater than 5, the assertion fails and raises an
AssertionError with the message "y should be greater than 5".

Example in a Function

def divide(a, b):
    assert b != 0, "The divisor 'b' must not be zero"
    return a / b

print(divide(10, 2))  # This works
print(divide(10, 0))  # This raises an AssertionError

In this example, the assert keyword ensures that the divisor is not zero
before performing the division. If b is zero, an AssertionError is raised with
the message "The divisor 'b' must not be zero".

Parameter types

We can use the assert statement to check the type of the parameters at the
beginning of the function.

def my_function(a, b):
    assert isinstance(a, int), "Parameter 'a' must be an integer"
    assert isinstance(b, str), "Parameter 'b' must be a string"
    # Function logic here
    print(f"a: {a}, b: {b}")

# Example usage
my_function(5, "hello")  # This works
my_function(5, 10)  # This raises AssertionError: Parameter 'b' must be a string

Example in Unit Tests

Assertions are often used in unit tests to validate the behavior of functions:

def test_sum():
    result = sum([1, 2, 3])
    assert result == 6, f"Expected 6 but got {result}"

test_sum()

In this unit test example, the assert keyword is used to ensure that the sum
function returns the expected result. If the result is not as expected, an
AssertionError is raised with a descriptive message.

Summary

The assert keyword is a handy tool for testing and debugging your code,
ensuring that certain conditions hold true. It provides a simple yet powerful
way to catch bugs early in the development process.

Unit tests

pip install pytest

The algo.py module:

def max(values):

  _max = values[0]

  for val in values:
      if val > _max:
          _max = val

  return _max


def min(values):

  _min = values[0]

  for val in values:
      if val < _min:
          _min = val

  return _min

The min_max_test.py:

import algo

def test_min():
    values = (2, 3, 1, 4, 6)

    val = algo.min(values)
    assert val == 1

def test_max():
    values = (2, 3, 1, 4, 6)

    val = algo.max(values)
    assert val == 6