Here's an example of a simple library that calculates the area and perimeter of a rectangle:

1. Create a new Python module named rectangle.py.

2. Write the following code inside rectangle.py:
   

def calculate_area(length, width):
    """Calculate the area of a rectangle."""
    return length * width

def calculate_perimeter(length, width):
    """Calculate the perimeter of a rectangle."""
    return 2 * (length + width)

1. Create another Python file named main.py to demonstrate how to use the library.

2. Write the following code inside main.py:
   

# Importing the library module
import rectangle

# Using the library functions
length = 5
width = 3

area = rectangle.calculate_area(length, width)
perimeter = rectangle.calculate_perimeter(length, width)

print("Rectangle Area:", area)
print("Rectangle Perimeter:", perimeter)

1. Save both rectangle.py and main.py in the same directory.

2. Run main.py using the Python interpreter:
   

$ python main.py

Output:

Rectangle Area: 15
Rectangle Perimeter: 16

In this example, we created a library module (rectangle.py) that defines two functions: calculate_area() and calculate_perimeter(). These functions perform calculations based on the given length and width of a rectangle.

In the main.py file, we imported the rectangle module and used its functions to calculate the area and perimeter of a rectangle with a length of 5 and width of 3. The results were then printed to the console.

By following this approach, you can create your own libraries/modules and import them into your code to reuse the defined functionality.