Chapter 9: Introduction to Python Modules

1. Introduction

When your program grows big, writing everything in a single file becomes messy. Modules help you organize code into separate files — neat, reusable, and easy to manage.

Example: greet.py Module

# greet.py - A simple greeting module
def say_hello(name):
    return f"Hello, {name}!"

def say_goodbye(name):
    return f"Goodbye, {name}!"

# Usage in main program:
# import greet
# print(greet.say_hello("Alice"))

2. Library or Package

A library is a collection of ready-made code — functions, classes, and modules that you can use directly in your program.

import math        # math is a standard library
print(math.sqrt(25))

import numpy as np  # importing the numpy package
print(np.array([1,2,3]))

Library vs Package Comparison

3. Modules in Python

A module is just a Python file (.py) containing variables, functions, or classes. Python already has hundreds of modules built-in — you do not need to write everything yourself!

Module Contents

Advantages of Python Modules

4. Module Name in Python

In Python, a module name is simply the name of the file without the .py extension.

Example 1 - Built-in module

import math
print(math.sqrt(25))

Module name: math (built-in module)

Example 2 - User-defined module

# Inside mytools.py
def greet():
    print("Hello from mytools module!")

# To use it in another program:
import mytools
mytools.greet()

Module name: mytools (filename without .py)

5. Importing Python Modules

To use a module, it must be imported into the main program or another module. Any Python file can be used as a module by importing it.

1. Basic Import (Most Common)

import module_name
# Gives access to all variables, constants, and functions in the module

import math
print(math.sqrt(16))  # Using dot notation

Note: The import statement does not give direct access to functions. Use dot notation: module_name.function_name()

2. Import Multiple Modules

import module1, module2, module3

3. From Import (Specific Functions)

from <module_name> import <function_name(s)>
# Or
from <module_name> import function1, function2, function3

from math import sqrt
print(sqrt(16))  # No need for math. prefix

4. Import All (Not Recommended)

from <module_name> import *
# Imports all names from module into current namespace

5. Member Aliasing

You can rename modules or functions while importing:

import math as m
print(m.pi)

from math import sqrt as s
print(s(25))

import area
dir(area)
# Output: Shows built-in attributes (__doc__, __name__, etc.)
# Also lists user-defined functions like circle_area, square_area, etc.

6. Locating Modules

Python looks for modules in a certain search path:

1. Current folder
2. Installed packages folder
3. System path for Python libraries

7. Standard Built-in Python Modules

Python comes with many ready-made modules for common tasks:

import math
print(math.sqrt(25))      # 5.0
print(math.pow(2, 3))     # 8.0
print(math.factorial(5))  # 120
print(math.pi)            # 3.14159...
print(math.ceil(4.3))     # 5
print(math.floor(4.7))    # 4

import random
print(random.random())        # 0.0 to 1.0
print(random.randint(1, 10))  # 1 to 10
print(random.choice([1,2,3])) # Random choice
random.shuffle([1,2,3,4])     # Shuffle list

import datetime
now = datetime.datetime.now()
print(now)                    # Current date/time
print(now.year)               # Current year
print(now.strftime("%Y-%m-%d")) # Format date

import os
print(os.getcwd())           # Current directory
os.chdir('/path/to/dir')     # Change directory
print(os.listdir('.'))       # List contents
os.mkdir('new_folder')       # Create directory
print(os.path.exists('file.txt'))  # Check file

import sys
print(sys.version)           # Python version
print(sys.path)              # Module search path
sys.exit()                   # Exit program

8. User-Defined Modules Example

You can create your own modules:

Step 1: Create mytools.py

# save this as mytools.py
def greet():
    print("Hello from mytools!")

def calculate_area(length, width):
    return length * width

PI = 3.14159

Step 2: Use in main program

# main program
import mytools

mytools.greet()
area = mytools.calculate_area(5, 3)
print(f"Area: {area}")
print(f"PI value: {mytools.PI}")

Key Points to Remember

Important Terms