A module in Python is a file containing functions, classes, and variables that can be used in other Python programs. Creating custom modules helps in code reuse, organization, and maintainability.

---

1. Why Use Custom Modules?

Code Reusability – Write once, use multiple times.
Better Code Organization – Split large programs into multiple files.
Easier Debugging – Isolate errors to specific modules.
Collaboration – Multiple developers can work on different modules.

---

2. Creating a Custom Module

A module is just a Python file (.py) containing functions, classes, and variables.

2.1 Creating mymodule.py

# mymodule.py

def greet(name):
    """Returns a greeting message."""
    return f"Hello, {name}!"

def add(a, b):
    """Returns the sum of two numbers."""
    return a + b

PI = 3.14159  # A constant variable

---

3. Importing a Custom Module

After creating mymodule.py, you can import and use it in another Python script.

3.1 Importing the Entire Module

import mymodule

print(mymodule.greet("Alice"))  # Output: Hello, Alice!
print(mymodule.add(5, 3))       # Output: 8
print(mymodule.PI)              # Output: 3.14159

Access functions using mymodule.function_name().

3.2 Importing Specific Functions

from mymodule import greet, add

print(greet("Bob"))  # Output: Hello, Bob!
print(add(10, 20))   # Output: 30

No need to use mymodule. prefix.

3.3 Importing with an Alias

import mymodule as mm

print(mm.greet("Charlie"))  # Output: Hello, Charlie!

mm is an alias for mymodule to shorten function calls.

---

4. Using if __name__ == "__main__"

This ensures that some parts of the module only run when the file is executed directly.

4.1 Modify mymodule.py

def greet(name):
    return f"Hello, {name}!"

if __name__ == "__main__":
    print(greet("Alice"))  # This runs only if executed directly

4.2 Running mymodule.py Directly

$ python mymodule.py
Hello, Alice!

4.3 Importing mymodule.py into Another Script

import mymodule
print(mymodule.greet("Bob"))

Only "Hello, Bob!" is printed because the if __name__ == "__main__": block is ignored when imported.

---

5. Storing Modules in a Package

A package is a directory containing multiple modules and a special __init__.py file.

5.1 Create a Folder (mypackage)

mypackage/
│── __init__.py   # Required to treat it as a package
│── math_utils.py
│── string_utils.py

5.2 math_utils.py

def add(a, b):
    return a + b

5.3 string_utils.py

def greet(name):
    return f"Hello, {name}!"

5.4 Importing from the Package

from mypackage import math_utils, string_utils

print(math_utils.add(10, 5))   # Output: 15
print(string_utils.greet("Eva"))  # Output: Hello, Eva!

Packages allow better structuring of multiple modules.

---

6. Reloading a Module (importlib)

If you modify a module, you may need to reload it without restarting Python.

import mymodule
import importlib

importlib.reload(mymodule)

---

7. Finding Module Location

Use the __file__ attribute to find where a module is stored.

import mymodule
print(mymodule.__file__)  # Output: /path/to/mymodule.py

---

8. Summary

Feature	Description
Creating a module	Save Python functions in a .py file
Importing a module	Use import module_name
Importing specific functions	Use from module import function
Using an alias	Use import module as alias
Checking if script is main	Use if __name__ == "__main__"
Creating a package	Use a folder with __init__.py