Modules in Python allow you to organize code into separate files, making it reusable and maintainable. A module is simply a Python file (.py) that contains functions, classes, and variables that can be used in other Python programs.

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1. Types of Modules in Python

Python provides three types of modules:
Built-in Modules – Pre-installed with Python (e.g., math, random, os)
User-Defined Modules – Custom modules created by the user
External Modules – Third-party modules installed via pip (e.g., numpy, requests)

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2. Importing Modules

Python provides different ways to import modules:

2.1 Importing the Entire Module

import math

print(math.sqrt(25))  # Output: 5.0
print(math.pi)        # Output: 3.141592653589793

Access functions using module_name.function_name()

2.2 Importing Specific Functions from a Module

from math import sqrt, pi

print(sqrt(25))  # Output: 5.0
print(pi)        # Output: 3.141592653589793

No need to use math. prefix when calling functions

2.3 Importing a Module with an Alias

import numpy as np

arr = np.array([1, 2, 3])
print(arr)  # Output: [1 2 3]

Shortens the module name for convenience

2.4 Importing All Functions from a Module (*)

from math import *

print(sin(0))  # Output: 0.0
print(log(10)) # Output: 2.302585092994046

Not recommended – Can cause conflicts with other function names.

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3. Creating and Importing User-Defined Modules

You can create your own module and import it into another script.

3.1 Creating a Custom Module (mymodule.py)

# mymodule.py
def greet(name):
    return f"Hello, {name}!"

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

3.2 Importing and Using the Module

import mymodule

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

3.3 Importing Specific Functions

from mymodule import greet

print(greet("Bob"))  # Output: Hello, Bob!

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4. Using dir() to List Module Contents

The dir() function lists all functions and variables in a module.

import math
print(dir(math))

🔍 Output (Partial List)

['acos', 'asin', 'atan', 'ceil', 'cos', 'degrees', 'e', 'exp', 'factorial', 'floor', 'log', 'pi', 'pow', 'radians', 'sin', 'sqrt', 'tan']

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5. Importing External Modules

External modules are installed using pip (Python package manager).

5.1 Installing an External Module

Example: Installing requests module

pip install requests

5.2 Using an External Module

import requests

response = requests.get("https://api.github.com")
print(response.status_code)  # Output: 200

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6. Understanding __name__ and __main__

In Python, every module has a built-in variable __name__.

• If a module is run directly, __name__ is set to "__main__".
• If a module is imported, __name__ is set to the module’s filename.

6.1 Example (mymodule.py)

# mymodule.py
def greet(name):
    return f"Hello, {name}!"

if __name__ == "__main__":
    print(greet("Alice"))  # Runs only when executed directly

6.2 Running mymodule.py Directly

$ python mymodule.py
Hello, Alice!

6.3 Importing mymodule.py into Another Script

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

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

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7. Reloading a Module (importlib)

If you modify a module and want to reload it without restarting Python, use importlib.reload().

import mymodule
import importlib

importlib.reload(mymodule)

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8. Finding Module Location

To find where a module is installed, use:

import os
print(os.__file__)  # Output: /usr/lib/python3.10/os.py

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9. Summary

Import Type	Syntax	Example
Import entire module	import module_name	import math
Import specific function(s)	from module import func	from math import sqrt
Import with alias	import module as alias	import numpy as np
Import all functions (not recommended)	from module import *	from math import *
Reload a module	importlib.reload(module)	importlib.reload(mymodule)