Modules and Packages in Python

🟡 Intermediate

📖 Definition

  • Module: A single Python source file (.py) containing functions, classes, and variables that can be re-used across other files.
  • Package: A directory containing multiple related Python module files and an __init__.py file (optional in modern Python 3.3+, but standard best practice).

🇮🇳 Hindi

Large programs ko simple aur clean rakhne ke liye code ko multiple .py files mein divide kiya jata hai. Code import karne ke liye import module_name ya from module import function ka use hota hai. File ko directly run kar rahe hain ya import karke use kar rahe hain—yeh check karne ke liye if __name__ == "__main__": idiom ka use hota hai.

🚩 Marathi

Code la alag .py files madhye organize karnyasathi Modules vaparatat. Import karnyasathi import keyword vaparatat.

📝 1. Import Statements & Aliasing

# 1. Importing complete module
import math
print(math.sqrt(16)) # 4.0

# 2. Importing specific functions with aliasing
from math import pi as PI_VAL, pow as power
print(PI_VAL) # 3.141592653589793

# 3. Importing custom module from package directory
from my_package.utils import format_currency

📝 2. The if __name__ == "__main__": Idiom

When Python runs a .py file:

  • If the file is executed directly from terminal, Python sets the hidden __name__ variable to "__main__".
  • If the file is imported as a module into another script, Python sets __name__ to the file’s actual module name (e.g. "utils").

💡 Why It Matters: Wrapping execution code inside if __name__ == "__main__": prevents testing code from executing automatically whenever another script imports your file!

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

# Runs ONLY when executed directly, NOT when imported!
if __name__ == "__main__":
    print("Testing add function:", add(10, 5))

📝 3. Essential Standard Library Modules

Python comes with a rich Standard Library (“Batteries Included”):

  • math: Trigonometry, logarithms, roots, constants (pi, e).
  • random: Random numbers (randint, choice, shuffle).
  • sys: System arguments (sys.argv), exit (sys.exit()), Python path.
  • os: Operating system files, directory creation, environment variables.
  • time / datetime: Time tracking and performance benchmarking.

💡 Complete Example: Custom Module & Package Architecture

File 1: calculator.py

# Module: calculator.py
import math

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

def square_root(x):
    if x < 0:
        raise ValueError("Cannot calculate square root of negative number.")
    return math.sqrt(x)

# Executable entry point check
if __name__ == "__main__":
    print("=== RUNNING CALCULATOR DIRECT MODULE TESTS ===")
    print("Add Test        :", add(10, 20))
    print("Square Root Test:", square_root(25))

File 2: main_app.py

# Main Application Script
import calculator as calc

print("=== MAIN APPLICATION LAUNCH ===")

val1 = 15
val2 = 25
sum_result = calc.add(val1, val2)
sqrt_result = calc.square_root(sum_result)

print(f"Sum of {val1} + {val2} = {sum_result}")
print(f"Square Root of {sum_result} = {sqrt_result:.2f}")

👀 Output

Running python3 main_app.py:

=== MAIN APPLICATION LAUNCH ===
Sum of 15 + 25 = 40
Square Root of 40 = 6.32

(Notice how the === RUNNING CALCULATOR DIRECT MODULE TESTS === block from calculator.py did NOT execute because of if __name__ == "__main__":!)

⚠️ Common Mistakes

  • Circular Imports: Module A imports Module B, and Module B imports Module A simultaneously, resulting in ImportError: cannot import name.
  • Naming custom Python files the exact same name as standard library modules (e.g., naming your test file math.py or random.py), which shadows the built-in library!

🛡️ Safety / Important Notes

Avoid using from module import * because it pollutes your current namespace with unknown variable names and increases the risk of accidental variable shadowing bugs.

🌍 Real-World Usage

All professional software architectures organize code into separate domain packages (models/, views/, controllers/, services/, utils/).

🧪 Try It Yourself

  1. Use Python’s built-in random module to pick a random item from a list ["Apple", "Banana", "Cherry"] using random.choice().
  2. Use math.pi to calculate the area of a circle with radius 7.

🎯 Mini Challenge

Create a custom module file string_utils.py containing a function reverse_and_capitalize(text). Import and use it in a separate app.py file.

🔗 Related Topics

🧭 Navigation

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