Q.Program 7-16: Create a user defined module basic_math that contains the following user defined functions:

1. To add two numbers and return their sum.
2. To subtract two numbers and return their difference.
3. To multiply two numbers and return their product.
4. To divide two numbers and return their quotient and print “Division by Zero” error if the denominator is zero.
5. Also add a docstring to describe the module. After creating module, import and execute functions.

Module Creation and Import

Imagine you're writing a long essay. You wouldn't write every single sentence in one giant, unbroken paragraph. You'd split it into sections: an introduction, a body, a conclusion. Each section has a clear purpose. If you later write another essay that needs a similar introduction, you'd copy-paste that section rather than rewrite it from scratch.

That's exactly what modules are in programming. A module is a separate file containing Python code — functions, classes, variables — that you can reuse in other programs. Instead of cramming everything into one massive script, you break your code into logical, self-contained pieces.

Why bother?

Three reasons. First, organisation. A 2000-line file is a nightmare to debug. Modules let you group related functionality: all math utilities in one file, all database operations in another. Second, reusability. Write a function once, import it into a hundred different projects. Third, namespace management. Each module has its own namespace, so you can have a function called calculate in two different modules without conflict.

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Creating a Module

Creating a module is trivial. Any .py file is a module. Its name is the filename without the .py extension.

Suppose you create a file called math_helpers.py:

# math_helpers.py

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

def multiply(a, b):
    return a * b

PI = 3.14159

That's it. You've just created a module called math_helpers. It contains two functions and a constant.

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Importing a Module

Now you want to use math_helpers in another file, say main.py. You have several ways to import it.

1. Import the entire module

import math_helpers

result = math_helpers.add(5, 3)
print(math_helpers.PI)

You access everything inside the module using dot notation: module_name.item_name. This is the cleanest approach because it keeps the module's namespace separate from your current namespace. You always know where add came from.

2. Import specific items

from math_helpers import add, PI

result = add(5, 3)   # No need for math_helpers.add
print(PI)

This pulls add and PI directly into your current namespace. Convenient, but dangerous — if you already have a variable called add in your program, it gets overwritten silently.

3. Import with an alias

import math_helpers as mh

result = mh.add(5, 3)

Useful when module names are long or when you want to avoid naming conflicts.

4. Import everything (avoid this)

from math_helpers import *

This dumps every public name from the module into your namespace. It's a recipe for disaster — you lose all trace of where names came from, and accidental overwrites become likely. Professional Python code rarely uses this.

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How Python Finds Modules

When you write import math_helpers, Python searches for math_helpers.py in a specific order:

1. The directory containing the script you're running
2. Each directory in the PYTHONPATH environment variable
3. The standard library directories
4. Site-packages directories (where third-party packages live)

This search path is stored in sys.path. You can inspect it:

import sys
print(sys.path)

If your module isn't in any of these locations, you get a ModuleNotFoundError.

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The __name__ Trick

Every module has a built-in attribute called __name__. When you run a file directly, __name__ is set to "__main__". When you import it, __name__ is set to the module's name.

This lets you write code that runs only when the file is executed directly, not when it's imported:

# math_helpers.py

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

if __name__ == "__main__":
    # This runs only when you run math_helpers.py directly
    print("Testing add:", add(2, 3))
``` …