Python Modules and Packages

# Python Modules and Packages

Welcome to Chapter 15! As programs grow, you need to organize code into reusable pieces. Modules and packages let you split code across files and reuse functionality efficiently.

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1. Learning Objectives #

• Import and use built-in modules.

• Create custom modules.

• Understand packages and _init.py.

```python id="py15ex1" # Import entire module import math print(math.sqrt(16)) # 4.0 print(math.pi) # 3.14159...

# Import specific items from math import sqrt, pi print(sqrt(25)) # 5.0

# Import with alias import datetime as dt print(dt.datetime.now())

# Import everything (avoid in production!) from math import * print(ceil(4.2)) # 5

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## 3. Useful Built-in Modules

python id="py15ex2" # os — operating system interface import os print(os.getcwd()) print(os.listdir("."))

# json import json data = {"name": "Alice", "age": 25} jsonstr = json.dumps(data, indent=2) print(json_str)

# collections from collections import Counter words = "hello world hello python hello".split() print(Counter(words)) # Counter({'hello': 3, 'world': 1, 'python': 1})

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## 4. Creating Custom Modules

Create a file `mymath.py`:

python id="py15ex3" # mymath.py PI = 3.14159

def circlearea(radius): return PI * radius ** 2

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

Use it in another file:

python id="py15ex4" # main.py import mymath

print(mymath.circlearea(5)) # 78.53975 print(mymath.rectangle_area(4, 6)) # 24 print(mymath.PI) # 3.14159

### The `__name__` Variable

python id="py15ex5" # mymath.py def add(a, b): return a + b

if name == "main_": # This only runs when file is executed directly # Not when imported as a module print("Testing: add(2, 3) =", add(2, 3))

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## 5. Packages

A package is a **directory** containing modules and an `__init__.py` file:

mypackage/ ├── init.py ├── mathutils.py ├── stringutils.py └── data/ ├── init_.py └── loader.py

python id="py15ex6" # Using a package from mypackage import mathutils from mypackage.data import loader

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## 6. pip — Package Manager

bash # Install a package pip install requests

# Install specific version pip install requests==2.31.0

# Upgrade pip install --upgrade requests

# List installed pip list

# Save dependencies pip freeze > requirements.txt

# Install from requirements pip install -r requirements.txt

# Uninstall pip uninstall requests

### Popular pip Packages

| Package | Purpose |
|---------|---------|
| requests | HTTP requests |
| flask | Web framework |
| django | Full web framework |
| pandas | Data analysis |
| numpy | Numerical computing |
| beautifulsoup4 | Web scraping |
| pytest | Testing |

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## 7. Virtual Environments

Virtual environments isolate project dependencies:

bash # Create virtual environment python -m venv myenv

# Activate (Windows) myenv\Scripts\activate

# Activate (macOS/Linux) source myenv/bin/activate

# Install packages (inside venv) pip install requests flask

# Deactivate deactivate

Without Virtual Environments: ┌────────────────────────────┐ │ System Python │ │ Project A: requests 2.28 │ │ Project B: requests 2.31 │ ← CONFLICT! └────────────────────────────┘

With Virtual Environments: ┌──────────────┐ ┌──────────────┐ │ Project A │ │ Project B │ │ venva/ │ │ venvb/ │ │ req. 2.28 │ │ req. 2.31 │ └──────────────┘ └──────────────┘ ← NO CONFLICT! ``

1. 2. Naming files same as modules: random.py shadows built-in random.

1. 4. Forgetting _init.py: Required for Python <3.3 packages.

Q3: name == "main" is True when: A) Module is imported B) File is run directly C) Always D) Never Answer: B

Q7: init.py makes a directory a: A) Module B) Package C) Script D) Library Answer: B

Q10: Which avoids running code on import? A) if name == "main": B) if import: C) if module: D) if file: Answer: A

1. 1. What is name in Python? A special variable that is "main" when the script runs directly, or the module name when imported.

1. 2. Module vs Package? A module is a single .py file. A package is a directory of modules with init.py.

• Packages are directories with init.py containing multiple modules.

• Use if name == "main_":` to guard executable code.

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12. Next Chapter Recommendation #

In Chapter 16: File Handling in Python, you'll learn to read, write, and manage files — a critical skill for real-world programming! 🚀