Module 3 Lesson 9: Python Standard Library

When people say Python is "Batteries Included," they mean that it comes pre-installed with a massive library of modules ready to use. You don't need to download anything extra to perform complex math, handle dates, or even download files from the internet. In this lesson, we will tour the most useful parts of the Python Standard Library.

Lesson Overview

In this lesson, we will cover:

• datetime: Working with time, dates, and time zones.
• os and pathlib: Navigating your computer’s file system.
• statistics: Performing basic data analysis.
• json: Reading and writing the web's most popular data format.
• shutil: Copying, moving, and deleting files.

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1. Dating with datetime

Dealing with leap years, time zones, and formats is a nightmare. The datetime module makes it easy.

from datetime import datetime

now = datetime.now()
print(f"Today is: {now.strftime('%B %d, %Y')}")

# Calculate how many days until your birthday
birthday = datetime(2025, 12, 31)
countdown = birthday - now
print(f"Days left: {countdown.days}")

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2. Working with your OS (os and pathlib)

Need to know what's in a folder or move a file? Python handles this safely across Windows, Mac, and Linux.

import os

# List all files in the current folder
print(os.listdir("."))

# Get the path to your current working directory
print(os.getcwd())

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3. Math and Statistics

Beyond the math module, the statistics module is perfect for quick data insights.

import statistics

grades = [88, 92, 75, 95, 80]
print(f"Mean (Average): {statistics.mean(grades)}")
print(f"Median: {statistics.median(grades)}")

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4. The json Module

JSON is the language of the web. If you want to save a dictionary into a file that other apps can read, you use JSON.

import json

user = {"name": "Alex", "id": 12345}
# Convert dictionary to a JSON string
user_json = json.dumps(user)
print(user_json) # Output: {"name": "Alex", "id": 12345}

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Practice Exercise: The File & Folder Tool

Create a file named sys_tool.py.

1. Import os, datetime, and statistics.
2. Print the names of all files in your current directory.
3. Print the current time in the format: HH:MM:SS.
4. Create a list of 10 random numbers and print their mean and standard deviation. (Hint: use random.randint for the numbers).

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Quick Knowledge Check

1. What does the phrase "Batteries Included" mean in Python?
2. Which module would you use to find the current date?
3. Why is the statistics module useful for beginners?
4. What type of data format is the json module used for?

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Key Takeaways

• The Standard Library is a huge collection of built-in modules.
• datetime is the go-to for time-based calculations.
• os allows your Python script to interact with your computer.
• statistics and json are essential for data-driven applications.

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What’s Next?

We’ve learned everything we need to build complex systems. But as you add more functions and modules, your code can get messy. In our final lesson of this module, Lesson 10, we’ll learn Refactoring Code—the art of cleaning up your work!