2.1.8 Modules and Packages

Where This Section Fits

In this section, you’ll learn how to split code into multiple files and reuse libraries written by others. Modules, packages, import, and pip are the gateway to the Python ecosystem. Once you understand them, you’ll be able to use tools like NumPy, Pandas, FastAPI, PyTorch, and more naturally.

Learning Objectives

• Understand the concepts of modules and packages
• Master different uses of import
• Learn about commonly used Python standard libraries
• Learn how to use pip to install third-party libraries
• Be able to create and use your own modules

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What Is a Module?

So far, all of your code has been written in a single file. But when a project grows, one file can easily become thousands of lines long — and that becomes hard to manage.

A module is a .py file. You can put related functions, classes, and variables into a module, then import and use them in other files.

Think about moving house:

• Put clothes in one box (clothes.py)
• Put books in one box (books.py)
• Put kitchenware in one box (kitchen.py)

Each box is a module. You open the one you need.

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Basic Uses of import

Import an Entire Module

import math

# You need to use the module name as a prefix
print(math.pi)          # 3.141592653589793
print(math.sqrt(16))    # 4.0
print(math.ceil(3.2))   # 4 (round up)
print(math.floor(3.8))  # 3 (round down)

Import Specific Items from a Module

from math import pi, sqrt

# Use directly without the module name prefix
print(pi)          # 3.141592653589793
print(sqrt(16))    # 4.0

Import with an Alias

import numpy as np            # give numpy a short alias
import pandas as pd           # standard alias for pandas

# Common aliases in the AI field
import matplotlib.pyplot as plt
import tensorflow as tf
import torch

Import Everything from a Module

from math import *

# You can use everything directly
print(pi)
print(sqrt(16))
print(sin(0))

Not Recommended: from xxx import *

Although it looks convenient, it imports all names from the module into the current file, which can cause name conflicts (for example, two modules having functions with the same name). It also makes your code harder to read, because others cannot easily tell which module a function came from.

Recommended approaches:

1. import math and then use math.sqrt() (most explicit)
2. from math import sqrt, pi (import only what you need)

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Common Python Standard Libraries

Python comes with many useful modules built in. After installing Python, you can use them right away without any extra installation.

math — Mathematical Operations

import math

print(math.pi)          # 3.141592653589793
print(math.e)           # 2.718281828459045
print(math.sqrt(144))   # 12.0
print(math.pow(2, 10))  # 1024.0
print(math.log(100, 10))  # 2.0 (logarithm with base 10)
print(math.sin(math.pi / 2))  # 1.0
print(math.factorial(5))  # 120 (5! = 5×4×3×2×1)

random — Random Numbers

import random

# Random integers
print(random.randint(1, 100))     # random integer between 1 and 100

# Random floating-point numbers
print(random.random())            # random float between 0 and 1
print(random.uniform(1.0, 10.0))  # between 1.0 and 10.0

# Randomly choose from a list
colors = ["red", "green", "blue", "yellow"]
print(random.choice(colors))       # choose one at random
print(random.sample(colors, 2))    # choose 2 at random (no duplicates)

# Shuffle a list
cards = list(range(1, 14))
random.shuffle(cards)
print(cards)  # shuffled list

# Set a random seed (to make results reproducible — commonly used in AI training)
random.seed(42)
print(random.randint(1, 100))  # same result every time you run it

os — Operating System Interaction

import os

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

# List files in a directory
print(os.listdir("."))

# Check whether a file/directory exists
print(os.path.exists("hello.py"))

# Join paths (cross-platform)
path = os.path.join("data", "train", "images")
print(path)  # data/train/images (macOS/Linux) or data\train\images (Windows)

# Get file name and extension
filename = "model_v2.pth"
name, ext = os.path.splitext(filename)
print(f"File name: {name}, extension: {ext}")  # File name: model_v2, extension: .pth

# Create directories
os.makedirs("output/results", exist_ok=True)  # exist_ok=True means no error if it already exists

datetime — Date and Time

from datetime import datetime, timedelta

# Get the current time
now = datetime.now()
print(now)                           # 2026-02-09 14:30:45.123456
print(now.strftime("%Y-%m-%d"))      # 2026-02-09
print(now.strftime("%Y-%m-%d"))      # 2026-02-09

# Time calculations
tomorrow = now + timedelta(days=1)
last_week = now - timedelta(weeks=1)
print(f"Tomorrow: {tomorrow.strftime('%Y-%m-%d')}")
print(f"Last week: {last_week.strftime('%Y-%m-%d')}")

# Parse a time string
date_str = "2026-01-15"
date = datetime.strptime(date_str, "%Y-%m-%d")
print(date)

json — JSON Data Handling

import json

# Python object → JSON string
data = {
    "name": "Xiao Ming",
    "age": 20,
    "scores": [85, 92, 78],
    "is_student": True
}

json_str = json.dumps(data, ensure_ascii=False, indent=2)
print(json_str)

# JSON string → Python object
parsed = json.loads(json_str)
print(parsed["name"])  # Xiao Ming

# Read and write JSON files
with open("data.json", "w", encoding="utf-8") as f:
    json.dump(data, f, ensure_ascii=False, indent=2)

with open("data.json", "r", encoding="utf-8") as f:
    loaded = json.load(f)
    print(loaded)

Standard Library Quick Reference

Module	Purpose	Common Functions
math	Mathematical operations	sqrt, pi, sin, log
random	Random numbers	randint, choice, shuffle
os	Operating system	getcwd, listdir, path.join
datetime	Date and time	now, strftime, timedelta
json	JSON handling	dumps, loads, dump, load
re	Regular expressions	search, findall, sub
collections	Advanced containers	Counter, defaultdict
pathlib	Path operations	Path, glob, mkdir
sys	System parameters	argv, path, exit
time	Time-related utilities	sleep, time

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Installing Third-Party Libraries

Python’s power comes largely from third-party libraries — modules written by others that you can install and use directly.

Install with pip

# Install a single library
pip install requests

# Install a specific version
pip install requests==2.28.0

# Install multiple libraries
pip install numpy pandas matplotlib

# Upgrade an installed library
pip install --upgrade requests

# Uninstall
pip uninstall requests

# List installed libraries
pip list

# Export all installed libraries (helpful for others to reproduce your environment)
pip freeze > requirements.txt

# Install in batch from a file
pip install -r requirements.txt

Common Third-Party Libraries for AI Development

Library	Install Command	Purpose
NumPy	pip install numpy	Foundation for numerical computing
Pandas	pip install pandas	Data analysis and processing
Matplotlib	pip install matplotlib	Data visualization
Requests	pip install requests	Network requests
scikit-learn	pip install scikit-learn	Traditional machine learning
PyTorch	pip install torch	Deep learning framework
Transformers	pip install transformers	Hugging Face pretrained models
FastAPI	pip install fastapi	Web API framework

conda vs pip

In Chapter 1, "Developer Tools Basics," you installed conda. Simple rule:

• conda: manage Python environments and install complex scientific computing libraries
• pip: install most Python packages

Usually, you first use conda to create and manage environments, and then use pip inside the environment to install the libraries you need.

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Creating Your Own Modules

A Basic Module

Create a file called my_math.py:

# my_math.py

PI = 3.14159

def circle_area(radius):
    """Calculate the area of a circle"""
    return PI * radius ** 2

def circle_perimeter(radius):
    """Calculate the circumference of a circle"""
    return 2 * PI * radius

def rectangle_area(width, height):
    """Calculate the area of a rectangle"""
    return width * height

Use it in another file:

# main.py
import my_math

print(my_math.circle_area(5))       # 78.53975
print(my_math.circle_perimeter(5))  # 31.4159

# Or:
from my_math import circle_area, PI
print(f"Circle area: {circle_area(3)}")
print(f"PI = {PI}")

What __name__ Does

You may have seen this mysterious pattern in other people’s code:

if __name__ == "__main__":
    print("This file is being run directly.")

What does it mean?

# my_math.py

def circle_area(radius):
    return 3.14159 * radius ** 2

# This code only runs when my_math.py is executed directly
# It will not run when imported by another file
if __name__ == "__main__":
    # test code
    print("Testing circle_area:")
    print(circle_area(5))  # 78.53975
    print("Test passed!")

# Run my_math.py directly → __name__ is "__main__", so the test code runs
python my_math.py
# Output:
# Testing circle_area:
# 78.53975
# Test passed!

# Import my_math in main.py → __name__ is "my_math", so the test code does not run

This is a clever Python design: a file can be both importable and directly runnable.

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Packages

When you have many modules, you can organize them into a package — a folder that contains an __init__.py file.

my_project/
├── main.py
└── utils/               ← this is a package
    ├── __init__.py      ← this file tells Python that utils is a package
    ├── math_utils.py
    ├── string_utils.py
    └── file_utils.py

Usage:

# main.py
from utils.math_utils import circle_area
from utils.string_utils import clean_text
from utils import file_utils

area = circle_area(5)
text = clean_text("  Hello  ")
file_utils.save_data(data, "output.json")

__init__.py can be an empty file, or it can define default behavior when the package is imported:

# utils/__init__.py
from .math_utils import circle_area, rectangle_area
from .string_utils import clean_text

# This lets users import directly from the package
# from utils import circle_area

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Comprehensive Example: A Personal Utility Library

Create a module that contains several useful functions:

# tools.py — my personal utility library

import random
import string
from datetime import datetime

def generate_id(length=8):
    """Generate a random ID"""
    chars = string.ascii_letters + string.digits
    return ''.join(random.choice(chars) for _ in range(length))

def timestamp():
    """Get the current timestamp string"""
    return datetime.now().strftime("%Y%m%d_%H%M%S")

def format_number(num):
    """Format a large number with thousands separators"""
    return f"{num:,.0f}"

def flatten_list(nested):
    """Flatten a nested list"""
    result = []
    for item in nested:
        if isinstance(item, list):
            result.extend(flatten_list(item))
        else:
            result.append(item)
    return result

def timer(func):
    """A simple timing decorator"""
    import time
    def wrapper(*args, **kwargs):
        start = time.time()
        result = func(*args, **kwargs)
        end = time.time()
        print(f"{func.__name__} took {end - start:.4f} seconds")
        return result
    return wrapper


if __name__ == "__main__":
    # test
    print(f"Random ID: {generate_id()}")
    print(f"Timestamp: {timestamp()}")
    print(f"Formatted: {format_number(1234567890)}")
    print(f"Flattened: {flatten_list([1, [2, 3], [4, [5, 6]]])}")

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Hands-On Practice

Exercise 1: Explore the Standard Library

Use math, random, and datetime to complete the following tasks:

# 1. Calculate how many digits are in 100!
# Hint: math.factorial() and len(str(...))

# 2. Generate 10 unique random numbers from 1 to 100
# Hint: random.sample()

# 3. Calculate how many days remain from today until January 1, 2027
# Hint: datetime

Exercise 2: Create Your Own Module

Create a string_tools.py module containing the following functions:

def count_words(text):
    """Count the number of words in an English text"""
    return len(text.split())

def reverse_words(text):
    """Reverse the order of words, not letters"""
    # "hello world" → "world hello"
    return " ".join(reversed(text.split()))

def is_palindrome(text):
    """Determine whether the text is a palindrome (ignore spaces and case)"""
    # "A man a plan a canal Panama" → True
    normalized = "".join(text.lower().split())
    return normalized == normalized[::-1]

Then import and test it in another file.

Exercise 3: Practice pip Operations

Run the following commands in the terminal:

# 1. Install the requests library
pip install requests

# 2. Write a simple script to test requests
python -c "import requests; print(requests.get('https://httpbin.org/get').status_code)"

# 3. Check which libraries are installed in the current environment
pip list

# 4. Export the dependency list
pip freeze > requirements.txt

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Summary

Concept	Description	Example
Module	A .py file	import math
Package	A folder containing __init__.py	from utils import helper
import	Import an entire module	import os
from...import	Import specific items	from math import pi
as	Create an alias	import numpy as np
pip	Install third-party libraries	pip install requests
__name__	Check whether a file is run directly	if __name__ == "__main__":

Core Idea

The module system lets you stand on the shoulders of giants. Python is powerful not because the language itself is overly complex, but because there are hundreds of thousands of modules — from data analysis and machine learning to web development and image processing — that cover almost any function you can imagine. Learning how to find and use these modules is a core skill for Python developers.