What is a library?

In Python, a library (often called a module or package) is a collection of ready-made code that you can reuse in your own programs. Instead of writing everything from scratch, you can "import" this existing code and use it directly.

Think of a library as a toolbox:

• Python itself is the workshop (the language and runtime).
• Your program is the project you're building.
• Libraries are toolboxes full of useful tools (functions, classes, constants).

When you use a library, you are saying: "Someone has already solved part of this problem. I will use their solution instead of reinventing it."

Why libraries exist

Programming problems repeat. For example:

• Working with dates and times
• Doing math beyond basic +, -, *, /
• Generating random numbers
• Reading and writing different file formats
• Talking to websites or APIs

Writing these features correctly and safely is hard. Libraries:

• Save time: you don't write everything yourself.
• Reduce bugs: widely used libraries are tested by many people.
• Make code clearer: instead of a long custom solution, you call a well-named function from a library.

How libraries fit into your code

You use libraries by importing them. Conceptually, this is:

1. Load the library into your program.
2. Use the functions, classes, or variables it provides.

In code, it looks like:

import math
result = math.sqrt(25)  # uses the sqrt function from the math library
print(result)

Here:

• math is the library (a module).
• sqrt is a function inside that library.
• math.sqrt(25) means: "call the sqrt function from the math library with argument 25."

You will learn the import statement and its variations in a later section; here, just understand that importing connects your program to a library.

Types of Python libraries

At a high level, there are two main kinds of libraries you will use:

1. Standard library
• Comes built-in with Python.
• You do not need to install anything extra.
• Always available once Python is installed.
• Examples (which you’ll explore later):
• math for mathematical functions
• random for random numbers
• datetime for working with dates and times
2. External (third-party) libraries
• Created by other people or organizations.
• You do need to install them (usually with pip).
• Add specialized features:
• Web development
• Data analysis
• Automation
• Machine learning, and more

You will learn how to install these with pip and how to choose good ones in later sections.

What is inside a library?

A typical Python library can contain:

• Functions – reusable pieces of code that do something specific.
• Example: math.sqrt(x) returns the square root of $x$.
• Classes – blueprints for creating objects with data and behavior.
• Constants and variables – useful predefined values.
• Example: math.pi for $\pi$.
• Submodules – smaller modules grouped inside a larger package.

You access these pieces using dot notation:

import math
print(math.pi)       # a constant
print(math.sqrt(9))  # a function

The pattern is:

$$
\text{library\_name}.\text{thing\_inside}
$$

Libraries vs your own code

You can think of code in three layers:

1. Python language
• The core rules: variables, loops, functions, etc.
2. Libraries
• Code built on top of the language that gives extra abilities.
3. Your program
• Uses the language + libraries to solve your specific problem.

Over time, you can even turn your own code into a small library that you reuse across multiple projects. That’s just code organized so others (or future you) can import and use it easily.

When should you use a library?

You usually reach for a library when:

• The task is common (dates, random numbers, math, file formats).
• The task is complex or low-level (network communication, encryption).
• Someone has already provided a well-tested solution.
• You want to focus on your problem, not the technical details.

For example, if you need random numbers, you could:

• Try to write your own random number generator (hard, error-prone), or
• Use the built-in random library:

import random
number = random.randint(1, 10)
print(number)

The second option is exactly why libraries are so valuable.

Advantages and trade-offs

Advantages:

• Faster development
• Less code to write and maintain
• More reliable features
• Access to advanced capabilities (data science, web, etc.)

Trade-offs:

• You need to learn the library’s interface (its functions and how to call them).
• For external libraries, you depend on:
• Them being installed
• Their updates and changes over time

For beginners, using the Python standard library is a gentle introduction, since it’s always available and well-documented.

How you will use libraries in this course

In this chapter and the next ones, you will:

• See how to import modules and access what’s inside them.
• Explore some important standard libraries:
• math, random, and datetime.
• Learn how to install external libraries with pip.

For now, the key idea to remember:

A library in Python is pre-written, reusable code that you import into your program so you can use its tools (functions, classes, constants) instead of writing everything from scratch.