5.1 Introduction to Data Types

The built-in data types covered in this chapter, grouped by category.

What Are Data Types?

What Is It?

A data type classifies what kind of value something is (a number, text, a collection) and, in turn, what operations are valid on it.

Why Does It Matter?

The type determines behavior — you can add two ints, but adding an int to a list raises a TypeError.

How Is It Used?

Every value in Python has exactly one type at any given moment, discoverable with type().

>>> type(42)
<class 'int'>
>>> type("hello")
<class 'str'>

Why Data Types Matter

Choosing the right type affects correctness (can this value be negative? can it have duplicates?), performance (list vs set membership testing), and memory usage. A large part of writing good Python is picking the type that matches your data’s real-world shape.

Dynamic Typing

What Is It?

A variable has no fixed type — its type is simply whatever value it currently holds, and reassignment can change that type freely. This was introduced in 4.8 Variables and is worth re-grounding here since it’s foundational to how types work in Python.

>>> x = 10
>>> type(x)
<class 'int'>
>>> x = "now a string"    # same name, completely different type -- perfectly legal
>>> type(x)
<class 'str'>

Real-World Examples

• A user’s age → int
• A server’s hostname → str
• A list of IP addresses to block → list or set
• An API response body → dict (parsed from JSON)
• A file’s raw contents → bytes

Quick Interview Answer

“A data type classifies what kind of value something is and what operations are valid on it — Python discovers a value’s type at runtime with type(), since it’s dynamically typed: a variable’s type is whatever value it currently holds, and reassigning it to a different type is perfectly legal. Picking the right type up front affects correctness, performance, and memory usage.”

Common Mistakes

• Assuming a variable’s type is fixed once assigned — reassignment to a different type is silent and completely legal in Python.
• Not checking type() or isinstance() before an operation, then getting a TypeError at runtime instead of catching a bad assumption early.