2.2. Values and Data Types¶

A value is one of the fundamental things — like a word or a number — that a program manipulates. Some example values are the number 5 and the text "Hello, World!". We often refer to these values as objects and we will use the words value and object interchangeably.

These objects are classified into different classes, or data types: 4 is an integer, and "Hello, World!" is a string, so-called because it contains a string or sequence of letters. You (and the Python interpreter) can identify strings because they are enclosed in quotation marks.

Not surprisingly, strings belong to the class str and integers belong to the class int.

Continuing with our discussion of data types, numbers with a decimal point belong to a class called float, because these numbers are represented in a format called floating-point. At this stage, you can treat the words class and type interchangeably. We’ll come back to a deeper understanding of what a class is in later chapters.

What about values like "17" and "3.2"? They look like numbers, but they are in quotation marks like strings.

They’re strings!

Strings in Python can be enclosed in either single quotes (') or double quotes (" - the double quote character).

Double quoted strings can contain single quotes inside them, as in "Bruce's beard", and single quoted strings can have double quotes inside them, as in 'The knights who say "Ni!"'.

Python doesn’t care whether you use single or double quotes to surround your strings. Once it has parsed the text of your program or command, the way it stores the value is identical in all cases, and the surrounding quotes are not part of the value.

Knowing the type of a value is important because operations may require a particular type. For example, the turtle.forward function requires inputs of type float:

import turtle
turtle.forward("one hundred") # Error: input must be a float

A type is also required when declaring a variable:

The type declaration is used to ensure that you don’t accidentally use a value with the wrong type. For example, you can’t assign a string to a float variable, otherwise it might cause an error later in the program when you try to use the variable:

import turtle

length: float
length = "one hundred" # Error: value type must match variable type
turtle.forward(length)

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