Variables and basic data types in Python

Variables are one of the fundamental building blocks of programs written in Python. Variables hold data in memory. They have names, and they can be referenced by name. Variables also have types which specify what type of data they can store, such as string and integer, and they can be used in expressions that use operators such as + and - to manipulate their values.

In Python, a variable is declared and assigned a value using the assignment operator =. The variable being assigned to is on the left-hand side of the operator, and the value being assigned, which can be an expression such as 2 + 2 and can even include other variables, is on the right-hand side:

x = 1         # assign variable x the value 1
y = x + 5     # assign variable y the value of x plus 5
z = y         # assign variable z the value of y

These examples assign numbers to variables, but numbers are just one of several data types that Python supports. Notice there is no type declared for the variables. This is because Python is a dynamically typed language, which means the variable type is determined by the data assigned to it. In the examples above, the x, y, and z variables will be integer types, capable of storing postive and negative whole numbers.

Variable names are case-sensitive and can use any letter, number, and the underscore (_) character, however they can't start with a number.

Working with numbers

Most programs manipulate numbers. Computers treat integer numbers and decimal numbers differently. Consider the following code:

x = 1       # integer
x = 1.0     # decimal (floating point)

Python creates integers from a built-in data typed called int, and decimals (floating-point numbers) as instances of float. Python's built-in type function returns a variable's data type. The following code outputs types to the screen:

x = 1
print(type(x)) # outputs: <class 'int'>

x = 1.0
print(type(x)) # outputs: <class 'float'>

The addition of the ".0" to the end of "1" makes a big difference in how the programming language treats a value. The data type impacts how the value is stored in memory, how the processor (CPU) handles the data when evaluating expressions, how the data relates to other data, and what kinds operations can be performed with it.

Another common data type is the Boolean type, which holds the value True or False:

x = True
print(type(x)) # outputs: <class 'bool'>

Internally, bool is treated as a special type of integer. Technically, True has a value of 1 and False has a value of 0. Typically, Booleans are not used to perform mathematical operations; rather, they are used to make decisions and perform branching. Nevertheless, it is interesting to understand the relationship between types. Many types are nothing more than specialized versions of more general types. Integers are a subset of floating point numbers, and Booleans are a subset of integers.

Working with strings

Along with numbers, strings are among the most commonly used data types. A string is a collection of zero or more characters. Strings are commonly declared using single quotation marks, but double quotation marks may be used, too:

x = 'This is a string'
print(x) # outputs: This is a string
print(type(x)) # outputs: <class 'str'>
y = "This is also a string"

Strings can be added to other strings (an operation known as "concatenation") with the same + operator that adds two numbers:

x = 'Hello' + ' ' + 'World!'
print(x) # outputs: Hello World!

You will learn more about strings in another lesson, including how to parse them and how to manipulate them in various ways. You will also learn about other important data types such as lists, which store collections of data and are frequently used to hold collections of strings.