2.2. Values and Data Types¶

A value is one of the fundamental things — like a word or a number — that a program manipulates. The values we have seen so far are 5 (the result when we added 2 + 3), and "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 interpreter) can identify strings because they are enclosed in quotation marks.

If you are not sure what class a value falls into, Python has a function called type which can tell you.

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

You may have used function notation in a math class, like y = f(x), likely only for functions that act on a single numeric value, and produce a single numeric value. Python has no such restrictions: Inputs and outputs may be of any type.

In the Python shell, it is not necessary to use the print function to see the values shown above. The shell evaluates the Python function and automatically prints the result. For example, consider the shell session shown below. When we ask the shell to evaluate type("Hello, World!"), it responds with the appropriate answer and then goes on to display the prompt for the next use.

Python 3.1.2 (r312:79360M, Mar 24 2010, 01:33:18)
[GCC 4.0.1 (Apple Inc. build 5493)] on darwin
Type "help", "copyright", "credits" or "license" for more information.
>>> type("Hello, World!")
<class 'str'>
>>> type(17)
<class 'int'>
>>> "Hello, World"
'Hello, World'
>>>

Note that in the last example, we simply ask the shell to evaluate the string “Hello, World”. The result is as you might expect, the string itself.

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), or three of the same separate quote characters (''' or """).

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!"'. Strings enclosed with three occurrences of either quote symbol are called triple quoted strings. They can contain either single or double quotes:

Triple quoted strings can even span multiple lines:

Python doesn’t care whether you use single or double quotes or the three-of-a-kind 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.

So the Python language designers usually chose to surround their strings by single quotes. What do you think would happen if the string already contained single quotes?

When you type a large integer, you might be tempted to use commas between groups of three digits, as in 42,000. This is not a legal integer in Python, but it does mean something else, which is legal:

Well, that’s not what we expected at all! Because of the comma, Python chose to treat this as a pair of values. In fact, the print function can print any number of values as long as you separate them by commas. Notice that the values are separated by spaces when they are displayed.

Remember not to put commas or spaces in your integers, no matter how big they are. Also revisit what we said in the previous chapter: formal languages are strict, the notation is concise, and even the smallest change might mean something quite different from what you intended.

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