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 seen so far is 5 (the result when 2 + 3 is added). These values are often referred as objects.

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.

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 the shell is asked to evaluate type("Hello, World!"), it responds with the appropriate answer and then goes on to display the prompt for the next use.

Python 3.6.9 (default, Dec  8 2021, 21:08:43)
[GCC 8.4.0] on linux
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, the shell is simply asked to evaluate the string “Hello, World”. The result is as you might expect, the string itself.

Continuing with the 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 ("), or three of each (''' 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?

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.

Check your understanding

© Copyright 2024 GS Ng.