And, as imagination bodies forth

The forms of things to unknown, and the poet's pen

Turns them to shapes, and gives to airy nothing

A local habitation and a name.

—William Shakespeare, A Midsummer-Night's Dream

To give Python a proper introduction, we will begin with an example that uses several language features. In the next section, we will start from scratch and build up the language piece by piece. Think of this section as a sneak preview of features to come.

Python has built-in support for a wide range of common programming activities, such as manipulating text, displaying graphics, and communicating over the Internet. The line of Python code

>>> from urllib.request import urlopen

is an import statement that loads functionality for accessing data on the Internet. In particular, it makes available a function called urlopen, which can access the content at a uniform resource locator (URL), a location of something on the Internet.

Statements & Expressions. Python code consists of expressions and statements. Broadly, computer programs consist of instructions to either

1. Compute some value
2. Carry out some action

Statements typically describe actions. When the Python interpreter executes a statement, it carries out the corresponding action. On the other hand, expressions typically describe computations. When Python evaluates an expression, it computes the value of that expression. This chapter introduces several types of statements and expressions.

The assignment statement

>>> shakespeare = urlopen('http://composingprograms.com/shakespeare.txt')

associates the name shakespeare with the value of the expression that follows =. That expression applies the urlopen function to a URL that contains the complete text of William Shakespeare's 37 plays, all in a single text document.

Functions. Functions encapsulate logic that manipulates data. urlopen is a function. A web address is a piece of data, and the text of Shakespeare's plays is another. The process by which the former leads to the latter may be complex, but we can apply that process using only a simple expression because that complexity is tucked away within a function. Functions are the primary topic of this chapter.

Another assignment statement

>>> words = set(shakespeare.read().decode().split())

associates the name words to the set of all unique words that appear in Shakespeare's plays, all 33,721 of them. The chain of commands to read, decode, and split, each operate on an intermediate computational entity: we read the data from the opened URL, then decode the data into text, and finally split the text into words. All of those words are placed in a set.

Objects. A set is a type of object, one that supports set operations like computing intersections and membership. An object seamlessly bundles together data and the logic that manipulates that data, in a way that manages the complexity of both. Objects are the primary topic of Chapter 2. Finally, the expression

>>> {w for w in words if len(w) == 6 and w[::-1] in words}
{'redder', 'drawer', 'reward', 'diaper', 'repaid'}

is a compound expression that evaluates to the set of all Shakespearian words that are simultaneously a word spelled in reverse. The cryptic notation w[::-1] enumerates each letter in a word, but the -1 dictates to step backwards. When you enter an expression in an interactive session, Python prints its value on the following line.

Interpreters. Evaluating compound expressions requires a precise procedure that interprets code in a predictable way. A program that implements such a procedure, evaluating compound expressions, is called an interpreter. The design and implementation of interpreters is the primary topic of Chapter 3.

When compared with other computer programs, interpreters for programming languages are unique in their generality. Python was not designed with Shakespeare in mind. However, its great flexibility allowed us to process a large amount of text with only a few statements and expressions.

In the end, we will find that all of these core concepts are closely related: functions are objects, objects are functions, and interpreters are instances of both. However, developing a clear understanding of each of these concepts and their role in organizing code is critical to mastering the art of programming.