Learning Python - Mark Lutz [182]
In the next chapter, we’ll continue our statement tour by filling in details about the if statement, Python’s main selection tool; there, we’ll also revisit Python’s syntax model in more depth and look at the behavior of Boolean expressions. Before we move on, though, the end-of-chapter quiz will test your knowledge of what you’ve learned here.
Test Your Knowledge: Quiz
Name three ways that you can assign three variables to the same value.
Why might you need to care when assigning three variables to a mutable object?
What’s wrong with saying L = L.sort()?
How might you use the print operation to send text to an external file?
Test Your Knowledge: Answers
You can use multiple-target assignments (A = B = C = 0), sequence assignment (A, B, C = 0, 0, 0), or multiple assignment statements on three separate lines (A = 0, B = 0, and C = 0). With the latter technique, as introduced in Chapter 10, you can also string the three separate statements together on the same line by separating them with semicolons (A = 0; B = 0; C = 0).
If you assign them this way:A = B = C = []
all three names reference the same object, so changing it in-place from one (e.g., A.append(99)) will affect the others. This is true only for in-place changes to mutable objects like lists and dictionaries; for immutable objects such as numbers and strings, this issue is irrelevant.
The list sort method is like append in that it makes an in-place change to the subject list—it returns None, not the list it changes. The assignment back to L sets L to None, not to the sorted list. As we’ll see later in this part of the book, a newer built-in function, sorted, sorts any sequence and returns a new list with the sorting result; because this is not an in-place change, its result can be meaningfully assigned to a name.
To print to a file for a single print operation, you can use 3.0’s print(X, file=F) call form, use 2.6’s extended print >> file, X statement form, or assign sys.stdout to a manually opened file before the print and restore the original after. You can also redirect all of a program’s printed text to a file with special syntax in the system shell, but this is outside Python’s scope.
Chapter 12. if Tests and Syntax Rules
This chapter presents the Python if statement, which is the main statement used for selecting from alternative actions based on test results. Because this is our first in-depth look at compound statements—statements that embed other statements—we will also explore the general concepts behind the Python statement syntax model here in more detail than we did in the introduction in Chapter 10. Because the if statement introduces the notion of tests, this chapter will also deal with Boolean expressions and fill in some details on truth tests in general.
if Statements
In simple terms, the Python if statement selects actions to perform. It’s the primary selection tool in Python and represents much of the logic a Python program possesses. It’s also our first compound statement. Like all compound Python statements, the if statement may contain other statements, including other ifs. In fact, Python lets you combine statements in a program sequentially (so that they execute one after another), and in an arbitrarily nested fashion (so that they execute only under certain conditions).
General Format
The Python if statement is typical of if statements in most procedural languages. It takes the form of an if test, followed by one or more optional elif (“else if”) tests and a final optional else block. The tests and the else part each have an associated block of nested statements, indented under a header line. When the if statement runs, Python executes the block of code associated with the first test that evaluates to true,