Beginning Python - From Novice to Professional

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CHAPTER 5 ■ CONDITIONALS, LOOPS, AND SOME OTHER STATEMENTS 85 >>> x = 2 >>> x += 1 >>> x *= 2 >>> x 6 It also works with other data types: >>> fnord = 'foo' >>> fnord += 'bar' >>> fnord 'foobar' Augmented assignments can make your code more compact and concise, and in many cases, more readable. ■Tip In general, you should not use += with strings, especially if you are building a large string piece by piece in a loop (see the section “Loops” later in this chapter for more information about loops). Each addition and assignment needs to create a new string, and that takes time, making your program slower. A much better approach is to append the small strings to a list, and use the string method join to create the big string when your list is finished. Blocks: The Joy of Indentation This isn’t really a type of statement but something you’re going to need when you tackle the next two sections. A block is a group of statements that can be executed if a condition is true (conditional statements), or executed several times (loops), and so on. A block is created by indenting a part of your code; that is, putting spaces in front of it. ■Note You can use tab characters to indent your blocks as well. Python interprets a tab as moving to the next tab stop, with one tab stop every eight spaces, but the standard and preferable style is to use spaces only, no tabs, and specifically four spaces per each level of indentation. Each line in a block must be indented by the same amount. The following is pseudocode (not real Python code) but shows how the indenting works: this is a line this is another line: this is another block continuing the same block the last line of this block phew, there we escaped the inner block
86 CHAPTER 5 ■ CONDITIONALS, LOOPS, AND SOME OTHER STATEMENTS In many languages a special word or character (for example, begin or {) is used to start a block, and another (such as end or }) is used to end it. In Python, a colon (:) is used to indicate that a block is about to begin, and then every line in that block is indented (by the same amount). When you go back to the same amount of indentation as some enclosing block, you know that the current block has ended. Now I’m sure you are curious to know how to use these blocks. So, without further ado, let’s have a look. Conditions and Conditional Statements Until now you’ve only written programs in which each statement is executed, one after the other. It’s time to move beyond that and let your program choose whether or not to execute a block of statements. So That’s What Those Boolean Values Are For Now you are finally going to need those truth values (also called Boolean values, after George Boole, who did a lot of smart stuff on truth values) that you’ve been bumping into repeatedly. ■Note If you’ve been paying close attention, you noticed the sidebar in Chapter 1, “Sneak Peek: The if Statement,” which describes the if statement. I haven’t really introduced it formally until now, and as you’ll see, there is a bit more to it than what I’ve told you so far. The following values are considered by the interpreter to mean false: False None 0 "" () [] {} In other words, the standard values False and None, numeric zero of all types (including float, long, and so on), empty sequences (such as empty strings, tuples, and lists), and empty dictionaries are all false. Everything else is interpreted as true, including the special value True. Laura Creighton describes this as discerning between something and nothing, rather than true and false. Got it? This means that every value in Python can be interpreted as a truth value, which can be a bit confusing at first, but it can also be extremely useful. And even though you have all these truth values to choose from, the “standard” truth values are True and False. In older versions of Python, the standard truth values were 0 (for false) and 1 (for true). In fact, True and False are just glorified versions of 0 and 1 that look different but act the same: >>> True True >>> False False >>> True == 1 True
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Beginning Python From Novice to Pro
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viii ■CONTENTS Strings . . . . .
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xxii ■CONTENTS Preparations . . .
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About the Technical Reviewer ■JER
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Introduction A C program is like a
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CHAPTER 9 ■ ■ ■ Magic Methods
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CHAPTER 9 ■ MAGIC METHODS, PROPER
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256 CHAPTER 11 ■ FILES AND STUFF
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270 CHAPTER 12 ■ GRAPHICAL USER I
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298 CHAPTER 14 ■ NETWORK PROGRAMM
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314 CHAPTER 15 ■ PYTHON AND THE W
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358 CHAPTER 17 ■ EXTENDING PYTHON
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392 CHAPTER 20 ■ PROJECT 1: INSTA
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APPENDIX A ■ ■ ■ The Short Ve
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558 APPENDIX B ■ PYTHON REFERENCE
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572 APPENDIX C ■ ONLINE RESOURCES
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Index ■Symbols - operator 558 !=
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■INDEX 595 Python C API 365 hand-
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