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CHAPTER 1 INSTANT HACKING: THE BASICS
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Note Variable names can consist of letters, digits, and underscore characters (_). A variable can t begin
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with a digit, so Plan9 is a valid variable name, whereas 9Plan is not.
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Until now we ve been working (almost) exclusively with expressions, the ingredients of the recipe. But what about statements the instructions In fact, I ve cheated. I ve introduced two types of statements already: the print statement, and assignments. So, what s the difference between a statement and an expression Well, an expression is something, while a statement does something (or, rather, tells the computer to do something). For example, 2*2 is 4, whereas print 2*2 prints 4. What s the difference, you may ask. After all, they behave very similarly. Consider the following: >>> 2*2 4 >>> print 2*2 4 As long as you execute this in the interactive interpreter the results are similar, but that is only because the interpreter always prints out the values of all expressions (using the same representation as repr see the section on string representations later in this chapter). That is not true of Python in general. Later in this chapter, you ll see how to make programs that run without this interactive prompt, and simply putting an expression such as 2*2 in your program won t do anything interesting.4 Putting print 2*2 in there, on the other hand, will in fact print out 4. The difference between statements and expressions may be more obvious when dealing with assignments. Because they are not expressions, they have no values that can be printed out by the interactive interpreter: >>> x = 3 >>> As you can see, you get a new prompt immediately. Something has changed, however; x is now bound to the value 3. This is a defining quality of statements in general: They change things. For example, assignments change variables, and print statements change how your screen looks. Assignments are, perhaps, the most important type of statement in any programming language; it may be difficult to grasp their importance right now. Variables may just seem like
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4. In case you re wondering yes, it does do something. It calculates the product of 2 and 2. However, the result isn t kept anywhere or shown to the user; it has no side effects, beyond the calculation itself.
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CHAPTER 1 INSTANT HACKING: THE BASICS
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temporary storage (like the pots and pans of a cooking recipe), but the real power of variables is that you needn t know what values they hold in order to manipulate them.5 For example, you know that x * y evaluates to the product of x and y even though you may have no knowledge of what x and y are. So, you may write programs that use variables in various ways without knowing the values they will eventually hold (or refer to) when the program is run.
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You ve seen that you can write programs with variables without knowing their values. Of course, the interpreter must know the values eventually. So how can it be that we don t The interpreter knows only what we tell it, right Not necessarily. You may have written a program, and somebody else may use it. You cannot predict what values they will supply to the program. Let s take a look at the useful function input. (I ll have more to say about functions in a minute.) >>> input("The meaning of life: ") The meaning of life: 42 42 What happens here is that the first line (input(...)) is executed in the interactive interpreter. It prints out the string "The meaning of life: " as a new prompt. I type 42 and press Enter. The resulting value of input is that very number, which is automatically printed out in the last line. Not very useful. But look at the following: >>> x = input("x: ") x: 34 >>> y = input("y: ") y: 42 >>> print x * y 1428 Here, the statements at the Python prompts (>>>) could be part of a finished program, and the values entered (34 and 42) would be supplied by some user. Your program would then print out the value 1428, which is the product of the two. And you didn t have to know these values when you wrote the program, right
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Note This is much more useful when you save your programs in a separate file so other users can execute
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