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CHAPTER 5 ■ CONDITIONALS, LOOPS, AND SOME OTHER STATEMENTS 107 A Quick Summary In this chapter you’ve seen several kinds of statements: Printing. You can use the print statement to print several values by separating them with commas. If you end the statement with a comma, later print statements will continue printing on the same line. Importing. Sometimes you don’t like the name of a function you want to import—perhaps you’ve already used the name for something else. You can use the import...as... statement, to locally rename a function. Assignments. You’ve seen that through the wonder of sequence unpacking and chained assignments, you can assign values to several variables at once, and that with augmented assignments you can change a variable in place. Blocks. Blocks are used as a means of grouping statements through indentation. They are used in conditionals and loops, and as you see later in the book, in function and class definitions, among other things. Conditionals. A conditional statement either executes a block or not, depending on a condition (Boolean expression). Several conditionals can be strung together with if/elif/else. Assertions. An assertion simply asserts that something (a Boolean expression) is true, optionally with a string explaining why it has to be so. If the expression happens to be false, the assertion brings your program to a halt (or actually raises an exception—more on that in Chapter 8). It’s better to find an error early than to let it sneak around your program until you don’t know where it originated. Loops. You either can execute a block for each element in a sequence (such as a range of numbers) or continue executing it while a condition is true. To skip the rest of the block and continue with the next iteration, use the continue statement; to break out of the loop, use the break statement. Optionally, you may add an else clause at the end of the loop, which will be executed if you didn’t execute any break statements inside the loop. List comprehension. These aren’t really statements—they are expressions that look a lot like loops, which is why I grouped them with the looping statements. Through list comprehension, you can build new lists from old ones, applying functions to the elements, filtering out those you don’t want, and so on. The technique is quite powerful, but in many cases using plain loops and conditionals (which will always get the job done) may be more readable. pass, del, exec, and eval. The pass statement does nothing, which can be useful as a placeholder, for example. The del statement is used to delete variables or parts of a datastructure, but cannot be used to delete values. The exec statement is used to execute a string as if it were a Python program. The built-in function eval evaluates an expression written in a string and returns the result.
108 CHAPTER 5 ■ CONDITIONALS, LOOPS, AND SOME OTHER STATEMENTS New Functions in This Chapter Function chr(n) eval(source[, globals[, locals]]) enumerate(seq) ord(c) range([start,] stop[, step]) reversed(seq) sorted(seq[, cmp][, key][, reverse]) xrange([start,] stop[, step]) zip(seq1, seq2,...) Description Returns a one-character string with ordinal n (0 ≤ n < 256) Evaluates a string as an expression and returns the value Yields (index, value) pairs suitable for iteration Returns the integer ordinal value of a onecharacter string Creates a list of integers Yields the values of seq in reverse order, suitable for iteration Returns a list with the values of seq in sorted order Creates an xrange object, used for iteration Creates a new sequence suitable for parallel iteration What Now? Now you’ve cleared the basics. You can implement any algorithm you can dream up; you can read in parameters and print out the results. In the next couple of chapters, you learn about something that will help you write larger programs without losing the big picture. That something is called abstraction.
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Beginning Python From Novice to Pro
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For Ranveig
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■CHAPTER 23 Project 4: In the New
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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, PROPER
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358 CHAPTER 17 ■ EXTENDING PYTHON
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572 APPENDIX C ■ ONLINE RESOURCES
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Index ■Symbols - operator 558 !=
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