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CHAPTER 5 CONDITIONALS, LOOPS, AND SOME OTHER STATEMENTS
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name = raw_input('Please enter your name: ') or '<unknown>' In other words, if the return value from raw_input is true (not an empty string), it is assigned to name (nothing changes); otherwise, the default '<unknown>' is assigned to name. This sort of short-circuit logic can be used to implement the so-called ternary operator (or conditional operator), found in languages such as C and Java. For a thorough explanation, see Alex Martelli s recipe on the subject in the Python Cookbook (http://aspn.activestate.com/ASPN/Cookbook/Python/Recipe/ 52310).
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There is a useful relative of the if statement, which works more or less like this (pseudocode): if not condition: crash program Now, why on earth would you want something like that Simply because it s better that your program crashes when an error condition emerges than at a much later time. Basically, you can require that certain things be true. The keyword used in the statement is assert: >>> age = 10 >>> assert 0 < age < 100 >>> age = -1 >>> assert 0 < age < 100 Traceback (most recent call last): File "<stdin>", line 1, in AssertionError It can be useful to put the assert statement in your program as a checkpoint, if you know something has to be true for your program to work correctly. A string may be added after the condition, to explain the assertion: >>> age = -1 >>> assert 0 < age < 100, 'The age must be realistic' Traceback (most recent call last): File "<stdin>", line 1, in AssertionError: The age must be realistic
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Now you know how to do something if a condition is true (or false), but how do you do something several times For example, you might want to create a program that reminds you to pay the rent every month, but with the tools we have looked at until now, you d have to write the program like this (pseudocode):
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CHAPTER 5 CONDITIONALS, LOOPS, AND SOME OTHER STATEMENTS
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send mail wait one month send mail wait one month send mail wait one month (...and so on) But what if you wanted it to continue doing this until you stopped it Basically, you want something like this (again, pseudocode): while we aren't stopped: send mail wait one month Or, let s take a simpler example. Let s say that you want to print out all the numbers from 1 to 100. Again, you could do it the stupid way: print 1 print 2 print 3 . . . and so on. But you didn t start using Python because you wanted to do stupid things, right
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In order to avoid the cumbersome code of the preceding example, it would be useful to be able to do something like this: x = 1 while x <= 100: print x x += 1 Now, how do you do that in Python You guessed it you do it just like that. Not that complicated is it You could also use a loop to ensure that the user enters a name, as follows: name = '' while not name: name = raw_input('Please enter your name: ') print 'Hello, %s!' % name Try running this, and then just pressing the Enter key when asked to enter your name: the question appears again because name is still an empty string, which evaluates to false.
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Tip What would happen if you entered just a space character as your name Try it. It is accepted because a string with one space character is not empty, and therefore not false. This is definitely a flaw in our little program, but easily corrected: just change while not name to while not name or name.isspace(), or, perhaps, while not name.strip().
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