Beginning Python - From Novice to Professional
Beginning Python - From Novice to Professional Beginning Python - From Novice to Professional
CHAPTER 5 ■ CONDITIONALS, LOOPS, AND SOME OTHER STATEMENTS 93 name = raw_input('Please enter your name: ') or '' 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 '' 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). Assertions 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 "", 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 "", line 1, in ? AssertionError: The age must be realistic Loops 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):
94 CHAPTER 5 ■ CONDITIONALS, LOOPS, AND SOME OTHER STATEMENTS 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? while Loops 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
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94 CHAPTER 5 ■ CONDITIONALS, LOOPS, AND SOME OTHER STATEMENTS<br />
send mail<br />
wait one month<br />
send mail<br />
wait one month<br />
send mail<br />
wait one month<br />
(...and so on)<br />
But what if you wanted it <strong>to</strong> continue doing this until you s<strong>to</strong>pped it? Basically, you want<br />
something like this (again, pseudocode):<br />
while we aren't s<strong>to</strong>pped:<br />
send mail<br />
wait one month<br />
Or, let’s take a simpler example. Let’s say that you want <strong>to</strong> print out all the numbers from<br />
1 <strong>to</strong> 100. Again, you could do it the stupid way:<br />
print 1<br />
print 2<br />
print 3<br />
. . . and so on. But you didn’t start using <strong>Python</strong> because you wanted <strong>to</strong> do stupid things, right?<br />
while Loops<br />
In order <strong>to</strong> avoid the cumbersome code of the preceding example, it would be useful <strong>to</strong> be able<br />
<strong>to</strong> do something like this:<br />
x = 1<br />
while x
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