Beginning Python - From Novice to Professional

104 CHAPTER 5 ■ CONDITIONALS, LOOPS, AND SOME OTHER STATEMENTS
>>> x = 1
>>> del x
>>> x
Traceback (most recent call last):
File "", line 1, in ?
x
NameError: name 'x' is not defined
This may seem easy, but it can actually be a bit tricky to understand at times. For instance,
in the following example, x and y refer to the same list:
>>> x = ["Hello", "world"]
>>> y = x
>>> y[1] = "Python"
>>> x
['Hello', 'Python']
You might assume that by deleting x, you would also delete y, but that is not the case:
>>> del x
>>> y
['Hello', 'Python']
Why is this? x and y referred to the same list, but deleting x didn’t affect y at all. The reason
for this is that you only delete the name, not the list itself (the value). In fact, there is no way to
delete values in Python (and you don’t really need to because the Python interpreter does it by
itself whenever you don’t use the value anymore).
Executing and Evaluating Strings with exec and eval
Sometimes you may want to create Python code “on the fly” and execute it as a statement or
evaluate it as an expression. This may border on dark magic at times—consider yourself warned.
■Caution In this section, you learn to execute Python code stored in a string. This is a potential security
hole of great dimensions. If you execute a string where parts of the contents have been supplied by a user,
you have little or no control over what code you are executing. This is especially dangerous in network applications,
such as CGI scripts, which you will learn about in Chapter 15.
exec
The statement for executing a string is exec:
>>> exec "print 'Hello, world!'"
Hello, world!
However, using this simple form of the exec statement is rarely a good thing; in most cases
you want to supply it with a namespace, a place where it can put its variables. You want to do
this so that the code doesn’t corrupt your namespace (that is, change your variables). For example,
let’s say that the code uses the name sqrt: