Beginning Python - From Novice to Professional

128 CHAPTER 6 ■ ABSTRACTION
■Note Use global variables only when you have to. They tend to make your code less readable and less
robust. Local variables make your program more abstract because they are “hidden” inside functions.
NESTED SCOPES
Python’s scopes may (from Python 2.2 on) be nested. This means that you can (among other things) write
functions like the following:
def multiplier(factor):
def multiplyByFactor(number):
return number*factor
return multiplyByFactor
One function is inside another, and the outer function returns the inner one. Each time the outer function
is called, the inner one gets redefined, and each time, the variable factor may have a new value. With nested
scopes, this variable from the outer local scope (of multiplier) is accessible in the inner function later on,
as follows:
>>> double = multiplier(2)
>>> double(5)
10
>>> triple = multiplier(3)
>>> triple(3)
9
>>> multiplier(5)(4)
20
A function such as multiplyByFactor that stores its enclosing scopes is called a closure.
If, for some reason, you’re using Python 2.1, you have to add the following line at the beginning of
your program:
from __future__ import nested_scopes
In older versions of Python, variables from surrounding nested scopes are not available. You get an error
message like this:
>>> double = multiplier(2)
>>> double(2)
Traceback (innermost last):
File "", line 1, in ?
File "", line 3, in multiplyByFactor
NameError: factor
Because old versions of Python only have local and global scopes, and factor is not a local variable in
multiplyByFactor, Python assumes that it must be a global variable. But it isn’t, so you get an exception.
To store a variable from an enclosing scope, you can use a trick—storing it as a default value: