Core Python Programming (2nd Edition)

count2 = counter(100)
>>> print count2()
101
>>> print count()
8
The one difference is that we were able to do something that previously required us to write a class, and
not only that, but to have to override the __call__() special method of that class to make its instances
callable. Here we were able to do it with a pair of functions.
Now, in many cases, a class is the right thing to use. Closures are more appropriate in cases whenever
you need a callback that has to have its own scope, especially if it is something small and simple, and
often, clever. As usual, if you use a closure, it is a good idea to comment your code and/or use doc
strings to explain what you are doing.
*Chasing Down Closure Lexical Variables
The next two sections contain material for advanced readers ... feel free to skip it if you wish. We will
discuss how you can track down free variables with a function's func_closure attribute. Here is a code
snippet that demonstrates it.
If we run this piece of code, we get the following output:
no f1 closure vars
f2 closure vars: ['']
f3 closure vars: ['', '']
Example 11.7. Tracking Closure Variables (closureVars.py)
This example shows how we can track closure variables by using a function's func_closure
variable.
1 #!/usr/bin/env python
2
3 output = ''
4 w = x = y = z = 1
5
6 def f1():
7 x = y = z = 2
8
9 def f2():
10 y = z = 3
11
12 def f3():
13 z = 4