Python local variables in methods

Question

I am learning Python 3 and I have very fundamental question regarding object oriented programming in Python. Here is my code.

class pet:
    number_of_legs = 0
    def count_legs(self):
        print("I have %s legs" %dog.number_of_legs)

dog = pet()
dog.number_of_legs = 4
dog.count_legs()

This code prints:

I have 4 legs

Why count_legs method does not give error like "Unknown variable dog in print." The variable dog exist outside the class.

How this code is working ? What is motivation behind such behavior ?

Answer 1

dog is looked up as a global at runtime; had you named your variable differently the code would have thrown an error:

>>> class pet:
...     number_of_legs = 0
...     def count_legs(self):
...         print("I have %s legs" %dog.number_of_legs)
... 
>>> cat = pet()
>>> cat.number_of_legs = 4
>>> cat.count_legs()
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "<stdin>", line 4, in count_legs
NameError: name 'dog' is not defined
>>> dog = cat
>>> cat.count_legs()
I have 4 legs

This makes it easy to refer to other globals in the module that are defined later; this includes recursive functions:

def fib(n):
    if n <= 1: return n
    return fib(n - 1) + fib(n - 2)

When defining fib() there is no global name fib yet; only when the function object has been created is the global added. If the name had to exist first you'd end up with code like:

fib = None  # placeholder for global reference

def fib(n):
    if n <= 1: return n
    return fib(n - 1) + fib(n - 2)

and that is just noise.