Python module globals versus __init__ globals

Question

Apologies, somewhat confused Python newbie question. Let's say I have a module called animals.py.......

globvar = 1

class dog: 
   def bark(self):
      print globvar

class cat:
   def miaow(self):
      print globvar

What is the difference between this and

class dog:
   def __init__(self):
      global globvar

   def bark(self):
      print globvar

class cat:
   def miaow(self):
      print globvar

Assuming I always instantiate a dog first?

I guess my question is, is there any difference? In the second example, does initiating the dog create a module level globvar just like in the first example, that will behave the same and have the same scope?

Answer 1

No, the global statement only matters when you're assigning to a global variable within a method or function. So that __init__ is irrelevant -- it does not create the global, because it's not assigning anything to it.

Answer 2

global doesn't create a new variable, it just states that this name should refer to a global variable instead of a local one. Usually assignments to variables in a function/class/... refer to local variables. For example take a function like this:

def increment(n)
  # this creates a new local m
  m = n+1
  return m

Here a new local variable m is created, even if there might be a global variable m already existing. This is what you usually want since some function call shouldn't unexpectedly modify variables in the surrounding scopes. If you indeed want to modify a global variable and not create a new local one, you can use the global keyword:

def increment(n)
  global increment_calls
  increment_calls += 1
  return n+1

In your case global in the constructor doesn't create any variables, further attempts to access globvar fail:

>>> import animals
>>> d = animals.dog()
>>> d.bark()
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "animals.py", line 7, in bark
    print globvar
NameError: global name 'globvar' is not defined

But if you would actually assign a value to globvar in the constructor, a module-global variable would be created when you create a dog:

class dog:
   def __init__(self):
      global globvar
      globvar = 1
...

Execution:

>>> import animals
>>> d = animals.dog()
>>> d.bark()
1
>>> print animals.globvar
1