python newbie

Neil Cerutti horpner at yahoo.com
Fri Nov 2 16:06:17 CET 2007


On 2007-11-02, Jim Hendricks <jim at bizcomputinginc.com> wrote:
> New to python, programming in 15 or so langs for 24 years.
>
> Couple of questions the tuts I've looked at don't explain:
>
> 1) global vars - python sets scope to the block a var is
> declared (1st set), I see the global keyword that allows access
> to global vars in a function, what I'm not clear on is does
> that global need to be declared in the global scope, or, when
> 1st set in a function where it is listed as a global, does that
> then declare the necessary global.

Names that are assigned in an assignment statement are assumed to
be defined in the current scope.

Names that are assigned to in a function are considered local
variables to that function. This allows them to be faster to
access than other attributes or global variables (which are just
attributes of a module).

def foo():
  x = 7

In function foo x is a local variable. This is true even if there
is an x defined in foo's enclosing scope.

x = 5

def foo():
  x = 7

x is a local variable, which shadows the module scope (global) x.

x = 5

If x is referenced, but not assigned to, then Python does not
create a local variable for it, and normal lookup rules will
cause x to be found in the enclosing scope of the bar function,
below:

def bar():
  print x
 
In bar, the x name refers to the global x.

In order to inform Python that you would like to make
modifications to a global variable in a function, you use the
global statement to declare your intation. This overrides
Python's assumption that that name is a local variable.

x = 5

def foo2():
  global x
  x = 7

> 2) Everything is an object.  So then, why the distinction between 
> functions/variables and fields/methods.  If a module is an object, would 
> not every function be a method of that module and every variable be a 
> field of that module?

You are almost correct. Every identifier/name in Python is
conceptually an attribute of an object. But identifiers in Python
are not typed. It is the objects that they refer to that are
typed.

-- 
Neil Cerutti



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