python newbie

Bruno Desthuilliers bruno.42.desthuilliers at wtf.websiteburo.oops.com
Fri Nov 2 17:27:26 CET 2007


Jim Hendricks a écrit :
> 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.

Didn't you try by yourself ? Would have been faster than writing down 
your question...

Python 2.5.1 (r251:54863, May  2 2007, 16:56:35)
[GCC 4.1.2 (Ubuntu 4.1.2-0ubuntu4)] on linux2
Type "help", "copyright", "credits" or "license" for more information.
 >>> def toto():
...     global p
...     p = 42
...
 >>> p
Traceback (most recent call last):
   File "<stdin>", line 1, in <module>
NameError: name 'p' is not defined
 >>> toto()
 >>> p
42
 >>>

Anyway, remember that in Python, 'global' really means 'module level'.

> 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

functions are *instance* attributes of the modules (obviously, since 
each module is a distinct instance of class 'module'...), and when 
functions are instance attributes of an object, the lookup mechanism 
doesn't turn them into methods.

> and every variable be a 
> field of that module?

There's nothing like a 'field' in Python - only attributes. And 
module-level (so-called 'global') vars *are* attributes of the module. 
So from 'the outside' (ie: from the importing code), a module is just an 
instance of class module, with it's own set of attributes - some of them 
callables (functions and classes), some other not.

HTH



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