On Sat, 18 Jul 2009 03:03:32 +0100, gabrielmonnerat
<gabrielmonne...@gmail.com> wrote:
Ronn Ross wrote:
How do you define a global variable in a class. I tried this with do
success:
class ClassName:
global_var = 1
def some_methos():
print global_var
This doesn't work. What am I doing wrong?
You need pass "self" to the function and "global_var" needs be called
with self.
class ClassName:
global_var = 1
def some_methos(self):
print self.global_var
If the OP really wants global_var to be global to the class, this
approach will only work as long as he never assigns to self.global_var.
The terminology is a bit confusing here. "Global variables" refers to
variables which are global to an entire module, ones that you would
use "global" to declare inside a function. Something like this:
x = 5
def set_x(y):
global x
x = y
set_x(3)
print x
...gives the result "3"
The idea of defining a global variable in a class is a bit of a
nonsense, since globals exist for the entire module. There are,
however, variables that exist and are the same for every instance
of a class, or "class attributes" as they are normally called. You
use them either by assignment in the class definition, or in a
method by prefixing them with the class name.
class ClassName:
global_var = 1
def some_method(self):
print ClassName.global_var
ClassName.global_var = 2
"Instance attributes", the ones prefixed by "self", are what get
used for most purposes. These are separate for each different
instance of the class.
class ClassName:
def __init__(self, val):
self.var = val
a = ClassName(1)
b = ClassName(2)
print a.var, b.var
...gives the result "1 2"
The wrinkle is that if you read from an instance attribute that
doesn't exist, Python will use a class attribute of the same name
if there is one. This is often exploited for setting default
values. Beware, though: the moment you assign, an instance
attribute is created -- the underlying class attribute is *not*
changed.
class ClassName:
global_var = 1
def some_method(self):
self.global_var = 2
a = ClassName()
b = ClassName()
b.some_method()
print a.global_var, b.global_var
...gives the result "1 2" again!
--
Rhodri James *-* Wildebeest Herder to the Masses
--
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