"bill.wu" <[EMAIL PROTECTED]> wrote
> i ask a easy question.
>
> why the first one have"x",the second one
> doesn't have "x". what is different?
The first is using x as the name of a parameter
of the function and is only used inside the function.
The second one takes no parameter and relies on
explicit knowlege that an variable called x
exists in the global namespace.
The second form is considered bad practice
unless you have a very good reason to use it
since it forces the function to know about
things outside its control.
> when write "x",when don't write "x".
Using a parameter (usually called something
more meaningful than x!) is normally the best
way. Version 2 should be avoided if possible.
> in my point,the second one don't def variable.
Corect it uses the global variable defined
at the module level. That is why the global
statement is used.
> (1)
>
> def func(x):
> print 'x is', x
> x = 2
> print 'Changed local x to', x
>
> x = 50
> func(x)
> print 'x is still', x (2)
This defines a global x with value 50.
It then calls func passing in the value
50 to the parameter x which acts like a
local variable, only seen inside the
function.
The func internally assigns a value
of 2 to that local x which does not
affect the global x. It then prints
the local value and exits
The next line of code then prints
the global x to show that it has
not changed.
If a different name had been used for the
parameter it would be much clearer but
I assume the author is trying to demonstrate
how names are controlled.
def func(y):
print 'y =',y
y = 2
print 'y =',y
x = 50
func(x)
print 'x =',x
The code here is identical in function
to the first version but because we
chose y as the parameter name it is
obvious that they are different variables.
> def func():
> global x
>
> print 'x is', x
> x = 2
> print 'Changed local x to', x
This function has no local variables and
instead acts on the global x. It could
be better written with a parameter like this:
def func(y)
print 'y =',y
y = 2
print 'y=',y
return y # allows it to affect the global
> x = 50
> func()
And this line becomes
x = func(x)
> print 'Value of x is', x
Now x will reflect the changes made by func()
You will find more about namespaces in the
"Whats in a name?" topic of my tutorial.
HTH,
--
Alan Gauld
Author of the Learn to Program web site
http://www.freenetpages.co.uk/hp/alan.gauld
_______________________________________________
Tutor maillist - Tutor@python.org
http://mail.python.org/mailman/listinfo/tutor
