Variable Scope

[Jens Thiede]

In the following terminal; could someone inform me as to why it is
posible to print a global variable without having to declare it using
global. This has affected some source of mine, and allows you to
modify a global in a local scope *without* the global keyword, for
instance, you can append to a global list, but *not* assign it a new
value, for then, you create a new local variable. -- Why.

Python 2.3.2 (#1, Jan 3 2004, 23:02:08)
[GCC 3.2.3 20030422 (Gentoo Linux 1.4 3.2.3-r3, propolice)] on linux2

IDLE 1.0[color=blue][color=green][color=darkred]
>>> x = 10;
>>> def test():[/color][/color][/color]
x += 1;

[color=blue][color=green][color=darkred]
>>> test();[/color][/color][/color]

Traceback (most recent call last):
File "<pyshell#5 >", line 1, in -toplevel-
test();
File "<pyshell#4 >", line 2, in test
x += 1;
UnboundLocalErr or: local variable 'x' referenced before assignment[color=blue][color=green][color=darkred]
>>> def test2():[/color][/color][/color]
print x;

[color=blue][color=green][color=darkred]
>>> test2();[/color][/color][/color]
10

Any help would be appreciated,

Jens Thiede.

[anton muhin]

Re: Variable Scope

Jens Thiede wrote:
[color=blue]
> In the following terminal; could someone inform me as to why it is
> posible to print a global variable without having to declare it using
> global. This has affected some source of mine, and allows you to
> modify a global in a local scope *without* the global keyword, for
> instance, you can append to a global list, but *not* assign it a new
> value, for then, you create a new local variable. -- Why.
>
> Python 2.3.2 (#1, Jan 3 2004, 23:02:08)
> [GCC 3.2.3 20030422 (Gentoo Linux 1.4 3.2.3-r3, propolice)] on linux2
>
> IDLE 1.0
>[color=green][color=darkred]
>>>>x = 10;
>>>>def test():[/color][/color]
>
> x += 1;
>
>
>[color=green][color=darkred]
>>>>test();[/color][/color]
>
>
> Traceback (most recent call last):
> File "<pyshell#5 >", line 1, in -toplevel-
> test();
> File "<pyshell#4 >", line 2, in test
> x += 1;
> UnboundLocalErr or: local variable 'x' referenced before assignment
>[color=green][color=darkred]
>>>>def test2():[/color][/color]
>
> print x;
>
>
>[color=green][color=darkred]
>>>>test2();[/color][/color]
>
> 10
>
> Any help would be appreciated,
>
> Jens Thiede.[/color]

I suppose because Python prohibits rebinding of global variables, not
modifications. You actually cannot change an integera (as an immutable
type) with rebinding a varibale. Lists (as mutable types) allows
modifications without rebinding. E.g.

l = []

def f1():
l.append(0)

def f2():
l = l + [0]

f1()
print l

f2()
print l

Python says:
[0]
Traceback (most recent call last):
File "1.py", line 12, in ?
f2()
File "1.py", line 7, in f2
l = l + [0]
UnboundLocalErr or: local variable 'l' referenced before assignment

regards,
anton.

[Peter Hansen]

Re: Variable Scope

Jens Thiede wrote:[color=blue]
>
> In the following terminal; could someone inform me as to why it is
> posible to print a global variable without having to declare it using
> global. This has affected some source of mine, and allows you to
> modify a global in a local scope *without* the global keyword, for
> instance, you can append to a global list, but *not* assign it a new
> value, for then, you create a new local variable. -- Why.[/color]

Have you read http://www.python.org/doc/faq/progra...-in-a-function ?

It should clarify some of these issues. If it does not, please let us
know what's still unclear and the FAQ can be improved.

-Peter