Trouble accessing global vars

[Fernando Rodríguez]

Hi,

I haven't used Python in quite some time, and I'm bit puzzled by this:

counter = 0

class Blah(object):
def run(self):
counter += 1

b = Blah()
b.run()

Traceback (most recent call last):
File "<pyshell#53>", line 1, in -toplevel-
b.run()
File "<pyshell#51>", line 3, in run
counter += 1
UnboundLocalError: local variable 'counter' referenced before assignment

However, counter is not a local var, it's a global one. :-? Shouldn't this
work?

[Gary]

On Sat, 04 Sep 2004 17:30:36 +0200, Fernando Rodríguez
<fe*******************@fernando-rodriguez.com> , created a minor stir
when he wrote:

Hi,

I haven't used Python in quite some time, and I'm bit puzzled by this:

counter = 0

class Blah(object):
def run(self): global counter <<----- try this here... counter += 1

b = Blah()
b.run()

Traceback (most recent call last):
File "<pyshell#53>", line 1, in -toplevel-
b.run()
File "<pyshell#51>", line 3, in run
counter += 1
UnboundLocalError: local variable 'counter' referenced before assignment

However, counter is not a local var, it's a global one. :-? Shouldn't this
work?

Check out Byte of Python here, too:
www.python.g2swaroop.net
It's proven very helpful.
--

[Troels Therkelsen]

In article <so********************************@4ax.com>, Fernando Rodríguez wrote:

Hi,

I haven't used Python in quite some time, and I'm bit puzzled by this:

counter = 0

class Blah(object):
def run(self):
counter += 1

b = Blah()
b.run()

Traceback (most recent call last):
File "<pyshell#53>", line 1, in -toplevel-
b.run()
File "<pyshell#51>", line 3, in run
counter += 1
UnboundLocalError: local variable 'counter' referenced before assignment

However, counter is not a local var, it's a global one. :-? Shouldn't this
work?

If you want to modify a global variable from inside a function/method scope,
you need explicitly tell Python that this is indeed your wish, by using the
global keyword, like this:

class Blah(object):
def run(self):
global counter
counter += 1

Hope this helps,

Troels Therkelsen

[Dennis Lee Bieber]

On Sat, 04 Sep 2004 17:30:36 +0200, Fernando Rodríguez
<fe*******************@fernando-rodriguez.com> declaimed the following
in comp.lang.python:

counter = 0

class Blah(object):
def run(self):
global counter
counter += 1

b = Blah()
b.run()
<snip>
UnboundLocalError: local variable 'counter' referenced before assignment

However, counter is not a local var, it's a global one. :-? Shouldn't this
work?
Undeclared globals can be READ from, but any assignment creates
a local of that name. Since

counter += 1

is effectively

counter = counter + 1

the left-hand side, during parsing, flags counter as a local; then, at
run time, it sees a right-hand side "counter" and objects that you
haven't given it an initial value.

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wl*****@ix.netcom.com | Wulfraed Dennis Lee Bieber KD6MOG <
wu******@dm.net | Bestiaria Support Staff <
================================================== ============ <
Home Page: <http://www.dm.net/~wulfraed/> <
Overflow Page: <http://wlfraed.home.netcom.com/> <

[Lenard Lindstrom]

Fernando Rodríguez <fe*******************@fernando-rodriguez.com> writes:

Hi,

I haven't used Python in quite some time, and I'm bit puzzled by this:

counter = 0

class Blah(object):
def run(self):
counter += 1

b = Blah()
b.run()

Traceback (most recent call last):
File "<pyshell#53>", line 1, in -toplevel-
b.run()
File "<pyshell#51>", line 3, in run
counter += 1
UnboundLocalError: local variable 'counter' referenced before assignment

However, counter is not a local var, it's a global one. :-? Shouldn't this
work?

Name counter has to be declared global in method run:

class Blah(object):
def run(self):
global counter
counter += 1

The augmented assignment statements such as counter += 1 bind a name to
a value. Binding a name within a function block makes the variable local
by default.

Lenard Lindstrom
<le***@telus.net>

[Miki Tebeka]

Hello Fernando,

counter = 0

class Blah(object):
def run(self):
counter += 1

b = Blah()
b.run()

Traceback (most recent call last):
File "<pyshell#53>", line 1, in -toplevel-
b.run()
File "<pyshell#51>", line 3, in run
counter += 1
UnboundLocalError: local variable 'counter' referenced before assignment

You need to declare it global using the "global" keyword.

counter = 0

class Blah(object):
def run(self):
counter += 1

b = Blah()
b.run()

Bye.
--
------------------------------------------------------------------------
Miki Tebeka <mi*********@zoran.com>
http://tebeka.spymac.net
The only difference between children and adults is the price of the toys

[Alex Martelli]

Troels Therkelsen <t_**********@hotmail.com> wrote:
...

If you want to modify a global variable from inside a function/method scope,
you need explicitly tell Python that this is indeed your wish, by using the
global keyword, like this:

_MODIFY_ (call a method that performs modification on a mutable object)
would be no problem. (bind or) _REBIND_ a global name, that's the
troublespot where 'global' is needed. The += operator, like any other
assignment, REBINDS the name (even when the object is mutable, so the
change is in-place, nevertheless the name-rebinding occurs, for
uniformity AND since that must be determined by the compiler which can't
rely on knowing the object type).
Alex