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]

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]

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]

counter = 0

class Blah(object):
def run(self):

global counter

counter += 1

b = Blah()
b.run()

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.

--

 

[Lenard Lindstrom]

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
<[email protected]>

 

[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.

 

[Alex Martelli]

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