Hi Nick,
Global variables in Python are global for *reading*, based in the
precedence order for looking into the namespace: locals, globals(module
scope actually), builtins
for writing, as variables are created on the fly, a local variable will
be created and will mask the global one.
That's why the keyword global "exists"
Do:
global x
whenever there is a variable that you need to *modify*, and that already
exists ourside the function.
In your example:
> Now this
> class T:
> def p(self):
global x
#this line brings x into existence for writing
> x += 1
> print x
>
> if __name__ == '__main__':
> x = 1
> t = T()
> t.p()
>
You'll get rid of the exception and the error.
Hope it helps.
Hugo
Nick Lunt wrote:
> Hi Folks,
>
> messing about with classes I've come across something basic that I don't
> understand.
>
> Take this class
>
> class T:
> def p(self):
> print x
>
> if __name__ == '__main__':
> x = 1
> t = T()
> t.p()
>
> This outputs 1
>
> Now this
> class T:
> def p(self):
> x += 1
> print x
>
> if __name__ == '__main__':
> x = 1
> t = T()
> t.p()
>
> This outputs
> UnboundLocalError: local variable 'x' referenced before assignment
>
> So I tried this
>
> class T:
> def p(self):
> x += 1
> print x
>
> if __name__ == '__main__':
> global x
> x = 1
> t = T()
> t.p()
>
> but that gives me the same UnboundLocalError exception.
>
> This has got me confused. Why does the first example work ok, but not
> the second ?
> And in the third example I get the same error even after declaring x to
> be global.
>
> No doubt the answer is staring me in the face ... but I still can't see it.
>
> Cheers
> Nick .
>
> _______________________________________________
> Tutor maillist - Tutor@python.org
> http://mail.python.org/mailman/listinfo/tutor
>
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