How to address a global variable in a function

[n179911]

HI,

I have a global variable

// line 8
tx  = 0

and then I have this function (start in line 12):
def handleTranslate(result):
	print line
	txStr, tyStr = result.group(1), result.group(2)
	print txStr, tyStr

	tx += int(txStr)
	ty += int(tyStr)

	return

But I am getting this error:
    handleTranslate(result)
  File "buildsvg.py", line 22, in handleTranslate
    tx += int(txStr)
UnboundLocalError: local variable 'tx' referenced before assignment

How can I fix it?
I have assigned 'tx' to 0 in line 8. I don't understand why i get this
error.

Thank you for any help.