Unexpected global variable behaviour

[RgeeK]

I'm seeing something which make me think I'm missing something  about 
how global var's behave.  I've defined a global string, right at the 
start of my .py file.

outXMLfile = "abc"

I define a class and do a bunch of stuff below that.  Then I have 
another class, and in it, there is a method 'def' that has:

   def OnOutfileButton(self,evt):
    (fPath, fName)=os.path.split(fullName)
     print "Selected output file: " + fName
     outXMLfile = fName

     print "output file: " + outXMLfile

Print statements in random other places in the project, show outXMLfile 
prints as "abc"  however, in this def, it comes out as the same as fName 
(e.g. "myfile.xml")   If the print line for outXMLfile is before the 
assignment to fName, it throws the error:

  UnboundLocalError: local variable 'outXMLfile' referenced before 
assignment

If I remove the line that says "outXMLfile = fName"
then the print statement gives me the value "abc"

What am I missing?  When I assign a value to update my global variable, 
it becomes a local variable. If I don't try to update it, it stays 
global.  I assume it's acting like a constant, though I have a couple of 
global lists and I seem to be able to append to them okay.

I guess I can move the variable into the parent class and get at it that 
way, but want to understand the error of my ways...

Ross.