ElementTree write attribute Not defined

I am kinda lost, can't figure out why it is giving me this Attribute error. I am running python 2.7.8 I am trying to test my Classes write to file and calling the ElementTree's write. However it says the write function isn't defined.

>>> import BMECATpreprocess as Bpp
>>>
>>> fibo = Bpp.BMECATpreprocess()
>>> fibo.filename()
Bitte geben Sie den Dateiname inklusive Phad ein: Zoro_CatMap.xml
'Zoro_CatMap.xml'
>>> fibo.writefile()
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "BMECATpreprocess.py", line 140, in writefile

AttributeError: 'str' object has no attribute 'write'

my BMECATpreprocess class looks like this:

import xml.etree.ElementTree as ET
tree = ''
root = ''
class BMECATpreprocess():
    #init 
    def __init__(self, keytag='', childkeytag='', tree='', write=''):
        self.keytag = keytag
        self.childkeytag = childkeytag
        self.tree = tree
        self.write = write

    #ask for filename/location  
    def filename(self):
        data = raw_input("Bitte geben sie den Datei Name inklusive Phad ein: ")
        return data

    def opentree(self): #open files and initialize the xml file for use
        global tree 
        tree = ET.parse(self.filename())


    def getroot(self):
        global root 
        root = tree.getroot()
        return root

    def writefile(self):
        tree.write(self.filename(), encoding="UTF-8", xml_declaration=True)