Python - global name is not defined when calling function from class __init__ method

I am new to Python and working on a class for storing meaningful data pieces about books. I have started as follows:

 class BookDisplay: 

     def __init__(self, _name, _isbn, _price, _picture, _link):
      self.name =    _name
      self.isbn =    _isbn
      self.price =   _price
      self.picture = _picture
      self.link =    _link
      self.xmlString = MakeXMLString(_name, _isbn, _price, _picture, _link)                    

    name = ""
    isbn = ""
    price = 0.0
    picture = ""  #a URL
    link = "" 

    xmlString = ""

I thought this __init__ method would just be able to call MakeXMLString, which I defined in the same file (bookdisplay.py), right below the BookDisplay class:

  def MakeXMLString(_name, _isbn, _price, _picture, _link): #python multi-line syntax  
    xmlString = "<name>" + _name + "</name>" \
                     + "<isbn>" + _isbn + "</isbn>" \
                     + "<price>" + str(_price) + "</price>" \
                     + "<picture>" + _picture + "</picture>" \
                     + "<link>" + _link + "</link>" 

    return xmlString 

Originally, I actually had MakeXMLString as a method within the class, like this:

 def MakeXMLString(self):  
    self.xmlString = "<name>" + self.name + "</name>" \
                     + "<isbn>" + self.isbn + "</isbn>" \
                     + "<price>" + str(self.price) + "</price>" \
                     + "<picture>" + self.picture + "</picture>" \
                     + "<link>" + self.link + "</link>" 

In that case, __init__ contained this call:

self.xmlString = self.MakeXMLString()

In both cases, when trying to instantiate BookDisplay from another file:

from bookdisplay import BookDisplay
...
...
thumbnails = []
...
thumbnails.append(BookDisplay(titleField, "-1", float(priceField), imgField, linkField))

...I get the following global name error (this traceback in particular for the not-within-class function):

Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "search.py", line 30, in ebaySearch
    handleDocument(doc)
  File "search.py", line 59, in handleDocument
    handleItems(items, outputFile)
  File "search.py", line 102, in handleItems
    thumbnails.append(BookDisplay(titleField, "-1", float(priceField), imgField, linkField))
  File "bookdisplay.py", line 15, in __init__
    self.xmlString = MakeXMLString(_name, _isbn, _price, _picture, _link) 
NameError: global name 'MakeXMLString' is not defined

What am I missing here? From what I can tell, MakeXMLString is perfectly accessible to the class.