参照: http://eli.thegreenplace.net/2012/03/15/processing-xml-in-python-with-elementtree/を使用
エラールートにはextend属性がありません
import xml.etree.cElementTree as ET
import sys
a = ET.Element('quotationinput version="1.0"')
b = ET.SubElement(a, 'authorisation')
c = ET.SubElement(b, 'password')
c.text = pwd
d = ET.SubElement(a, 'productdetail')
e = ET.SubElement(d, 'producttype')
e.text = ""
f = ET.SubElement(d, 'accommodationCode')
f.text = ""
g = ET.SubElement(d, 'startDate')
g.text = start_date
h = ET.SubElement(d, 'duration')
h.text = duration
i = ET.SubElement(d, 'unitCode')
i.text = unit
j = ET.SubElement(a, 'partydetail')
k = ET.SubElement(j, 'adultCount')
k.text = adults
l = ET.SubElement(j, 'childCount')
l.text = children
m = ET.SubElement(j, 'babyCount')
m.text = baby
n = ET.SubElement(j, 'petCount')
n.text = pets
root = ET.Element('root')
root.extend([a])
tree = ET.ElementTree(root)
tree.write(sys.stdout)