Python: Update XML-file using ElementTree while conserving layout as much as possible

Question

I have a document which uses an XML namespace for which I want to increase /group/house/dogs by one: (the file is called houses.xml)

Answer 1

An XML based solution to this problem is to write a helper class for ElementTree which:

• Grabs the XML-declaration line before parsing as ElementTree at the time of writing is unable to write an XML-declaration line without also writing an encoding attribute(I checked the source).
• Parses the input file once, grabs the namespace of the root element. Registers that namespace with ElementTree as having the empty string as prefix. When that is done the source file is parsed using ElementTree again, with that new setting.

It has one major drawback:

• XML-comments are lost. Which I have learned is not acceptable for this situation(I initially didn´t think the input data had any comments, but it turns out it has).

My helper class with example:

from xml.etree import ElementTree as ET
import re


class ElementTreeHelper():
    def __init__(self, xml_file_name):
        xml_file = open(xml_file_name, "rb")

        self.__parse_xml_declaration(xml_file)

        self.element_tree = ET.parse(xml_file)
        xml_file.seek(0)

        root_tag_namespace = self.__root_tag_namespace(self.element_tree)
        self.namespace = None
        if root_tag_namespace is not None:
            self.namespace = '{' + root_tag_namespace + '}'
            # Register the root tag namespace as having an empty prefix, as
            # this has to be done before parsing xml_file we re-parse.
            ET.register_namespace('', root_tag_namespace)
            self.element_tree = ET.parse(xml_file)

    def find(self, xpath_query):
        return self.element_tree.find(xpath_query)

    def write(self, xml_file_name):
        xml_file = open(xml_file_name, "wb")
        if self.xml_declaration_line is not None:
            xml_file.write(self.xml_declaration_line + '\n')

        return self.element_tree.write(xml_file)

    def __parse_xml_declaration(self, xml_file):
        first_line = xml_file.readline().strip()
        if first_line.startswith(''):
            self.xml_declaration_line = first_line
        else:
            self.xml_declaration_line = None
        xml_file.seek(0)

    def __root_tag_namespace(self, element_tree):
        namespace_search = re.search('^{(\S+)}', element_tree.getroot().tag)
        if namespace_search is not None:
            return namespace_search.group(1)
        else:
            return None


def __main():
    el_tree_hlp = ElementTreeHelper('houses.xml')

    dogs_tag = el_tree_hlp.element_tree.getroot().find(
                   '{ns}house/{ns}dogs'.format(
                         ns=el_tree_hlp.namespace))
    one_dog_added = int(dogs_tag.text.strip()) + 1
    dogs_tag.text = str(one_dog_added)

    el_tree_hlp.write('hejsan.xml')

if __name__ == '__main__':
    __main()

The output:

    
            2821
            3
    

If someone has an improvement to this solution please don´t hesitate to grab the code and improve it.