Keep Existing Namespaces when overwriting XML file with ElementTree and Python

Question

I have an XML file in the following format

<?xml version="1.0" encoding="utf-8"?>
<foo>
   <bar>
      <bat>1</bat>
   </bar>
   <a>
      <b xmlns="urn:schemas-microsoft-com:asm.v1">
         <c>1</c>
      </b>
   </a>
</foo>

I want to change the value of bat to '2' and change the file to this:

<?xml version="1.0" encoding="utf-8"?>
<foo>
   <bar>
      <bat>2</bat>
   </bar>
   <a>
      <b xmlns="urn:schemas-microsoft-com:asm.v1">
         <c>1</c>
      </b>
   </a>
</foo>

I open this file by doing this

tree = ET.parse(filePath)
root = tree.getroot()

I then change the value of bat to '2' and save the file like this:

tree.write(filePath, "utf-8", True, None, "xml")

The value of bat successfully changes to 2, but the XML file now looks like this.

<?xml version="1.0" encoding="utf-8"?>
<foo xmlns:ns0="urn:schemas-microsoft-com:asm.v1">
   <bar>
      <bat>2</bat>
   </bar>
   <a>
      <ns0:b>
         <ns0:c>1</ns0:c>
      </ns0:b>
   </a>
</foo>

In order to fix the issue of having a namespace named ns0, I do the following before parsing the document

ET.register_namespace('', "urn:schemas-microsoft-com:asm.v1")

This gets rid of the ns0 namepace but the xml file now looks like this

<?xml version="1.0" encoding="utf-8"?>
<foo xmlns="urn:schemas-microsoft-com:asm.v1">
   <bar>
      <bat>2</bat>
   </bar>
   <a>
      <b>
         <c>1</c>
      </b>
   </a>
</foo>

What do I do to get the output I need?

Answer 1

As far as i know there isn't a way by the means of xml.etree.ElementTree methods to achieve your goal. By digging in the xml.etree source code and the xml specification I found that the library behaviour is not wrong, nor unreasonable. Anyway it does not allows the output you are looking for.

To achieve your goal using that library you have to customize rendering behaviour. To best suite your needs I have written the following render function.

from xml.etree import ElementTree as ET
from re import findall, sub

def render(root, buffer='', namespaces=None, level=0, indent_size=2, encoding='utf-8'):
    buffer += f'<?xml version="1.0" encoding="{encoding}" ?>\n' if not level else ''
    root = root.getroot() if isinstance(root, ET.ElementTree) else root
    _, namespaces = ET._namespaces(root) if not level else (None, namespaces)
    for element in root.iter():
        indent = ' ' * indent_size * level
        tag = sub(r'({[^}]+}\s*)*', '', element.tag)
        buffer += f'{indent}<{tag}'
        for ns in findall(r'{[^}]+}', element.tag):
            ns_key = ns[1:-1]
            if ns_key not in namespaces: continue
            buffer += ' xmlns' + (f':{namespaces[ns_key]}' if namespaces[ns_key] != '' else '') + f'="{ns_key}"'
            del namespaces[ns_key]
        for k, v in element.attrib.items():
            buffer += f' {k}="{v}"'
        buffer += '>' + element.text.strip() if element.text else '>'
        children = list(element)
        for child in children:
            sep = '\n' if buffer[-1] != '\n' else ''
            buffer += sep + render(child, level=level+1, indent_size=indent_size, namespaces=namespaces)
        buffer += f'{indent}</{tag}>\n' if 0 != len(children) else f'</{tag}>\n'
    return buffer

By supplying to the above render() function your xml input data as follows:

data =\ 
'''<?xml version="1.0" encoding="utf-8"?>
<foo>
   <bar>
      <bat>1</bat>
   </bar>
   <a>
      <b xmlns="urn:schemas-microsoft-com:asm.v1">
         <c>1</c>
      </b>
   </a>
</foo>'''

root = ET.ElementTree(ET.fromstring(data))
ET.register_namespace('', "urn:schemas-microsoft-com:asm.v1")
print(render(root))

It prints out the output your are looking for:

<?xml version="1.0" encoding="utf-8" ?>
<foo>
  <bar>
    <bat>1</bat>
  </bar>
  <a>
    <b xmlns="urn:schemas-microsoft-com:asm.v1">
      <c>1</c>
    </b>
  </a>
</foo>