[Python] How to replace xml node value in Python, without changing the whole file

Doing my first steps in python I try to parse and update a xml file. The xml is as follows:

<?xml version="1.0" encoding="utf-8"?>
<?xml-stylesheet href="util/style/aaaa-2-0.xsl" type="text/xsl"?>
<test dtd-version="3.2" xmlns:test="http://www.ich.org/test" xmlns:xlink="http://www.w3c.org/1999/xlink">
<mr>
<leaf checksum="88ed245997a341a4c7d1e40d614eb14f" >
<title>book name</title>
</leaf>
</mr>
</test>


I would like to update the value of the checksum.I have written a class with one method:

@staticmethod
def replace_checksum_in_index_xml(xml_file_path, checksum):
logging.debug(f"ReplaceChecksumInIndexXml xml_file_path: {xml_file_path}")
try:
from xml.etree import ElementTree as et
tree = et.parse(xml_file_path)
tree.find('.//leaf').set("checksum", checksum)
tree.write(xml_file_path)
except Exception as e:
logging.error(f"Error updating checksum in {xml_file_path}: {e}")


I call the method:

xml_file_path = "index.xml"
checksum = "aaabbb"
Hashes.replace_checksum_in_index_xml(xml_file_path, checksum)


The checksum is indeed updated. But also the whole xml structure is changed:

<test dtd-version="3.2">
<mr>
<leaf checksum="aaabbb">
<title>book name</title>
</leaf>
</mr>
</test>


How can I update only given node, without changing anything else in given xml file?

UPDATE

Solution provided by LRD_Clan is better than my original one. But it is still changing a bit the structure of xml. Also when I take more complex example I see again part of xml is removed. More complex example with additional DOCTYPE:

<?xml version="1.0" encoding="utf-8"?>
<!DOCTYPE eu:eu-backbone SYSTEM "../../util/dtd/eu-regional.dtd"[]>
<?xml-stylesheet href="util/style/aaaa-2-0.xsl" type="text/xsl"?>

<test dtd-version="3.2" xmlns:test="http://www.ich.org/test" xmlns:xlink="http://www.w3c.org/1999/xlink">
<mr>
<leaf checksum="88ed245997a341a4c7d1e40d614eb14f" >
<title>book name</title>
</leaf>

</mr>
</test>


after running updated script the result is:

<?xml version='1.0' encoding='UTF-8'?>
<?xml-stylesheet href="util/style/aaaa-2-0.xsl" type="text/xsl"?><test xmlns:test="http://www.ich.org/test" xmlns:xlink="http://www.w3c.org/1999/xlink" dtd-version="3.2">
<mr>
<leaf checksum="aaabbb">
<title>book name</title>
</leaf>
</mr>
</test>


I would really like to see only this one xml element being changed and the other part of document to be left intact.

I had similiar solution written in powershell which looks like

[xml]$xmlContent = Get-Content -Path $xmlFilePath
$element = $xmlContent.SelectSingleNode("//leaf")
$element.SetAttribute("checksum, "new text")
$xmlContent.Save((Resolve-Path "$xmlFilePath").Path)


I was hoping I will find something at least same elegant in python.

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