How to correctly use escape characters to write a mathematical string to a XML file using Python?

I have an XML file which I am trying to edit using Python. This is an example of the content of the XML file:

<?xml version="1.0" encoding="utf-8"?>
<NEKTAR>
    <CONDITIONS>
        <FUNCTION NAME="Baseflow">
            <E VAR="u0" VALUE="(x>3) * (y <= 3) * 4 * x"/>
        </FUNCTION>
    </CONDITIONS>
</NEKTAR>

In the original file, the value for u0 is set to 0. Using Python, I want to change this value to the mathematical expression shown in the code-block. I have tried two approaches un-successfully:

Write the in-equalities using the escape characters such as < and >
Try to use CDATA

What would be the right approach to do something like this using xml.etree.ElementTree ?

Solution

Edit

The op needs a version that doesn't escape and leave the equation as is. This can be done with string replacement by regular expressions (re). Beware that the processed string is never a valid XML and cannot be parsed by any xml package by far as I know.

import re

ORIGINAL = """<?xml version="1.0" encoding="utf-8"?>
<NEKTAR>
    <CONDITIONS>
        <FUNCTION NAME="Baseflow">
            <E VAR="u0" VALUE="0"/>
        </FUNCTION>
    </CONDITIONS>
</NEKTAR>"""

TARGET = '(x>3) * (y <= 3) * 4 * x'

replaced_match = re.search(r'VALUE="(.+?)"', ORIGINAL)
if replaced_match:
    result = ORIGINAL[:replaced_match.start(1)]
    result += TARGET
    result += ORIGINAL[replaced_match.end(1):]
print(result)

"""
<?xml version="1.0" encoding="utf-8"?>
<NEKTAR>
    <CONDITIONS>
        <FUNCTION NAME="Baseflow">
            <E VAR="u0" VALUE="(x>3) * (y <= 3) * 4 * x"/>
        </FUNCTION>
    </CONDITIONS>
</NEKTAR>
"""

xml package automatically do the escaping jobs for you. Does this satisfy your need? (replace the VALUE of E tag with given equation that needs escaping)

from xml.etree import ElementTree

ORIGINAL = """<?xml version="1.0" encoding="utf-8"?>
<NEKTAR>
    <CONDITIONS>
        <FUNCTION NAME="Baseflow">
            <E VAR="u0" VALUE="0"/>
        </FUNCTION>
    </CONDITIONS>
</NEKTAR>"""

TARGET = '(x>3) * (y <= 3) * 4 * x'

x = ElementTree.fromstring(ORIGINAL)

# find all E tags
els = x.findall('.//E')

for el in els:
    el.set('VALUE', TARGET)
x_mod = ElementTree.tostring(x).decode()

print(x_mod)

result:

<NEKTAR>
    <CONDITIONS>
        <FUNCTION NAME="Baseflow">
            <E VAR="u0" VALUE="(x&gt;3) * (y &lt;= 3) * 4 * x" />
        </FUNCTION>
    </CONDITIONS>
</NEKTAR>