parsing a xml file with multiple root components with python - xml-parsing

Question

My xml file is test.xml. See below
<?xml version="1.0" ?>
<Output>
<partialstore /> <!-- The code writes the spectrum at Sun position for each species on a FITS file (optional) -->
<fullstore /> <!-- The code writes the complete (r,z,p) grid of propagated particles for each species on a FITS file (optional) -->
<feedback value="2" /> </Output>
<Diffusion type="Constant"> <!-- Spatial distribution of the diffusion coefficient; options: Constant, Exp, Qtau -->
<!-- In Constant mode: D(Rigidity) = beta^etaT * D0 * (Rigidity/4GV)^delta -->
<!-- In Exp mode: D(Rigidity,z) = beta^etaT * D0 * (Rigidity/4GV)^delta * exp(z/zt) -->
<D0_1e28 value="2.7" /> <!-- Normalization of the diffusion coefficient at reference rigidity DiffRefRig Unit: 10^28 cm^2/s -->
<DiffRefRig value = "4" /> <!-- Reference rigidity for the normalization of the diffusion coefficient -->
<!-- NOTE: the reference rigidity 4 GV is stored in the const D_ref_rig defined in include/constants.h -->
<Delta value="0.6" /> <!-- Slope of the diffusion coefficient spectrum -->
<zt value="4" /> <!-- Scale heigth of the diffusion coefficient, useful in Exp mode: D(z) \propto exp(z/zt) (optional) -->
<etaT value="1." /> <!-- Low energy correction factor of the diffusion coefficient: D \propto beta^etaT --> </Diffusion>
I want to parse this xml with python. There are two root elements output and diffusion. I want to add 0.6 with D0_1e28 value and Delta_value so that those become 3.1 and 1.2 respectively and save the test.xml as a test_mod.xml with the modification. How can I do it with python code?

Answer 1

If I understand you correctly, this should work, using xpath with lxml:
xml = """
<root>
[your xml above; note that it was an invalid xml file, so I added a root]
</root>
"""
import lxml.etree as et
tree = et.fromstring(xml)
to_change = ['//Diffusion/D0_1e28','//Delta'] #those are the items you are changing
for i in range(len(to_change)):
item=tree.xpath(to_change[i])[0] #xpath returns a list of 1 item, so [0] selects it
item.attrib['value']= str(float(item.attrib['value'])+0.6)
#the "value" attribute has a value of type 'string' which needs to be cast to
#'float' to increment it by 0.6, and then back to 'string` to change the value
#of the 'value' attribute
mydata=et.tostring(tree)
with open("test_mod.xml", "wb") as myfile:
myfile.write(mydata)