xpath-expression 适用于文字但不适用于变量

Question

I have an XML file containing data looking as follows:

<?xml version="1.0" encoding="UTF-8"?>
<document date="2021-03-01" name="dataExport">

<CASHFLOW CASHFLOW_ID="001" C001="06-08"/><CASHFLOW_FIELD CASHFLOW_ID="001" C001="06-08" FIELDNAME="A001" VALUE="0"/><CASHFLOW_FIELD CASHFLOW_ID="001" F001="04-08-01" FIELDNAME="C254" VALUE="15.78"/>

<CASHFLOW CASHFLOW_ID="002" C001="06-08"/><CASHFLOW_FIELD CASHFLOW_ID="001" C001="06-08" FIELDNAME="A001" VALUE="2"/><CASHFLOW_FIELD CASHFLOW_ID="002" C001="06-08" FIELDNAME="C254" VALUE="10.55"/>

</document>

Using XPath, I want - for each entry - to get the value of attribute "VALUE" where attribute "FIELDNAME" has value "C254".

I tried the following to achieve my goal (using python library lxml):

import lxml.etree as etree


global tree
tree = etree.parse('filepath/filename.xml')


cashflow_ids = tree.xpath("//CASHFLOW/@CASHFLOW_ID")

for cashflow_id in cashflow_ids:
    print(cashflow_id)
   
    value_I_want = tree.xpath("//CASHFLOW_FIELD[@CASHFLOW_ID=cashflow_id and @FIELDNAME='C254']/@VALUE")

    print(value_I_want)

Unfortunately, with this code value_I_want equals an empty list in each case.

The problem is the variable "cashflow_id".

If I execute the code for a specific value, it works. For example, the following works fine:

value_I_want = tree.xpath("//CASHFLOW_FIELD[@CASHFLOW_ID='002' and @FIELDNAME='C254']/@VALUE")

(value_I_want in this case equals 10.55)

How can I make the code work with a variable instead of a literal?

Answer 1

You're passing in the string cashflow_id whereas you want to have the value, convert it to a f-string and pass in the variable cashflow_id like this and it works:

value_I_want = tree.xpath(f"//CASHFLOW_FIELD[@CASHFLOW_ID={cashflow_id} and @FIELDNAME='C254']/@VALUE")