Create Smaller XML based on value of element - lxml

Question

On Python 3.7, I am looking to create a subset of a XML. For example, the larger XML is:
<data>
<student>
<result>
<grade>A</grade>
</result>
<details>
<name>John</name>
<id>100</id>
<age>16</age>
<email>john#mail.com</email>
</details>
</student>
<student>
<result>
<grade>B</grade>
</result>
<details>
<name>Alice</name>
<id>101</id>
<age>17</age>
<email>alice#mail.com</email>
</details>
</student>
<student>
<result>
<grade>F</grade>
</result>
<details>
<name>Bob</name>
<id>102</id>
<age>16</age>
<email>bob#mail.com</email>
</details>
</student>
<student>
<result>
<grade>A</grade>
</result>
<details>
<name>Hannah</name>
<id>103</id>
<age>17</age>
<email>hannah#mail.com</email>
</details>
</student>
</data>
and am looking for a new XML like below, the condition to create a smaller subset depends on a list of ids in this case 101 and 102. All other student blocks will be deleted.
<data>
<student>
<result>
<grade>B</grade>
</result>
<details>
<name>Alice</name>
<id>101</id>
<age>17</age>
<email>alice#mail.com</email>
</details>
</student>
<student>
<result>
<grade>F</grade>
</result>
<details>
<name>Bob</name>
<id>102</id>
<age>16</age>
<email>bob#mail.com</email>
</details>
</student>
</data>
i.e. The output XML will depend on a list of id's, in this case ['101',102']
This is what I tried:
import lxml.etree
#Original Large XML
tree = etree.parse(open('students.xml'))
root = tree.getroot()
results = root.findall('student')
textnumbers = [r.find('details/id').text for r in results]
print(textnumbers)
required_ids = ['101','102']
wanted = tree.xpath("//student/details/[not(#id in required_ids)]")
for node in unwanted:
node.getparent().remove(node)
#New Smaller XML
tree.write(open('student_output.xml', 'wb'))
But I am getting an expected error of "Invalid expression" for
wanted = tree.xpath("//student/details/[not(#id in required_ids)]")
I know it's a read, but i am fairly new to Python, thanks in advance for your help.

Answer 1

I think you can do it like this:
from lxml import etree as ET
required_ids = ['101','102']
for event, element in ET.iterparse('students.xml'):
if element.tag == 'student' and not(element.xpath('.//id/text()')[0] in required_ids):
element.clear()
element.getparent().remove(element)
if element.tag == 'data':
ET.dump(element)
Instead of the dump you would of course want to write to a file, that is use
if element.tag == 'data':
tree = ET.ElementTree(element)
tree.write('student_output.xml')
Your attempt fails as you can't simply use a Python list variable in XPath and in is not an XPath 1.0 operator.