I'm adding elements to an xml file.

The document's root is as follows

<Root xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xmlns:xsd="http://www.w3.org/2001/XMLSchema">

And elements to add look like

<Element xsi:type="some type">
 <Sub1>Some text</Sub1>
 <Sub2>More text</Sub2>
 ...
</Element>

I'm trying to find a way for lxml to write 'xsi:' in front of my Element's attibute. This xml file is used by a program to which's source code I do not have access to. I read in a few other questions how to do it by declaring the nsmap of the xml's root, and then again in the child's attribute, which I tried but it didn't work. So far I have (that's what didn't work, the ouput file did not contain the xsi prefix):

element = SubElement(_parent=parent, 
 _tag='some tag', 
 attrib='%stype' % XSI: 'some type'
 nsmap='xsi': XSI) # Where XSI = namespace address

The namespace is declared properly in the xml file I parse, so I don't know why this isn't working.
The output I get is the element as shown above without the 'xsi:' prefix and all on one line:

<Element type="some type"><Sub1>Some text</Sub1><Sub2>More text</Sub2>...</Element>

If anyone can also point out why in this line

self.tree.write(self.filename, pretty_print=True, encoding='utf-8')

the 'pretty_print' option doesn't work (all printed out in one line), it would be greatly appreciated.

Here is a code example of my script:

from math import floor
from lxml import etree
from lxml.etree import SubElement


def Element(root, sub1: str):
 if not isinstance(sub1, str):
 raise TypeError
 else:
 element = SubElement(root, 'Element')
 element_sub1 = SubElement(element, 'Sub1')
 element_sub1.text = sub1
 # ...
 # Omitted additional SubElements
 # ...
 return element


def Sub(root, sub5_sub: str):
 XSI = "http://www.w3.org/2001/XMLSchema-instance"
 if not isinstance(sub5_sub, str):
 raise TypeError
 else:
 sub = SubElement(root, 'Sub5_Sub', '%stype' % XSI: 'SomeType', nsmap='xsi': XSI)
 # ...
 # Omitted additional SubElements
 # ...
 return sub


class Generator:
 def __init__(self) -> None:
 self.filename = None
 self.csv_filename = None
 self.csv_content = []
 self.tree = None
 self.root = None
 self.panel = None
 self.panels = None

 def mainloop(self) -> None:
 """App's mainloop"""
 while True:
 # Getting files from user
 xml_filename = input('Enter path to xml file : ')

 # Parsing files
 csv_content = ['field1': 'ElementSub1', 'field2': 'something',
 'field1': 'ElementSub1', 'field2': 'something',
 'field1': 'ElementSub2', 'field2': 'something'] # Replaces csv file that I use
 tree = etree.parse(xml_filename)
 root = tree.getroot()

 elements = root.find('Elements')

 for element in elements:
 if element.find('Sub1').text in ['ElementSub1', 'ElementSub2']:
 for line in csv_content:
 if element.find('Sub5') is not None:
 Sub(root=element.find('Sub5'),
 sub5_sub=line['field2'])

 tree.write(xml_filename, pretty_print=True, encoding='utf-8')

 if input('Continue? (Y) Quit (n)').upper().startswith('Y'):
 elements.clear()
 continue
 else:
 break

 @staticmethod
 def get_x(x: int) -> str:
 if not isinstance(x, int):
 x = int(x)
 return str(int(floor(9999 / 9 * x)))

 @staticmethod
 def get_y(y: int) -> str:
 if not isinstance(y, int):
 y = int(y)
 return str(int(floor(999 / 9 * y)))

 def quit(self) -> None:
 quit()


if __name__ == "__main__":
 app = Generator()
 app.mainloop()
 app.quit()

Here is what it outputs:

<Root xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xmlns:xsd="http://www.w3.org/2001/XMLSchema">
 <Elements>
 <Element>
 <Sub1>ElementSub1</Sub1>
 <Sub5>
 <Sub5_Sub xsi:type="SomeType"/>
 <Sub5_Sub xsi:type="SomeType"/><Sub5_Sub xsi:type="SomeType"/><Sub5_Sub xsi:type="SomeType"/></Sub5>
 </Element>
 <Element>
 <Sub1>ElementSub1</Sub1>
 <Sub5>
 <Sub5_Sub xsi:type="SomeType"/>
 <Sub5_Sub xsi:type="SomeType"/>
 <Sub5_Sub xsi:type="SomeType"/><Sub5_Sub xsi:type="SomeType"/><Sub5_Sub xsi:type="SomeType"/></Sub5>
 </Element>
 <Element>
 <Sub1>ElementSub1</Sub1>
 </Element>
 </Elements>
</Root>

For some reason, this piece of code does what I want but my real code doesn't. I've come to realize that it does put a prefix on some sub elements with the type attribute, but not all and on those it puts the prefix, it isn't always just 'xsi:'. I found a quick and dirty way to fix this problem which is less than ideal (find and replace through the file for xsi-type -> accepted by lxml's api to xsi:type). What still isn't working though is that it's all printed out in one line despite the pretty_print parameter being true.

I just recently encountered this scenario and was able to successfully create an attribute with the xsi:

qname = etree.QName("http://www.w3.org/2001/XMLSchema-instance", "type")
element = etree.Element('Element', {qname: "some type")

root.append(element)

this outputs something like

<Element xsi:type="some type">