[Python] Pretty print in lxml is failing when I add tags to a parsed tree

I have an xml file that I'm using etree from lxml to work with, but when I add tags to it, pretty printing doesn't seem to work.

>>> from lxml import etree
>>> root = etree.parse('file.xml').getroot()
>>> print etree.tostring(root, pretty_print = True)

<root>
<x>
<y>test1</y>
</x>
</root>


So far so good. But now

>>> x = root.find('x')
>>> z = etree.SubElement(x, 'z')
>>> etree.SubElement(z, 'z1').attrib['value'] = 'val1'
>>> print etree.tostring(root, pretty_print = True)

<root>
<x>
<y>test1</y>
<z><z1 value="val1"/></z></x>
</root>


it's no longer pretty. I've also tried to do it "backwards" where I create the z1 tag, then create the z tag and append z1 to it, then append the z tag to the x tag. But I get the same result.

If I don't parse the file and just create all the tags in one go, it'll print correctly. So I think it has something to do with parsing the file.

How can I get pretty printing to work?

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