If you use lxml library and need to pretty print XML, here is a snippet that works even if you have existing indentation in your XML.

#!/usr/bin/env python
import sys
import StringIO

from lxml import etree

def main():
    xml_text = sys.stdin.read()
    parser = etree.XMLParser(remove_blank_text=True)
    file_obj = StringIO.StringIO(xml_text)
    tree = etree.parse(file_obj, parser)
    print(etree.tostring(tree, pretty_print=True))

if __name__ == '__main__':

Save the above code to a file ppxml and make it executable.
After that you could use it on the command line like this, for example:

$ cat << EOF | ppxml
> <root>
> <child1/>
> <child2/>
>        <child3/>
>   </root>

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