I would use lxml (see http://lxml.de/api.html).

Then you can:

from lxml import etree
document = etree.Element('outer')
node = etree.SubElement(document, 'inner')
print(etree.tostring(document, xml_declaration=True))

I am surprised to find that there doesn't seem to be a way with ElementTree.tostring(). You can however use ElementTree.ElementTree.write() to write your XML document to a fake file:

from io import BytesIO
from xml.etree import ElementTree as ET


document = ET.Element('outer')
node = ET.SubElement(document, 'inner')
et = ET.ElementTree(document)


f = BytesIO()
et.write(f, encoding='utf-8', xml_declaration=True)
print(f.getvalue())  # your XML file, encoded as UTF-8

See this question. Even then, I don't think you can get your 'standalone' attribute without writing prepending it yourself.

I encounter this issue recently, after some digging of the code, I found the following code snippet is definition of function ElementTree.write

def write(self, file, encoding="us-ascii"):
assert self._root is not None
if not hasattr(file, "write"):
file = open(file, "wb")
if not encoding:
encoding = "us-ascii"
elif encoding != "utf-8" and encoding != "us-ascii":
file.write("<?xml version='1.0' encoding='%s'?>\n" %
encoding)
self._write(file, self._root, encoding, {})

So the answer is, if you need write the XML header to your file, set the encoding argument other than utf-8 or us-ascii, e.g. UTF-8

I would use ET:

try:
from lxml import etree
print("running with lxml.etree")
except ImportError:
try:
# Python 2.5
import xml.etree.cElementTree as etree
print("running with cElementTree on Python 2.5+")
except ImportError:
try:
# Python 2.5
import xml.etree.ElementTree as etree
print("running with ElementTree on Python 2.5+")
except ImportError:
try:
# normal cElementTree install
import cElementTree as etree
print("running with cElementTree")
except ImportError:
try:
# normal ElementTree install
import elementtree.ElementTree as etree
print("running with ElementTree")
except ImportError:
print("Failed to import ElementTree from any known place")


document = etree.Element('outer')
node = etree.SubElement(document, 'inner')
print(etree.tostring(document, encoding='UTF-8', xml_declaration=True))

This works if you just want to print. Getting an error when I try to send it to a file...

import xml.dom.minidom as minidom
import xml.etree.ElementTree as ET
from xml.etree.ElementTree import Element, SubElement, Comment, tostring


def prettify(elem):
rough_string = ET.tostring(elem, 'utf-8')
reparsed = minidom.parseString(rough_string)
return reparsed.toprettyxml(indent="  ")

If you include the encoding='utf8', you will get an XML header:

xml.etree.ElementTree.tostring writes a XML encoding declaration with encoding='utf8'

Sample Python code (works with Python 2 and 3):

import xml.etree.ElementTree as ElementTree


tree = ElementTree.ElementTree(
ElementTree.fromstring('<xml><test>123</test></xml>')
)
root = tree.getroot()


print('without:')
print(ElementTree.tostring(root, method='xml'))
print('')
print('with:')
print(ElementTree.tostring(root, encoding='utf8', method='xml'))

Python 2 output:

$ python2 example.py
without:
<xml><test>123</test></xml>


with:
<?xml version='1.0' encoding='utf8'?>
<xml><test>123</test></xml>

With Python 3 you will note the b prefix indicating byte literals are returned (just like with Python 2):

$ python3 example.py
without:
b'<xml><test>123</test></xml>'


with:
b"<?xml version='1.0' encoding='utf8'?>\n<xml><test>123</test></xml>"

The minimal working example with ElementTree package usage:

import xml.etree.ElementTree as ET


document = ET.Element('outer')
node = ET.SubElement(document, 'inner')
node.text = '1'
res = ET.tostring(document, encoding='utf8', method='xml').decode()
print(res)

the output is:

<?xml version='1.0' encoding='utf8'?>
<outer><inner>1</inner></outer>

Including 'standalone' in the declaration

I didn't found any alternative for adding the standalone argument in the documentation so I adapted the ET.tosting function to take it as an argument.

from xml.etree import ElementTree as ET


# Sample
document = ET.Element('outer')
node = ET.SubElement(document, 'inner')
et = ET.ElementTree(document)


# Function that you need
def tostring(element, declaration, encoding=None, method=None,):
class dummy:
pass
data = []
data.append(declaration+"\n")
file = dummy()
file.write = data.append
ET.ElementTree(element).write(file, encoding, method=method)
return "".join(data)
# Working example
xdec = """<?xml version="1.0" encoding="UTF-8" standalone="no" ?>"""
xml = tostring(document, encoding='utf-8', declaration=xdec)

Another pretty simple option is to concatenate the desired header to the string of xml like this:

xml = (bytes('<?xml version="1.0" encoding="UTF-8"?>\n', encoding='utf-8') + ET.tostring(root))
xml = xml.decode('utf-8')
with open('invoice.xml', 'w+') as f:
f.write(xml)

Easy

Sample for both Python 2 and 3 (encoding parameter must be utf8):

import xml.etree.ElementTree as ElementTree


tree = ElementTree.ElementTree(ElementTree.fromstring('<xml><test>123</test></xml>'))
root = tree.getroot()
print(ElementTree.tostring(root, encoding='utf8', method='xml'))

From Python 3.8 there is xml_declaration parameter for that stuff:

New in version 3.8: The xml_declaration and default_namespace parameters.

xml.etree.ElementTree.tostring(element, encoding="us-ascii", method="xml", *, xml_declaration=None, default_namespace=None, short_empty_elements=True) Generates a string representation of an XML element, including all subelements. element is an Element instance. encoding 1 is the output encoding (default is US-ASCII). Use encoding="unicode" to generate a Unicode string (otherwise, a bytestring is generated). method is either "xml", "html" or "text" (default is "xml"). xml_declaration, default_namespace and short_empty_elements has the same meaning as in ElementTree.write(). Returns an (optionally) encoded string containing the XML data.

Sample for Python 3.8 and higher:

import xml.etree.ElementTree as ElementTree


tree = ElementTree.ElementTree(ElementTree.fromstring('<xml><test>123</test></xml>'))
root = tree.getroot()
print(ElementTree.tostring(root, encoding='unicode', method='xml', xml_declaration=True))

xml_declaration Argument

Is there a proper method for rendering the XML declaration in an ElementTree?

YES, and there is no need of using .tostring function. According to ElementTree Documentation, you should create an ElementTree object, create Element and SubElements, set the tree's root, and finally use xml_declaration argument in .write function, so the declaration line is included in output file.

You can do it this way:

import xml.etree.ElementTree as ET


tree = ET.ElementTree("tree")


document = ET.Element("outer")
node1 = ET.SubElement(document, "inner")
node1.text = "text"


tree._setroot(document)
tree.write("./output.xml", encoding = "UTF-8", xml_declaration = True)

And the output file is:

<?xml version='1.0' encoding='UTF-8'?>
<outer><inner>text</inner></outer>