How do I set attributes for an XML element with Python?

You can specify attributes for an Element or SubElement during creation with keyword arguments.

import xml.etree.ElementTree as ET

root = ET.Element('Summary')
ET.SubElement(root, 'TextSummary', Status='Completed')

XML:

<Summary>
    <TextSummary Status="Completed"/>
</Summary>

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Alternatively, you can use .set to add attributes to an existing element.

import xml.etree.ElementTree as ET

root = ET.Element('Summary')
sub = ET.SubElement(root, 'TextSummary')
sub.set('Status', 'Completed')

XML:

<Summary>
    <TextSummary Status="Completed"/>
</Summary>

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Technical Explanation:

The constructors for Element and SubElement include **extra, which accepts attributes as keyword arguments.

xml.etree.ElementTree.Element(tag, attrib={}, **extra)
xml.etree.ElementTree.SubElement(parent, tag, attrib={}, **extra)

This allows you to add an arbitrary number of attributes.

root = ET.Element('Summary', Date='2018/07/02', Timestamp='11:44am')
# <Summary Date = "2018/07/02" Timestamp = "11:44am">

You can also use use .set to add attributes to a pre-existing element. However, this can only add one element at a time. (As suggested by Thomas Orozco).

root = ET.Element('Summary')
root.set('Date', '2018/07/02')
root.set('Timestamp', '11:44am')
# <Summary Date = "2018/07/02" Timestamp = "11:44am">

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Full Example:

import xml.etree.ElementTree as ET

root = ET.Element('school', name='Willow Creek High')
ET.SubElement(root, 'student', name='Jane Doe', grade='9')
print(ET.tostring(root).decode())
# <school name="Willow Creek High"><student grade="9" name="Jane Doe" /></school>

You should be doing:

ET.SubElement(root,'TextSummary').set('Status','Completed')

The Etree documentation shows usage.