How do I save a modified HTML or XML tree back to a file with lxml?

After modifying an HTML or XML tree with lxml, you can save it back to a file using the write() method. This guide covers the different approaches for XML and HTML documents.

Basic XML Example

from lxml import etree

# Parse the XML file
tree = etree.parse('example.xml')
root = tree.getroot()

# Modify the tree - add a new element
new_element = etree.SubElement(root, "new_element")
new_element.text = "This is a new element"
new_element.set("id", "123")  # Add attribute

# Save the modified tree
tree.write('modified_example.xml', 
           pretty_print=True, 
           xml_declaration=True, 
           encoding="UTF-8")

Basic HTML Example

from lxml import html

# Parse the HTML file
tree = html.parse('example.html')
root = tree.getroot()

# Modify the tree - add a new div to body
body = root.find('.//body')
new_div = etree.SubElement(body, "div")
new_div.text = "This is a new div"
new_div.set("class", "new-content")

# Save the modified tree
tree.write('modified_example.html', 
           pretty_print=True, 
           method="html", 
           encoding="UTF-8")

Advanced Modification Examples

Modifying Existing Elements

from lxml import etree

tree = etree.parse('data.xml')
root = tree.getroot()

# Find and modify existing elements
for element in root.xpath('//item[@id="1"]'):
    element.text = "Updated content"
    element.set("status", "modified")

# Remove elements
elements_to_remove = root.xpath('//item[@status="obsolete"]')
for element in elements_to_remove:
    element.getparent().remove(element)

tree.write('updated_data.xml', pretty_print=True, xml_declaration=True)

Working with Namespaces

from lxml import etree

# Parse XML with namespaces
tree = etree.parse('namespaced.xml')
root = tree.getroot()

# Define namespace map
nsmap = {'ns': 'http://example.com/namespace'}

# Modify elements with namespaces
for element in root.xpath('//ns:item', namespaces=nsmap):
    element.text = "Modified namespace content"

# Preserve namespaces when writing
tree.write('modified_namespaced.xml', 
           pretty_print=True, 
           xml_declaration=True, 
           encoding="UTF-8")

Write Method Options

The write() method accepts several important parameters:

| Parameter | Description | Default | |-----------|-------------|---------| | pretty_print | Format output for readability | False | | xml_declaration | Add XML declaration (XML only) | False | | encoding | Output file encoding | "ascii" | | method | Serialization method ("xml", "html", "text") | "xml" | | doctype | Custom DOCTYPE declaration | None | | with_tail | Include tail text | True |

Alternative: Using tostring()

For more control, use etree.tostring():

from lxml import etree

tree = etree.parse('example.xml')
# ... make modifications ...

# Convert to string first
xml_string = etree.tostring(tree, 
                           pretty_print=True, 
                           xml_declaration=True, 
                           encoding="UTF-8")

# Write to file manually
with open('output.xml', 'wb') as f:
    f.write(xml_string)

Working with Fragments

from lxml import etree, html

# Create new XML document from scratch
root = etree.Element("root")
child = etree.SubElement(root, "child")
child.text = "Hello World"

# Save as new document
tree = etree.ElementTree(root)
tree.write('new_document.xml', pretty_print=True, xml_declaration=True)

# For HTML fragments
html_fragment = html.fromstring('<div><p>Content</p></div>')
html_fragment.append(html.Element('span'))
html_fragment[-1].text = "Added span"

# Convert back to string
result = html.tostring(html_fragment, pretty_print=True, encoding='unicode')
with open('fragment.html', 'w') as f:
    f.write(result)

Error Handling

from lxml import etree

try:
    tree = etree.parse('input.xml')
    # ... make modifications ...
    tree.write('output.xml', pretty_print=True, xml_declaration=True)
    print("File saved successfully")
except etree.XMLSyntaxError as e:
    print(f"XML parsing error: {e}")
except IOError as e:
    print(f"File I/O error: {e}")

Best Practices

1. Always specify encoding explicitly when writing files
2. Use pretty_print=True for human-readable output
3. Validate modifications before saving to avoid malformed documents
4. Handle exceptions when working with file I/O operations
5. Preserve original formatting when possible by using appropriate parser settings
6. Test with sample data before processing large files