xml4h includes a document builder tool that makes it easy to create valid, well-formed XML documents using relatively sparse python code. It makes it so easy to create XML that you will no longer be tempted to cobble together documents with error-prone methods like manual string concatenation or a templating library.

Internally, the builder uses the DOM-building features of an underlying XML library which means it is (almost) impossible to construct an invalid document.

Here is some example code to build a document about Monty Python films:

>>> import xml4h
>>> xmlb = ('MontyPythonFilms')
...     .attributes({'source': ''})
...     .element('Film')
...         .attributes({'year': 1971})
...         .element('Title')
...             .text('And Now for Something Completely Different')
...             .up()
...         .elem('Description').t(
...             "A collection of sketches from the first and second TV"
...             " series of Monty Python's Flying Circus purposely"
...             " re-enacted and shot for film.")
...             .up()
...         .up()
...     .elem('Film')
...         .attrs(year=1974)
...         .e('Title')
...             .t('Monty Python and the Holy Grail')
...             .up()
...         .e('Description').t(
...             "King Arthur and his knights embark on a low-budget search"
...             " for the Holy Grail, encountering humorous obstacles along"
...             " the way. Some of these turned into standalone sketches."
...             ).up()
...     )

The code above produces the following XML document (abbreviated):

>>> xmlb.write_doc(indent=True)  
<?xml version="1.0" encoding="utf-8"?>
<MontyPythonFilms source="">
    <Film year="1971">
        <Title>And Now for Something Completely Different</Title>
        <Description>A collection of sketches from the first and second...
    <Film year="1974">
        <Title>Monty Python and the Holy Grail</Title>
        <Description>King Arthur and his knights embark on a low-budget...

Getting Started

You typically create a new XML document builder by calling the function with the name of the root element:

>>> root_b ='RootElement')

The function returns a Builder object that represents the RootElement and allows you to manipulate this element’s attributes or to add child elements.

Once you have the first builder instance, every action you perform to add content to the XML document will return another instance of the Builder class:

>>> # Add attributes to the root element's Builder
>>> root_b = root_b.attributes({'a': 1, 'b': 2}, c=3)

>>> root_b  
<xml4h.builder.Builder object ...

The Builder class always represents an underlying element in the DOM. The dom_element attribute returns the element node:

>>> root_b.dom_element
<xml4h.nodes.Element: "RootElement">

>>> root_b.dom_element.attributes
<xml4h.nodes.AttributeDict: [('a', '1'), ('b', '2'), ('c', '3')]>

When you add a new child element, the result is a builder instance representing that child element, not the original element:

>>> child1_b = root_b.element('ChildElement1')
>>> child2_b = root_b.element('ChildElement2')

>>> # The element method returns a Builder wrapping the new child element
>>> child2_b.dom_element
<xml4h.nodes.Element: "ChildElement2">
>>> child2_b.dom_element.parent
<xml4h.nodes.Element: "RootElement">

This feature of the builder can be a little confusing, but it allows for the very convenient method-chaining feature that gives the builder its power.

Method Chaining

Because every builder method that adds content to the XML document returns a builder instance representing the nearest (or newest) element, you can chain together many method calls to construct your document without any need for intermediate variables.

For example, the example code in the previous section used the variables root_b, child1_b and child2_b to represent builder instances but this is not necessary. Here is how you can use method-chaining to build the same document with less code:

>>> b = (xml4h
...     .build('RootElement').attributes({'a': 1, 'b': 2}, c=3)
...         .element('ChildElement1').up()  # NOTE the up() method
...         .element('ChildElement2')
...     )

>>> b.write_doc(indent=4)
<?xml version="1.0" encoding="utf-8"?>
<RootElement a="1" b="2" c="3">

Notice how you can use chained method calls to write code with a structure that mirrors that of the XML document you want to produce? This makes it much easier to spot errors in your code than it would be if you were to concatenate strings.


It is a good idea to wrap the build() function call and all following chained methods in parentheses, so you don’t need to put backslash (\) characters at the end of every line.

The code above introduces a very important builder method: up(). This method returns a builder instance representing the current element’s parent, or indeed any ancestor.

Without the up() method, every time you created a child element with the builder you would end up deeper in the document structure with no way to return to prior elements to add sibling nodes or hierarchies.

To help reduce the number of up() method calls you need to include in your code, this method can also jump up multiple levels or to a named ancestor element:

>>> # A builder that references a deeply-nested element:
>>> deep_b = ('Root')
...     .element('Deep')
...         .element('AndDeeper')
...             .element('AndDeeperStill')
...                 .element('UntilWeGetThere')
...     )
>>> deep_b.dom_element
<xml4h.nodes.Element: "UntilWeGetThere">

>>> # Jump up 4 levels, back to the root element
>>> deep_b.up(4).dom_element
<xml4h.nodes.Element: "Root">

>>> # Jump up to a named ancestor element
>>> deep_b.up('Root').dom_element
<xml4h.nodes.Element: "Root">

Shorthand Methods

To make your XML-producing code even less verbose and quicker to type, the builder has shorthand “alias” methods corresponding to the full names.

For example, instead of calling element() to create a new child element, you can instead use the equivalent elem() or e() methods. Similarly, instead of typing attributes() you can use attrs() or a().

Here are the methods and method aliases for adding content to an XML document:

XML Node Created Builder method Aliases
Element element elem, e
Attribute attributes attrs, a
Text text t
CDATA cdata data, d
Comment comment c
Process Instruction processing_instruction inst, i

These shorthand method aliases are convenient and lead to even less cruft around the actual XML content you are interested in. But on the other hand they are much less explicit than the longer versions, so use them judiciously.

Access the DOM

The XML builder is merely a layer of convenience methods that sits on the xml4h.nodes DOM API. This means you can quickly access the underlying nodes from a builder if you need to inspect them or manipulate them in a way the builder doesn’t allow:

  • The dom_element attribute returns a builder’s underlying Element
  • The root attribute returns the document’s root element.
  • The document attribute returns a builder’s underlying Document.

See the DOM Nodes API documentation to find out how to work with DOM element nodes once you get them.

Building on an Existing DOM

When you are building an XML document from scratch you will generally use the build() function described in Getting Started. However, what if you want to add content to a parsed XML document DOM you have already?

To wrap an Element DOM node with a builder you simply provide the element node to the same builder() method used previously and it will do the right thing.

Here is an example of parsing an existing XML document, locating an element of interest, constructing a builder from that element, and adding some new content. Luckily, the code is simpler than that description...

>>> # Parse an XML document
>>> doc = xml4h.parse('tests/data/monty_python_films.xml')

>>> # Find an Element node of interest
>>> lob_film_elem = doc.MontyPythonFilms.Film[2]
>>> lob_film_elem.Title.text
"Monty Python's Life of Brian"

>>> # Construct a builder from the element
>>> lob_builder =

>>> # Add content
>>> b = (lob_builder.attrs(stars=5)
...     .elem('Review').t('One of my favourite films!').up())

>>> # See the results
>>> lob_builder.write(indent=True)  
<Film stars="5" year="1979">
    <Title>Monty Python's Life of Brian</Title>
    <Description>Brian is born on the first Christmas, in the stable...
    <Review>One of my favourite films!</Review>


Because each builder class instance is independent, an advanced technique for constructing complex documents is to use multiple builders anchored at different places in the DOM. In some situations, the ability to add content to different places in the same document can be very handy.

Here is a trivial example of this technique:

>>> # Create two Elements in a doc to store even or odd numbers
>>> odd_b ='EvenAndOdd').elem('Odd')
>>> even_b = odd_b.up().elem('Even')

>>> # Populate the numbers from a loop
>>> for i in range(1, 11):  
...     if i % 2 == 0:
...         even_b.elem('Number').text(i)
...     else:
...         odd_b.elem('Number').text(i)

>>> # Check the final document
>>> odd_b.write_doc(indent=True)
<?xml version="1.0" encoding="utf-8"?>

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