Library Documentation

This section documents the package as a Python library. To learn about the page template language, consult the language reference.

Getting started

There are several template constructor classes available, one for each of the combinations text or xml, and string or file.

The file-based constructor requires an absolute path. To set up a templates directory once, use the template loader class:

import os

path = os.path.dirname(__file__)

from chameleon import PageTemplateLoader
templates = PageTemplateLoader(os.path.join(path, "templates"))

Then, to load a template relative to the provided path, use dictionary syntax:

template = templates['']

Alternatively, use the appropriate template class directly. Let’s try with a string input:

from chameleon import PageTemplate
template = PageTemplate("<div>Hello, ${name}.</div>")

All template instances are callable. Provide variables by keyword argument:

>>> template(name='John')
'<div>Hello, John.</div>'


The template engine compiles (or translates) template source code into Python byte-code. In simple templates this yields an increase in performance of about 7 times in comparison to the reference implementation.

In benchmarks for the content management system Plone, switching to Chameleon yields a request to response improvement of 20-50%.


You can extend the language through the expression engine by writing your own expression compiler.

Let’s try and write an expression compiler for an expression type that will simply uppercase the supplied value. We’ll call it upper.

You can write such a compiler as a closure:

import ast

def uppercase_expression(string):
    def compiler(target, engine):
        uppercased = self.string.uppercase()
        value = ast.Str(uppercased)
        return [ast.Assign(targets=[target], value=value)]
    return compiler

To make it available under a certain prefix, we’ll add it to the expression types dictionary.

from chameleon import PageTemplate
PageTemplate.expression_types['upper'] = uppercase_expression

Alternatively, you could subclass the template class and set the attribute expression_types to a dictionary that includes your expression:

from chameleon import PageTemplateFile
from chameleon.tales import PythonExpr

class MyPageTemplateFile(PageTemplateFile):
    expression_types = {
        'python': PythonExpr,
        'upper': uppercase_expression

You can now uppercase strings natively in your templates:

<div tal:content="upper: hello, world" />

It’s probably best to stick with a Python expression:

<div tal:content="'hello, world'.upper()" />

API reference

This section describes the documented API of the library.


Use the PageTemplate* template classes to define a template from a string or file input:

class chameleon.PageTemplate(body, **config)

Constructor for the page template language.

Takes a string input as the only positional argument:

template = PageTemplate("<div>Hello, ${name}.</div>")

Configuration (keyword arguments):


Set the default expression type. The default setting is python.


The default text substitution value is a unicode string on Python 2 or simply string on Python 3.

Pass an encoding to allow encoded byte string input (e.g. UTF-8).


Attributes are not dropped for a value of False. Instead, the value is coerced to a string.

This setting exists to provide compatibility with the reference implementation.


Attributes included in this set are treated as booleans: if a true value is provided, the attribute value is the attribute name, e.g.:

boolean_attributes = {"selected"}

If we insert an attribute with the name “selected” and provide a true value, the attribute will be rendered:


If a false attribute is provided (including the empty string), the attribute is dropped.

The special return value default drops or inserts the attribute based on the value element attribute value.


Use this option to set a translation function.


def translate(msgid, domain=None, mapping=None, default=None, context=None):
    return translation

Note that if target_language is provided at render time, the translation function must support this argument.


Enables implicit translation for text appearing inside elements. Default setting is False.

While implicit translation does work for text that includes expression interpolation, each expression must be simply a variable name (e.g. ${foo}); otherwise, the text will not be marked for translation.


Any attribute contained in this set will be marked for implicit translation. Each entry must be a lowercase string.


implicit_i18n_attributes = set(['alt', 'title'])


Enabled by default. If disabled, expressions are only required to be valid at evaluation time.

This setting exists to provide compatibility with the reference implementation which compiles expressions at evaluation time.


If set, additional attribute whitespace will be stripped.

Output is unicode on Python 2 and string on Python 3.

Note: The remaining classes take the same general configuration arguments.

render(encoding=None, translate=None, **vars)

Render template to string.

The encoding and translate arguments are documented in the template class constructor. If passed to this method, they are used instead of the class defaults.

Additional arguments:


This argument will be partially applied to the translation function.

An alternative is thus to simply provide a custom translation function which includes this information or relies on a different mechanism.

class chameleon.PageTemplateFile(filename, **config)

File-based constructor.

Takes a string input as the only positional argument:

template = PageTemplateFile(absolute_path)

Note that the file-based template class comes with the expression type load which loads templates relative to the provided filename.

Below are listed the configuration arguments specific to file-based templates; see the string-based template class for general options and documentation:

Configuration (keyword arguments):


The provided class will be used to create the template loader object. The default implementation supports relative and absolute path specs.

The class must accept keyword arguments search_path (sequence of paths to search for relative a path spec) and default_extension (if provided, this should be added to any path spec).


Inserts the path relative to the provided template file path into the template search path.

The default setting is True.


If provided, this is used as the search path for the load: expression. It must be a string or an iterable yielding a sequence of strings.
class chameleon.PageTextTemplate(body, **config)

Text-based template class.

Takes a non-XML input:

template = PageTextTemplate("Hello, ${name}.")

This is similar to the standard library class string.Template, but uses the expression engine to substitute variables.

class chameleon.PageTextTemplateFile(filename, search_path=None, loader_class=<class 'chameleon.loader.TemplateLoader'>, **config)

File-based constructor.


Some systems have framework support for loading templates from files. The following loader class is directly compatible with the Pylons framework and may be adapted to other frameworks:

class chameleon.PageTemplateLoader(search_path=None, default_extension=None, **config)

Load templates from search_path (must be a string or a list of strings):

templates = PageTemplateLoader(path)
example = templates['']

If default_extension is provided, this will be added to inputs that do not already have an extension:

templates = PageTemplateLoader(path, ".pt")
example = templates['example']

Any additional keyword arguments will be passed to the template constructor:

templates = PageTemplateLoader(path, debug=True, encoding="utf-8")
PageTemplateLoader.load(filename, format=None)

Load and return a template file.

The format parameter determines will parse the file. Valid options are xml and text.


Chameleon may raise exceptions during both the cooking and the rendering phase, but those raised during the cooking phase (parse and compile) all inherit from a single base class:

class chameleon.TemplateError(msg, token)

This exception is the base class of all exceptions raised by the template engine in the case where a template has an error.

It may be raised during rendering since templates are processed lazily (unless eager loading is enabled).

An error that occurs during the rendering of a template is wrapped in an exception class to disambiguate the two cases:

class chameleon.RenderError(*args)

Indicates an exception that resulted from the evaluation of an expression in a template.

A complete traceback is attached to the exception beginning with the expression that resulted in the error. The traceback includes a string representation of the template variable scope for further reference.


For advanced integration, the compiler module provides support for dynamic expression evaluation:

class chameleon.compiler.ExpressionEvaluator(engine, builtins)

Evaluates dynamic expression.

This is not particularly efficient, but supported for legacy applications.

>>> from chameleon import tales
>>> parser = tales.ExpressionParser({'python': tales.PythonExpr}, 'python')
>>> engine = functools.partial(ExpressionEngine, parser)
>>> evaluate = ExpressionEvaluator(engine, {
...     'foo': 'bar',
...     })

The evaluation function is passed the local and remote context, the expression type and finally the expression.

>>> evaluate({'boo': 'baz'}, {}, 'python', 'foo + boo')

The cache is now primed:

>>> evaluate({'boo': 'baz'}, {}, 'python', 'foo + boo')

Note that the call method supports currying of the expression argument:

>>> python = evaluate({'boo': 'baz'}, {}, 'python')
>>> python('foo + boo')