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Library Documentation

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


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:

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")


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:

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:

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: