[jinja2.git] / docs / api.rst

API
===

.. module:: jinja2
    :synopsis: public Jinja2 API

This document describes the API to Jinja2 and not the template language.  It
will be most useful as reference to those implementing the template interface
to the application and not those who are creating Jinja2 templates.

Basics
------

Jinja2 uses a central object called the template :class:`Environment`.
Instances of this class are used to store the configuration, global objects
and are used to load templates from the file system or other locations.
Even if you are creating templates from string by using the constructor of
:class:`Template` class, an environment is created automatically for you.

Most applications will create one :class:`Environment` object on application
initialization and use that to load templates.  In some cases it's however
useful to have multiple environments side by side, if different configurations
are in use.

The simplest way to configure Jinja2 to load templates for your application
looks roughly like this::

    from jinja2 import Environment, PackageLoader
    env = Environment(loader=PackageLoader('yourapplication', 'templates'))

This will create a template environment with the default settings and a
loader that looks up the templates in the `templates` folder inside the
`yourapplication` python package.  Different loaders are available
and you can also write your own if you want to load templates from a
database or other resources.

To load a template from this environment you just have to call the
:meth:`get_template` method which then returns the loaded :class:`Template`::

    template = env.get_template('mytemplate.html')

To render it with some variables, just call the :meth:`render` method::

    print template.render(the='variables', go='here')


High Level API
--------------

.. autoclass:: jinja2.environment.Environment([options])
    :members: from_string, get_template, join_path, parse, lex, extend

    .. attribute:: shared

        If a template was created by using the :class:`Template` constructor
        an environment is created automatically.  These environments are
        created as shared environments which means that multiple templates
        may have the same anonymous environment.  For all shared environments
        this attribute is `True`, else `False`.

    .. attribute:: sandboxed

        If the environment is sandboxed this attribute is `True`.  For the
        sandbox mode have a look at the documentation for the
        :class:`~jinja2.sandbox.SandboxedEnvironment`.

    .. attribute:: filters

        A dict of filters for this environment.  As long as no template was
        loaded it's safe to add new filters or remove old.  For custom filters
        see :ref:`writing-filters`.  For valid filter names have a look at
        :ref:`identifier-naming`.

    .. attribute:: tests

        A dict of test functions for this environment.  As long as no
        template was loaded it's safe to modify this dict.  For custom tests
        see :ref:`writing-tests`.  For valid test names have a look at
        :ref:`identifier-naming`.

    .. attribute:: globals

        A dict of global variables.  These variables are always available
        in a template.  As long as no template was loaded it's safe
        to modify this dict.  For more details see :ref:`global-namespace`.
        For valid object names have a look at :ref:`identifier-naming`.

    .. automethod:: overlay([options])


.. autoclass:: jinja2.Template
    :members: make_module, module, new_context

    .. attribute:: globals

        The dict with the globals of that template.  It's unsafe to modify
        this dict as it may be shared with other templates or the environment
        that loaded the template.

    .. attribute:: name

        The loading name of the template.  If the template was loaded from a
        string this is `None`.

    .. automethod:: render([context])

    .. automethod:: generate([context])

    .. automethod:: stream([context])


.. autoclass:: jinja2.environment.TemplateStream
    :members: disable_buffering, enable_buffering


.. _identifier-naming:

Notes on Identifiers
~~~~~~~~~~~~~~~~~~~~

Jinja2 uses the regular Python 2.x naming rules.  Valid identifiers have to
match ``[a-zA-Z_][a-zA-Z0-9_]*``.  As a matter of fact non ASCII characters
are currently not allowed.  This limitation will probably go away as soon as
unicode identifiers are fully specified for Python 3.

Filters and tests are looked up in separate namespaces and have slightly
modified identifier syntax.  Filters and tests may contain dots to group
filters and tests by topic.  For example it's perfectly valid to add a
function into the filter dict and call it `to.unicode`.  The regular
expression for filter and test identifiers is
``[a-zA-Z_][a-zA-Z0-9_]*(\.[a-zA-Z_][a-zA-Z0-9_]*)*```.


Undefined Types
---------------

These classes can be used as undefined types.  The :class:`Environment`
constructor takes an `undefined` parameter that can be one of those classes
or a custom subclass of :class:`Undefined`.  Whenever the template engine is
unable to look up a name or access an attribute one of those objects is
created and returned.  Some operations on undefined values are then allowed,
others fail.

The closest to regular Python behavior is the `StrictUndefined` which
disallows all operations beside testing if it's an undefined object.

.. autoclass:: jinja2.runtime.Undefined

.. autoclass:: jinja2.runtime.DebugUndefined

.. autoclass:: jinja2.runtime.StrictUndefined


The Context
-----------

.. autoclass:: jinja2.runtime.Context
    :members: resolve, get_exported, get_all

    .. attribute:: parent

        A dict of read only, global variables the template looks up.  These
        can either come from another :class:`Context`, from the
        :attr:`Environment.globals` or :attr:`Template.globals`.  It must not
        be altered.

    .. attribute:: vars

        The template local variables.  This list contains environment and
        context functions from the :attr:`parent` scope as well as local
        modifications and exported variables from the template.  The template
        will modify this dict during template evaluation but filters and
        context functions are not allowed to modify it.

    .. attribute:: environment

        The environment that loaded the template.

    .. attribute:: exported_vars

        This set contains all the names the template exports.  The values for
        the names are in the :attr:`vars` dict.  In order to get a copy of the
        exported variables as dict, :meth:`get_exported` can be used.

    .. attribute:: name

        The load name of the template owning this context.

    .. attribute:: blocks

        A dict with the current mapping of blocks in the template.  The keys
        in this dict are the names of the blocks, and the values a list of
        blocks registered.  The last item in each list is the current active
        block (latest in the inheritance chain).


.. _loaders:

Loaders
-------

Loaders are responsible for loading templates from a resource such as the
file system.  The environment will keep the compiled modules in memory like
Python's `sys.modules`.  Unlike `sys.modules` however this cache is limited in
size by default and templates are automatically reloaded.
All loaders are subclasses of :class:`BaseLoader`.  If you want to create your
own loader, subclass :class:`BaseLoader` and override `get_source`.

.. autoclass:: jinja2.loaders.BaseLoader
    :members: get_source, load

Here a list of the builtin loaders Jinja2 provides:

.. autoclass:: jinja2.loaders.FileSystemLoader

.. autoclass:: jinja2.loaders.PackageLoader

.. autoclass:: jinja2.loaders.DictLoader

.. autoclass:: jinja2.loaders.FunctionLoader

.. autoclass:: jinja2.loaders.PrefixLoader

.. autoclass:: jinja2.loaders.ChoiceLoader


Utilities
---------

These helper functions and classes are useful if you add custom filters or
functions to a Jinja2 environment.

.. autofunction:: jinja2.filters.environmentfilter

.. autofunction:: jinja2.filters.contextfilter

.. autofunction:: jinja2.utils.environmentfunction

.. autofunction:: jinja2.utils.contextfunction

.. function:: escape(s)

    Convert the characters &, <, >, and " in string s to HTML-safe sequences.
    Use this if you need to display text that might contain such characters
    in HTML.  This function will not escaped objects that do have an HTML
    representation such as already escaped data.

.. autofunction:: jinja2.utils.clear_caches

.. autoclass:: jinja2.utils.Markup


Exceptions
----------

.. autoexception:: jinja2.exceptions.TemplateError

.. autoexception:: jinja2.exceptions.UndefinedError

.. autoexception:: jinja2.exceptions.TemplateNotFound

.. autoexception:: jinja2.exceptions.TemplateSyntaxError

.. autoexception:: jinja2.exceptions.TemplateAssertionError


.. _writing-filters:

Custom Filters
--------------

Custom filters are just regular Python functions that take the left side of
the filter as first argument and the the arguments passed to the filter as
extra arguments or keyword arguments.

For example in the filter ``{{ 42|myfilter(23) }}`` the function would be
called with ``myfilter(42, 23)``.  Here for example a simple filter that can
be applied to datetime objects to format them::

    def datetimeformat(value, format='%H:%M / %d-%m-%Y'):
        return value.strftime(format)

You can register it on the template environment by updating the
:attr:`~Environment.filters` dict on the environment::

    environment.filters['datetimeformat'] = datetimeformat

Inside the template it can then be used as follows:

.. sourcecode:: jinja

    written on: {{ article.pub_date|datetimeformat }}
    publication date: {{ article.pub_date|datetimeformat('%d-%m-%Y') }}

Filters can also be passed the current template context or environment.  This
is useful if a filters wants to return an undefined value or check the current
:attr:`~Environment.autoescape` setting.  For this purpose two decorators
exist: :func:`environmentfilter` and :func:`contextfilter`.

Here a small example filter that breaks a text into HTML line breaks and
paragraphs and marks the return value as safe HTML string if autoescaping is
enabled::

    import re
    from jinja2 import environmentfilter, Markup, escape

    _paragraph_re = re.compile(r'(?:\r\n|\r|\n){2,}')

    @environmentfilter
    def nl2br(environment, value):
        result = u'\n\n'.join(u'<p>%s</p>' % p.replace('\n', '<br>\n')
                              for p in _paragraph_re.split(escape(value)))
        if environment.autoescape:
            result = Markup(result)
        return result

Context filters work the same just that the first argument is the current
active :class:`Context` rather then the environment.


.. _writing-tests:

Custom Tests
------------

Tests work like filters just that there is no way for a filter to get access
to the environment or context and that they can't be chained.  The return
value of a filter should be `True` or `False`.  The purpose of a filter is to
give the template designers the possibility to perform type and conformability
checks.

Here a simple filter that checks if a variable is a prime number::

    import math

    def is_prime(n):
        if n == 2:
            return True
        for i in xrange(2, int(math.ceil(math.sqrt(n))) + 1):
            if n % i == 0:
                return False
        return True
        

You can register it on the template environment by updating the
:attr:`~Environment.tests` dict on the environment::

    environment.tests['prime'] = is_prime

A template designer can then use the test like this:

.. sourcecode:: jinja

    {% if 42 is prime %}
        42 is a prime number
    {% else %}
        42 is not a prime number
    {% endif %}


.. _global-namespace:

The Global Namespace
--------------------

Variables stored in the :attr:`Environment.globals` dict are special as they
are available for imported templates too, even if they are imported without
context.  This is the place where you can put variables and functions
that should be available all the time.  Additionally :attr:`Template.globals`
exist that are variables available to a specific template that are available
to all :meth:`~Template.render` calls.


Jinja2 Semantics
----------------

Jinja2 behaves like regular Python code in the runtime with some additional
restrictions that allow the template engine to optimize some common idoms.
The context is semi immutable.  Only Jinja2 itself may modify it, if other
code (such as :func:`contextfunction`\s) modify it the changes won't show up
as expected.

Another change to regular Python is that Jinja2 sees attributes as immutable.
It assumes that every time it accesses `foo.bar`, bar will look the same as
long as `foo` was not overriden.  In some situations Jinja will take advantage
of that to internally substitute continuous lookups to the same attributes
with a copy of the attribute.

For example in the following template the `post.user` attribute is looked up
once:

.. sourcecode:: html+jinja

    {% if post.user %}
        <h3>{{ post.user.username }}</h3>
        <p>Location: {{ post.user.location }}</p>
    {% endif %}

That said, the objects passed to the template may not generate different
values every time a property is accessed.