1. API¶

1.1. Basics¶

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

Most applications will create one Environment object on application initialization and use that to load templates. In some cases however, it’s 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 get_template() method which then returns the loaded Template:

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

To render it with some variables, just call the render() method:

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

Using a template loader rather than passing strings to Template or Environment.from_string() has multiple advantages. Besides being a lot easier to use it also enables template inheritance.

1.2. Unicode¶

Jinja2 is using Unicode internally which means that you have to pass Unicode objects to the render function or bytestrings that only consist of ASCII characters. Additionally newlines are normalized to one end of line sequence which is per default UNIX style (\n).

Python 2.x supports two ways of representing string objects. One is the str type and the other is the unicode type, both of which extend a type called basestring. Unfortunately the default is str which should not be used to store text based information unless only ASCII characters are used. With Python 2.6 it is possible to make unicode the default on a per module level and with Python 3 it will be the default.

To explicitly use a Unicode string you have to prefix the string literal with a u: u'Hänsel und Gretel sagen Hallo'. That way Python will store the string as Unicode by decoding the string with the character encoding from the current Python module. If no encoding is specified this defaults to ‘ASCII’ which means that you can’t use any non ASCII identifier.

To set a better module encoding add the following comment to the first or second line of the Python module using the Unicode literal:

# -*- coding: utf-8 -*-

We recommend utf-8 as Encoding for Python modules and templates as it’s possible to represent every Unicode character in utf-8 and because it’s backwards compatible to ASCII. For Jinja2 the default encoding of templates is assumed to be utf-8.

It is not possible to use Jinja2 to process non-Unicode data. The reason for this is that Jinja2 uses Unicode already on the language level. For example Jinja2 treats the non-breaking space as valid whitespace inside expressions which requires knowledge of the encoding or operating on an Unicode string.

For more details about Unicode in Python have a look at the excellent Unicode documentation.

Another important thing is how Jinja2 is handling string literals in templates. A naive implementation would be using Unicode strings for all string literals but it turned out in the past that this is problematic as some libraries are typechecking against str explicitly. For example datetime.strftime does not accept Unicode arguments. To not break it completely Jinja2 is returning str for strings that fit into ASCII and for everything else unicode:

>>> m = Template(u"{% set a, b = 'foo', 'föö' %}").module
>>> m.a
'foo'
>>> m.b
u'f\xf6\xf6'

1.3. High Level API¶

The high-level API is the API you will use in the application to load and render Jinja2 templates. The Low Level API on the other side is only useful if you want to dig deeper into Jinja2 or develop extensions.

1.4. Autoescaping¶

As of Jinja 2.4 the preferred way to do autoescaping is to enable the Autoescape Extension and to configure a sensible default for autoescaping. This makes it possible to enable and disable autoescaping on a per-template basis (HTML versus text for instance).

Here a recommended setup that enables autoescaping for templates ending in '.html', '.htm' and '.xml' and disabling it by default for all other extensions:

def guess_autoescape(template_name):
    if template_name is None or '.' not in template_name:
        return False
    ext = template_name.rsplit('.', 1)[1]
    return ext in ('html', 'htm', 'xml')

env = Environment(autoescape=guess_autoescape,
                  loader=PackageLoader('mypackage'),
                  extensions=['jinja2.ext.autoescape'])

When implementing a guessing autoescape function, make sure you also accept None as valid template name. This will be passed when generating templates from strings.

Inside the templates the behaviour can be temporarily changed by using the autoescape block (see Autoescape Extension).

1.5. 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_]*)*`.

1.6. Undefined Types¶

These classes can be used as undefined types. The Environment constructor takes an undefined parameter that can be one of those classes or a custom subclass of 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.

class NullUndefined(Undefined):
    def __int__(self):
        return 0
    def __float__(self):
        return 0.0

class NonIterableUndefined(Undefined):
    __iter__ = Undefined._fail_with_undefined_error

1.7. The Context¶

1.8. 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 BaseLoader. If you want to create your own loader, subclass BaseLoader and override get_source.

1.9. Bytecode Cache¶

Jinja 2.1 and higher support external bytecode caching. Bytecode caches make it possible to store the generated bytecode on the file system or a different location to avoid parsing the templates on first use.

This is especially useful if you have a web application that is initialized on the first request and Jinja compiles many templates at once which slows down the application.

To use a bytecode cache, instantiate it and pass it to the Environment.

1.10. Utilities¶

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

1.11. Exceptions¶

1.12. Custom Filters¶

Custom filters are just regular Python functions that take the left side of the filter as first argument and 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 filters dict on the environment:

environment.filters['datetimeformat'] = datetimeformat

Inside the template it can then be used as follows:

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 filter wants to return an undefined value or check the current autoescape setting. For this purpose three decorators exist: environmentfilter(), contextfilter() and evalcontextfilter().

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 evalcontextfilter, Markup, escape

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

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

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

1.13. Evaluation Context¶

The evaluation context (short eval context or eval ctx) is a new object introduced in Jinja 2.4 that makes it possible to activate and deactivate compiled features at runtime.

Currently it is only used to enable and disable the automatic escaping but can be used for extensions as well.

In previous Jinja versions filters and functions were marked as environment callables in order to check for the autoescape status from the environment. In new versions it’s encouraged to check the setting from the evaluation context instead.

Previous versions:

@environmentfilter
def filter(env, value):
    result = do_something(value)
    if env.autoescape:
        result = Markup(result)
    return result

In new versions you can either use a contextfilter() and access the evaluation context from the actual context, or use a evalcontextfilter() which directly passes the evaluation context to the function:

@contextfilter
def filter(context, value):
    result = do_something(value)
    if context.eval_ctx.autoescape:
        result = Markup(result)
    return result

@evalcontextfilter
def filter(eval_ctx, value):
    result = do_something(value)
    if eval_ctx.autoescape:
        result = Markup(result)
    return result

The evaluation context must not be modified at runtime. Modifications must only happen with a nodes.EvalContextModifier and nodes.ScopedEvalContextModifier from an extension, not on the eval context object itself.

1.14. Custom Tests¶

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

Here a simple test 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 tests dict on the environment:

environment.tests['prime'] = is_prime

A template designer can then use the test like this:

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

1.15. The Global Namespace¶

Variables stored in the 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 Template.globals exist that are variables available to a specific template that are available to all render() calls.

1.16. Low Level API¶

The low level API exposes functionality that can be useful to understand some implementation details, debugging purposes or advanced extension techniques. Unless you know exactly what you are doing we don’t recommend using any of those.

Environment.lex(source, name=None, filename=None)[source]¶

Lex the given sourcecode and return a generator that yields tokens as tuples in the form (lineno, token_type, value). This can be useful for extension development and debugging templates.

This does not perform preprocessing. If you want the preprocessing of the extensions to be applied you have to filter source through the preprocess() method.

Environment.parse(source, name=None, filename=None)[source]¶

Parse the sourcecode and return the abstract syntax tree. This tree of nodes is used by the compiler to convert the template into executable source- or bytecode. This is useful for debugging or to extract information from templates.

If you are developing Jinja2 extensions this gives you a good overview of the node tree generated.

Environment.preprocess(source, name=None, filename=None)[source]¶

Preprocesses the source with all extensions. This is automatically called for all parsing and compiling methods but not for lex() because there you usually only want the actual source tokenized.

Template.new_context(vars=None, shared=False, locals=None)[source]¶

Create a new Context for this template. The vars provided will be passed to the template. Per default the globals are added to the context. If shared is set to True the data is passed as it to the context without adding the globals.

locals can be a dict of local variables for internal usage.

Template.root_render_func(context)¶

This is the low level render function. It’s passed a Context that has to be created by new_context() of the same template or a compatible template. This render function is generated by the compiler from the template code and returns a generator that yields unicode strings.

If an exception in the template code happens the template engine will not rewrite the exception but pass through the original one. As a matter of fact this function should only be called from within a render() / generate() / stream() call.

Template.blocks¶

A dict of block render functions. Each of these functions works exactly like the root_render_func() with the same limitations.

Template.is_up_to_date¶

This attribute is False if there is a newer version of the template available, otherwise True.

1.17. The Meta API¶

The meta API returns some information about abstract syntax trees that could help applications to implement more advanced template concepts. All the functions of the meta API operate on an abstract syntax tree as returned by the Environment.parse() method.

jinja2.meta.find_undeclared_variables(ast)[source]¶

Returns a set of all variables in the AST that will be looked up from the context at runtime. Because at compile time it’s not known which variables will be used depending on the path the execution takes at runtime, all variables are returned.

>>> from jinja2 import Environment, meta
>>> env = Environment()
>>> ast = env.parse('{% set foo = 42 %}{{ bar + foo }}')
>>> meta.find_undeclared_variables(ast) == set(['bar'])
True

Implementation

Internally the code generator is used for finding undeclared variables. This is good to know because the code generator might raise a TemplateAssertionError during compilation and as a matter of fact this function can currently raise that exception as well.

jinja2.meta.find_referenced_templates(ast)[source]¶

Finds all the referenced templates from the AST. This will return an iterator over all the hardcoded template extensions, inclusions and imports. If dynamic inheritance or inclusion is used, None will be yielded.

>>> from jinja2 import Environment, meta
>>> env = Environment()
>>> ast = env.parse('{% extends "layout.html" %}{% include helper %}')
>>> list(meta.find_referenced_templates(ast))
['layout.html', None]

This function is useful for dependency tracking. For example if you want to rebuild parts of the website after a layout template has changed.