API

jinja2

This document describes the API to Jinja and not the template language (for that, see /templates). It will be most useful as reference to those implementing the template interface to the application and not those who are creating Jinja templates.

Basics

Jinja 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 Jinja to load templates for your application looks roughly like this:

from jinja2 import Environment, PackageLoader, select_autoescape
env = Environment(
    loader=PackageLoader('yourapplication', 'templates'),
    autoescape=select_autoescape(['html', 'xml'])
)

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. This also enables autoescaping for HTML and XML files.

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.

Notes on Autoescaping

In future versions of Jinja we might enable autoescaping by default for security reasons. As such you are encouraged to explicitly configure autoescaping now instead of relying on the default.

Unicode

Jinja 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 Jinja the default encoding of templates is assumed to be utf-8.

It is not possible to use Jinja to process non-Unicode data. The reason for this is that Jinja uses Unicode already on the language level. For example Jinja 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 Jinja 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 Jinja 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'fxf6xf6'

High Level API

The high-level API is the API you will use in the application to load and render Jinja templates. The low-level-api on the other side is only useful if you want to dig deeper into Jinja or develop extensions <jinja-extensions>.

Environment([options])

shared

If a template was created by using the 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.

sandboxed

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

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 writing-filters. For valid filter names have a look at identifier-naming.

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 writing-tests. For valid test names have a look at identifier-naming.

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 global-namespace. For valid object names have a look at identifier-naming.

policies

A dictionary with policies. These can be reconfigured to change the runtime behavior or certain template features. Usually these are security related.

code_generator_class

The class used for code generation. This should not be changed in most cases, unless you need to modify the Python code a template compiles to.

context_class

The context used for templates. This should not be changed in most cases, unless you need to modify internals of how template variables are handled. For details, see ~jinja2.runtime.Context.

overlay([options])
undefined([hint, obj, name, exc])

Creates a new Undefined object for name. This is useful for filters or functions that may return undefined objects for some operations. All parameters except of hint should be provided as keyword parameters for better readability. The hint is used as error message for the exception if provided, otherwise the error message will be generated from obj and name automatically. The exception provided as exc is raised if something with the generated undefined object is done that the undefined object does not allow. The default exception is UndefinedError. If a hint is provided the name may be omitted.

The most common way to create an undefined object is by providing a name only:
return environment.undefined(name='some_name')
This means that the name some_name is not defined. If the name was from an attribute of an object it makes sense to tell the undefined object the holder object to improve the error message:
if not hasattr(obj, 'attr'):
    return environment.undefined(obj=obj, name='attr')
For a more complex example you can provide a hint. For example the first filter creates an undefined object that way:
return environment.undefined('no first item, sequence was empty')
If it the name or obj is known (for example because an attribute was accessed) it should be passed to the undefined object, even if a custom hint is provided. This gives undefined objects the possibility to enhance the error message.

Template

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.

name

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

filename

The filename of the template on the file system if it was loaded from there. Otherwise this is None.

render([context])

generate([context])

stream([context])

render_async([context])

generate_async([context])

jinja2.environment.TemplateStream()

Autoescaping

2.4

Jinja now comes with autoescaping support. As of Jinja 2.9 the autoescape extension is removed and built-in. However autoescaping is not yet enabled by default though this will most likely change in the future. It's recommended 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).

jinja2.select_autoescape

Here a recommended setup that enables autoescaping for templates ending in '.html', '.htm' and '.xml' and disabling it by default for all other extensions. You can use the ~jinja2.select_autoescape function for this:

from jinja2 import Environment, PackageLoader, select_autoescape
env = Environment(autoescape=select_autoescape(['html', 'htm', 'xml']),
                  loader=PackageLoader('mypackage'))

The ~jinja.select_autoescape function returns a function that works roughly like this:

def autoescape(template_name):
    if template_name is None:
        return False
    if template_name.endswith(('.html', '.htm', '.xml'))

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. You should always configure autoescaping as defaults in the future might change.

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

Notes on Identifiers

Jinja uses Python naming rules. Valid identifiers can be any combination of Unicode characters accepted by Python.

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 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.

jinja2.Undefined()

_undefined_hint

Either None or an unicode string with the error message for the undefined object.

_undefined_obj

Either None or the owner object that caused the undefined object to be created (for example because an attribute does not exist).

_undefined_name

The name for the undefined variable / attribute or just None if no such information exists.

_undefined_exception

The exception that the undefined object wants to raise. This is usually one of UndefinedError or SecurityError.

_fail_with_undefined_error(*args, **kwargs)

When called with any arguments this method raises _undefined_exception with an error message generated from the undefined hints stored on the undefined object.

jinja2.ChainableUndefined()

jinja2.DebugUndefined()

jinja2.StrictUndefined()

There is also a factory function that can decorate undefined objects to implement logging on failures:

jinja2.make_logging_undefined

Undefined objects are created by calling undefined.

Implementation

Undefined objects are implemented by overriding the special __underscore__ methods. For example the default Undefined class implements __unicode__ in a way that it returns an empty string, however __int__ and others still fail with an exception. To allow conversion to int by returning 0 you can implement your own:

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

To disallow a method, just override it and raise ~Undefined._undefined_exception. Because this is a very common idiom in undefined objects there is the helper method ~Undefined._fail_with_undefined_error that does the error raising automatically. Here a class that works like the regular Undefined but chokes on iteration:

class NonIterableUndefined(Undefined):
    __iter__ = Undefined._fail_with_undefined_error

The Context

jinja2.runtime.Context()

parent

A dict of read only, global variables the template looks up. These can either come from another Context, from the Environment.globals or Template.globals or points to a dict created by combining the globals with the variables passed to the render function. It must not be altered.

vars

The template local variables. This list contains environment and context functions from the 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.

environment

The environment that loaded the template.

exported_vars

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

name

The load name of the template owning this context.

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).

eval_ctx

The current eval-context.

jinja2.runtime.Context.call(callable, *args, **kwargs)

Implementation

Context is immutable for the same reason Python's frame locals are immutable inside functions. Both Jinja and Python are not using the context / frame locals as data storage for variables but only as primary data source.

When a template accesses a variable the template does not define, Jinja looks up the variable in the context, after that the variable is treated as if it was defined in the template.

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.

jinja2.BaseLoader

Here a list of the builtin loaders Jinja provides:

jinja2.FileSystemLoader

jinja2.PackageLoader

jinja2.DictLoader

jinja2.FunctionLoader

jinja2.PrefixLoader

jinja2.ChoiceLoader

jinja2.ModuleLoader

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.

jinja2.BytecodeCache

jinja2.bccache.Bucket

environment

The Environment that created the bucket.

key

The unique cache key for this bucket

code

The bytecode if it's loaded, otherwise None.

Builtin bytecode caches:

jinja2.FileSystemBytecodeCache

jinja2.MemcachedBytecodeCache

Async Support

2.9

Jinja supports the Python async and await syntax. For the template designer, this support (when enabled) is entirely transparent, templates continue to look exactly the same. However, developers should be aware of the implementation as it affects what types of APIs you can use.

By default, async support is disabled. Enabling it will cause the environment to compile different code behind the scenes in order to handle async and sync code in an asyncio event loop. This has the following implications:

Template rendering requires an event loop to be available to the current thread. asyncio.get_event_loop must return an event loop.
The compiled code uses await for functions and attributes, and uses async for loops. In order to support using both async and sync functions in this context, a small wrapper is placed around all calls and access, which add overhead compared to purely async code.
Sync methods and filters become wrappers around their corresponding async implementations where needed. For example, render invokes async_render, and |map supports async iterables.

Awaitable objects can be returned from functions in templates and any function call in a template will automatically await the result. The await you would normally add in Python is implied. For example, you can provide a method that asynchronously loads data from a database, and from the template designer's point of view it can be called like any other function.

Policies

Starting with Jinja 2.9 policies can be configured on the environment which can slightly influence how filters and other template constructs behave. They can be configured with the ~jinja2.Environment.policies attribute.

Example:

env.policies['urlize.rel'] = 'nofollow noopener'

compiler.ascii_str:: This boolean controls on Python 2 if Jinja should store ASCII only literals as bytestring instead of unicode strings. This used to be always enabled for Jinja versions below 2.9 and now can be changed. Traditionally it was done this way since some APIs in Python 2 failed badly for unicode strings (for instance the datetime strftime API). Now however sometimes the inverse is true (for instance str.format). If this is set to False then all strings are stored as unicode internally.
truncate.leeway:: Configures the leeway default for the truncate filter. Leeway as introduced in 2.9 but to restore compatibility with older templates it can be configured to 0 to get the old behavior back. The default is 5.
urlize.rel:: A string that defines the items for the rel attribute of generated links with the urlize filter. These items are always added. The default is noopener.
urlize.target:: The default target that is issued for links from the urlize filter if no other target is defined by the call explicitly.
json.dumps_function:: If this is set to a value other than None then the tojson filter will dump with this function instead of the default one. Note that this function should accept arbitrary extra arguments which might be passed in the future from the filter. Currently the only argument that might be passed is indent. The default dump function is json.dumps.
json.dumps_kwargs:: Keyword arguments to be passed to the dump function. The default is {'sort_keys': True}.

ext.i18n.trimmed:: If this is set to True, {% trans %} blocks of the i18n-extension will always unify linebreaks and surrounding whitespace as if the trimmed modifier was used.

Utilities

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

jinja2.environmentfilter

jinja2.contextfilter

jinja2.evalcontextfilter

jinja2.environmentfunction

jinja2.contextfunction

jinja2.evalcontextfunction

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.

The return value is a Markup string.

jinja2.clear_caches

jinja2.is_undefined

jinja2.Markup([string])

Note

The Jinja Markup class is compatible with at least Pylons and Genshi. It's expected that more template engines and framework will pick up the __html__ concept soon.

Exceptions

jinja2.TemplateError

jinja2.UndefinedError

jinja2.TemplateNotFound

jinja2.TemplatesNotFound

jinja2.TemplateSyntaxError

message

The error message as utf-8 bytestring.

lineno

The line number where the error occurred

name

The load name for the template as unicode string.

filename

The filename that loaded the template as bytestring in the encoding of the file system (most likely utf-8 or mbcs on Windows systems).

The reason why the filename and error message are bytestrings and not unicode strings is that Python 2.x is not using unicode for exceptions and tracebacks as well as the compiler. This will change with Python 3.

jinja2.TemplateRuntimeError

jinja2.TemplateAssertionError

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 ~Environment.filters dict on the environment:

environment.filters['datetimeformat'] = datetimeformat

Inside the template it can then be used as follows:

jinja

written on: {{ article.pub_datedatetimeformat('%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 ~Environment.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)|\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 than the environment.

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.

jinja2.nodes.EvalContext

autoescape

True or False depending on if autoescaping is active or not.

volatile

True if the compiler cannot evaluate some expressions at compile time. At runtime this should always be False.

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 range(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 ~Environment.tests dict on the environment:

environment.tests['prime'] = is_prime

A template designer can then use the test like this:

jinja

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

{% endif %}

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 ~Template.render calls.

Low Level API

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

Environment.lex

Environment.parse

Environment.preprocess

Template.new_context

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.

Note

The low-level API is fragile. Future Jinja versions will try not to change it in a backwards incompatible way but modifications in the Jinja core may shine through. For example if Jinja introduces a new AST node in later versions that may be returned by ~Environment.parse.

The Meta API

2.2

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

jinja2.meta.find_referenced_templates

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api.rst

api.rst

API

Basics

Unicode

High Level API

Autoescaping

Notes on Identifiers

Undefined Types

The Context

Loaders

Bytecode Cache

Async Support

Policies

Utilities

Exceptions

Custom Filters

Evaluation Context

Custom Tests

The Global Namespace

Low Level API

The Meta API

Files

api.rst

Latest commit

History

api.rst

File metadata and controls

API

Basics

Unicode

High Level API

Autoescaping

Notes on Identifiers

Undefined Types

The Context

Loaders

Bytecode Cache

Async Support

Policies

Utilities

Exceptions

Custom Filters

Evaluation Context

Custom Tests

The Global Namespace

Low Level API

The Meta API