docs/api.rst
.. module:: jinja2 :noindex: :synopsis: public Jinja API
This document describes the API to Jinja and not the template language
(for that, see :doc:/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.
Jinja uses a central object called the template :class: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
:class:Template class, an environment is created automatically for you,
albeit a shared one.
Most applications will create one :class: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 is to use :class:~loaders.PackageLoader.
.. code-block:: python
from jinja2 import Environment, PackageLoader, select_autoescape
env = Environment(
loader=PackageLoader("yourapp"),
autoescape=select_autoescape()
)
This will create a template environment with a loader that looks up
templates in the templates folder inside the yourapp Python
package (or next to the yourapp.py Python module). It also enables
autoescaping for HTML files. This loader only requires that yourapp
is importable, it figures out the absolute path to the folder for you.
Different loaders are available to load templates in other ways or from
other locations. They're listed in the Loaders_ section below. You can
also write your own if you want to load templates from a source that's
more specialized to your project.
To load a template from this environment, call the :meth:get_template
method, which returns the loaded :class:Template.
.. code-block:: python
template = env.get_template("mytemplate.html")
To render it with some variables, call the :meth:render method.
.. code-block:: python
print(template.render(the="variables", go="here"))
Using a template loader rather than passing strings to :class:Template
or :meth:Environment.from_string has multiple advantages. Besides being
a lot easier to use it also enables template inheritance.
.. admonition:: 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.
The high-level API is the API you will use in the application to load and
render Jinja templates. The :ref:low-level-api on the other side is only
useful if you want to dig deeper into Jinja or :ref:develop extensions <jinja-extensions>.
.. autoclass:: Environment([options]) :members: from_string, get_template, select_template, get_or_select_template, join_path, extend, compile_expression, compile_templates, list_templates, add_extension
.. 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 variables that are available in every template loaded
by the environment. As long as no template was loaded it's safe
to modify this. For more details see :ref:`global-namespace`.
For valid object names see :ref:`identifier-naming`.
.. attribute:: policies
A dictionary with :ref:`policies`. These can be reconfigured to
change the runtime behavior or certain template features. Usually
these are security related.
.. attribute:: 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.
.. attribute:: 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
:class:`~jinja2.runtime.Context`.
.. automethod:: overlay([options])
.. method:: undefined([hint, obj, name, exc])
Creates a new :class:`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 :exc:`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 :func:`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.
.. autoclass:: Template :members: module, make_module
.. attribute:: globals
A dict of variables that are available every time the template
is rendered, without needing to pass them during render. This
should not be modified, as depending on how the template was
loaded it may be shared with the environment and other
templates.
Defaults to :attr:`Environment.globals` unless extra values are
passed to :meth:`Environment.get_template`.
Globals are only intended for data that is common to every
render of the template. Specific data should be passed to
:meth:`render`.
See :ref:`global-namespace`.
.. attribute:: name
The loading name of the template. If the template was loaded from a
string this is `None`.
.. attribute:: filename
The filename of the template on the file system if it was loaded from
there. Otherwise this is `None`.
.. automethod:: render([context])
.. automethod:: generate([context])
.. automethod:: stream([context])
.. automethod:: render_async([context])
.. automethod:: generate_async([context])
.. autoclass:: jinja2.environment.TemplateStream() :members: disable_buffering, enable_buffering, dump
.. versionchanged:: 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).
.. autofunction:: 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 :func:~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 :func:~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 :ref:autoescape-overrides).
.. _identifier-naming:
Jinja uses Python naming rules. Valid identifiers can be any combination of 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.str. The regular
expression for filter and test identifiers is
[a-zA-Z_][a-zA-Z0-9_]*(\.[a-zA-Z_][a-zA-Z0-9_]*)*.
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 :class:StrictUndefined which
disallows all operations beside testing if it's an undefined object.
.. autoclass:: jinja2.Undefined()
.. attribute:: _undefined_hint
Either `None` or a string with the error message for the
undefined object.
.. attribute:: _undefined_obj
Either `None` or the owner object that caused the undefined object
to be created (for example because an attribute does not exist).
.. attribute:: _undefined_name
The name for the undefined variable / attribute or just `None`
if no such information exists.
.. attribute:: _undefined_exception
The exception that the undefined object wants to raise. This
is usually one of :exc:`UndefinedError` or :exc:`SecurityError`.
.. method:: _fail_with_undefined_error(\*args, \**kwargs)
When called with any arguments this method raises
:attr:`_undefined_exception` with an error message generated
from the undefined hints stored on the undefined object.
.. autoclass:: jinja2.ChainableUndefined()
.. autoclass:: jinja2.DebugUndefined()
.. autoclass:: jinja2.StrictUndefined()
There is also a factory function that can decorate undefined objects to implement logging on failures:
.. autofunction:: jinja2.make_logging_undefined
Undefined objects are created by calling :attr:undefined.
.. admonition:: Implementation
:class:`Undefined` is implemented by overriding the special
``__underscore__`` methods. For example the default
:class:`Undefined` class implements ``__str__`` to returns an empty
string, while ``__int__`` and others fail with an exception. To
allow conversion to int by returning ``0`` you can implement your
own subclass.
.. code-block:: python
class NullUndefined(Undefined):
def __int__(self):
return 0
def __float__(self):
return 0.0
To disallow a method, override it and raise
:attr:`~Undefined._undefined_exception`. Because this is very
common there is the helper method
:meth:`~Undefined._fail_with_undefined_error` that raises the error
with the correct information. Here's a class that works like the
regular :class:`Undefined` but fails on iteration::
class NonIterableUndefined(Undefined):
def __iter__(self):
self._fail_with_undefined_error()
.. autoclass:: jinja2.runtime.Context() :members: get, resolve, resolve_or_missing, 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` or points
to a dict created by combining the globals with the variables
passed to the render function. 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).
.. attribute:: eval_ctx
The current :ref:`eval-context`.
.. automethod:: jinja2.runtime.Context.call(callable, \*args, \**kwargs)
The context is immutable, it prevents modifications, and if it is modified somehow despite that those changes may not show up. For performance, Jinja does not use the context as data storage for, only as a primary data source. Variables that the template does not define are looked up in the context, but variables the template does define are stored locally.
Instead of modifying the context directly, a function should return a value that can be assigned to a variable within the template itself.
.. code-block:: jinja
{% set comments = get_latest_comments() %}
.. _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.BaseLoader :members: get_source, load
Here a list of the builtin loaders Jinja provides:
.. autoclass:: jinja2.FileSystemLoader
.. autoclass:: jinja2.PackageLoader
.. autoclass:: jinja2.DictLoader
.. autoclass:: jinja2.FunctionLoader
.. autoclass:: jinja2.PrefixLoader
.. autoclass:: jinja2.ChoiceLoader
.. autoclass:: 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 :class:Environment.
.. autoclass:: jinja2.BytecodeCache :members: load_bytecode, dump_bytecode, clear
.. autoclass:: jinja2.bccache.Bucket :members: write_bytecode, load_bytecode, bytecode_from_string, bytecode_to_string, reset
.. attribute:: environment
The :class:`Environment` that created the bucket.
.. attribute:: key
The unique cache key for this bucket
.. attribute:: code
The bytecode if it's loaded, otherwise `None`.
Builtin bytecode caches:
.. autoclass:: jinja2.FileSystemBytecodeCache
.. autoclass:: jinja2.MemcachedBytecodeCache
.. versionadded:: 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:
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 adds overhead compared to purely async
code.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
:attr:~jinja2.Environment.policies attribute.
Example::
env.policies['urlize.rel'] = 'nofollow noopener'
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.
urlize.extra_schemes:
Recognize URLs that start with these schemes in addition to the
default http://, https://, and mailto:.
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:
ext.i18n.trimmed:
If this is set to True, {% trans %} blocks of the
:ref:i18n-extension will always unify linebreaks and surrounding
whitespace as if the trimmed modifier was used.
These helper functions and classes are useful if you add custom filters or functions to a Jinja environment.
.. autofunction:: jinja2.pass_context
.. autofunction:: jinja2.pass_eval_context
.. autofunction:: jinja2.pass_environment
.. autofunction:: jinja2.clear_caches
.. autofunction:: jinja2.is_undefined
.. autoexception:: jinja2.TemplateError
.. autoexception:: jinja2.UndefinedError
.. autoexception:: jinja2.TemplateNotFound
.. autoexception:: jinja2.TemplatesNotFound
.. autoexception:: jinja2.TemplateSyntaxError
.. attribute:: message
The error message.
.. attribute:: lineno
The line number where the error occurred.
.. attribute:: name
The load name for the template.
.. attribute:: filename
The filename that loaded the template in the encoding of the
file system (most likely utf-8, or mbcs on Windows systems).
.. autoexception:: jinja2.TemplateRuntimeError
.. autoexception:: jinja2.TemplateAssertionError
.. _writing-filters:
Filters are Python functions that take the value to the left of the filter as the first argument and produce a new value. Arguments passed to the filter are passed after the value.
For example, the filter {{ 42|myfilter(23) }} is called behind the
scenes as myfilter(42, 23).
Jinja comes with some :ref:built-in filters <builtin-filters>. To use
a custom filter, write a function that takes at least a value
argument, then register it in :attr:Environment.filters.
Here's a filter that formats datetime objects:
.. code-block:: python
def datetime_format(value, format="%H:%M %d-%m-%y"):
return value.strftime(format)
environment.filters["datetime_format"] = datetime_format
Now it can be used in templates:
.. sourcecode:: jinja
{{ article.pub_date|datetime_format }}
{{ article.pub_date|datetime_format("%B %Y") }}
Some decorators are available to tell Jinja to pass extra information to the filter. The object is passed as the first argument, making the value being filtered the second argument.
pass_environment passes the :class:Environment.pass_eval_context passes the :ref:eval-context.pass_context passes the current
:class:~jinja2.runtime.Context.Here's a filter that converts line breaks into HTML and <p>
tags. It uses the eval context to check if autoescape is currently
enabled before escaping the input and marking the output safe.
.. code-block:: python
import re
from jinja2 import pass_eval_context
from markupsafe import Markup, escape
@pass_eval_context
def nl2br(eval_ctx, value):
br = "
\n"
if eval_ctx.autoescape:
value = escape(value)
br = Markup(br)
result = "\n\n".join(
f"<p>{br.join(p.splitlines())}</p>"
for p in re.split(r"(?:\r\n|\r(?!\n)|\n){2,}", value)
)
return Markup(result) if eval_ctx.autoescape else result
.. _writing-tests:
Test are Python functions that take the value to the left of the test as
the first argument, and return True or False. Arguments passed
to the test are passed after the value.
For example, the test {{ 42 is even }} is called behind the scenes
as is_even(42).
Jinja comes with some :ref:built-in tests <builtin-tests>. To use a
custom tests, write a function that takes at least a value argument,
then register it in :attr:Environment.tests.
Here's a test that checks if a value is a prime number:
.. code-block:: python
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
environment.tests["prime"] = is_prime
Now it can be used in templates:
.. sourcecode:: jinja
{% if value is prime %}
{{ value }} is a prime number
{% else %}
{{ value }} is not a prime number
{% endif %}
Some decorators are available to tell Jinja to pass extra information to the test. The object is passed as the first argument, making the value being tested the second argument.
pass_environment passes the :class:Environment.pass_eval_context passes the :ref:eval-context.pass_context passes the current
:class:~jinja2.runtime.Context... _eval-context:
The evaluation context (short eval context or eval ctx) makes it possible to activate and deactivate compiled features at runtime.
Currently it is only used to enable and disable automatic escaping, but it can be used by extensions as well.
The autoescape setting should be checked on the evaluation context,
not the environment. The evaluation context will have the computed value
for the current template.
Instead of pass_environment:
.. code-block:: python
@pass_environment
def filter(env, value):
result = do_something(value)
if env.autoescape:
result = Markup(result)
return result
Use pass_eval_context if you only need the setting:
.. code-block:: python
@pass_eval_context
def filter(eval_ctx, value):
result = do_something(value)
if eval_ctx.autoescape:
result = Markup(result)
return result
Or use pass_context if you need other context behavior as well:
.. code-block:: python
@pass_context
def filter(context, value):
result = do_something(value)
if context.eval_ctx.autoescape:
result = Markup(result)
return result
The evaluation context must not be modified at runtime. Modifications
must only happen with a :class:nodes.EvalContextModifier and
:class:nodes.ScopedEvalContextModifier from an extension, not on the
eval context object itself.
.. autoclass:: jinja2.nodes.EvalContext
.. attribute:: autoescape
`True` or `False` depending on if autoescaping is active or not.
.. attribute:: volatile
`True` if the compiler cannot evaluate some expressions at compile
time. At runtime this should always be `False`.
.. _global-namespace:
The global namespace stores variables and functions that should be
available without needing to pass them to :meth:Template.render. They
are also available to templates that are imported or included without
context. Most applications should only use :attr:Environment.globals.
:attr:Environment.globals are intended for data that is common to all
templates loaded by that environment. :attr:Template.globals are
intended for data that is common to all renders of that template, and
default to :attr:Environment.globals unless they're given in
:meth:Environment.get_template, etc. Data that is specific to a
render should be passed as context to :meth:Template.render.
Only one set of globals is used during any specific rendering. If
templates A and B both have template globals, and B extends A, then
only B's globals are used for both when using b.render().
Environment globals should not be changed after loading any templates, and template globals should not be changed at any time after loading the template. Changing globals after loading a template will result in unexpected behavior as they may be shared between the environment and other templates.
.. _low-level-api:
The low level API exposes functionality that can be useful to understand some
implementation details, debugging purposes or advanced :ref:extension <jinja-extensions> techniques. Unless you know exactly what you are doing we
don't recommend using any of those.
.. automethod:: Environment.lex
.. automethod:: Environment.parse
.. automethod:: Environment.preprocess
.. automethod:: Template.new_context
.. method:: Template.root_render_func(context)
This is the low level render function. It's passed a :class:`Context`
that has to be created by :meth:`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
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
:meth:`render` / :meth:`generate` / :meth:`stream` call.
.. attribute:: Template.blocks
A dict of block render functions. Each of these functions works exactly
like the :meth:`root_render_func` with the same limitations.
.. attribute:: Template.is_up_to_date
This attribute is `False` if there is a newer version of the template
available, otherwise `True`.
.. admonition:: 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 :meth:`~Environment.parse`.
.. versionadded:: 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 :meth:Environment.parse method.
.. autofunction:: jinja2.meta.find_undeclared_variables
.. autofunction:: jinja2.meta.find_referenced_templates