API === .. 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. Basics ------ 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. High Level API -------------- 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 `. .. 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 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`. .. 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 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`. .. 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 Autoescaping ------------ .. 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, 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: Notes on Identifiers -------------------- 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_]*)*```. 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 :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() 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` 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) .. admonition:: 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 ------- 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: 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 Async Support ------------- .. 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: - Template rendering requires an event loop to be available to the current thread. :func:`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: 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. ``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. Utilities --------- These helper functions and classes are useful if you add custom filters or functions to a Jinja environment. .. autofunction:: jinja2.environmentfilter .. autofunction:: jinja2.contextfilter .. autofunction:: jinja2.evalcontextfilter .. autofunction:: jinja2.environmentfunction .. autofunction:: jinja2.contextfunction .. autofunction:: jinja2.evalcontextfunction .. 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. The return value is a :class:`Markup` string. .. autofunction:: jinja2.clear_caches .. autofunction:: jinja2.is_undefined .. autoclass:: jinja2.Markup([string]) :members: escape, unescape, striptags .. admonition:: Note The Jinja :class:`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 ---------- .. 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: 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 :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 filter wants to return an undefined value or check the current :attr:`~Environment.autoescape` setting. For this purpose three decorators exist: :func:`environmentfilter`, :func:`contextfilter` and :func:`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 = "\n\n".join( f"

{p.replace('\n', Markup('
\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 :class:`Context` rather than the environment. .. _eval-context: 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 :func:`contextfilter` and access the evaluation context from the actual context, or use a :func:`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 :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`. .. _writing-tests: 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 :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. .. _low-level-api: Low Level API ------------- The low level API exposes functionality that can be useful to understand some implementation details, debugging purposes or advanced :ref:`extension ` 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`. The Meta API ------------ .. 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