django/docs/ref/utils.txt

============
Django Utils
============

.. module:: django.utils
   :synopsis: Django's built-in utilities.

This document covers all stable modules in ``django.utils``. Most of the
modules in ``django.utils`` are designed for internal use and only the
following parts can be considered stable and thus backwards compatible as per
the :ref:`internal release deprecation policy <internal-release-deprecation-policy>`.

``django.utils.cache``
======================

.. module:: django.utils.cache
   :synopsis: Helper functions for controlling caching.

This module contains helper functions for controlling HTTP caching. It does so
by managing the ``Vary`` header of responses. It includes functions to patch
the header of response objects directly and decorators that change functions to
do that header-patching themselves.

For information on the ``Vary`` header, see :rfc:`7231#section-7.1.4`.

Essentially, the ``Vary`` HTTP header defines which headers a cache should take
into account when building its cache key. Requests with the same path but
different header content for headers named in ``Vary`` need to get different
cache keys to prevent delivery of wrong content.

For example, :doc:`internationalization </topics/i18n/index>` middleware would
need to distinguish caches by the ``Accept-language`` header.

.. function:: patch_cache_control(response, **kwargs)

    This function patches the ``Cache-Control`` header by adding all keyword
    arguments to it. The transformation is as follows:

    * All keyword parameter names are turned to lowercase, and underscores
      are converted to hyphens.
    * If the value of a parameter is ``True`` (exactly ``True``, not just a
      true value), only the parameter name is added to the header.
    * All other parameters are added with their value, after applying
      ``str()`` to it.

.. function:: get_max_age(response)

    Returns the max-age from the response Cache-Control header as an integer
    (or ``None`` if it wasn't found or wasn't an integer).

.. function:: patch_response_headers(response, cache_timeout=None)

    Adds some useful headers to the given ``HttpResponse`` object:

    * ``Expires``
    * ``Cache-Control``

    Each header is only added if it isn't already set.

    ``cache_timeout`` is in seconds. The :setting:`CACHE_MIDDLEWARE_SECONDS`
    setting is used by default.

.. function:: add_never_cache_headers(response)

    Adds a ``Cache-Control: max-age=0, no-cache, no-store, must-revalidate``
    header to a response to indicate that a page should never be cached.

.. function:: patch_vary_headers(response, newheaders)

    Adds (or updates) the ``Vary`` header in the given ``HttpResponse`` object.
    ``newheaders`` is a list of header names that should be in ``Vary``.
    Existing headers in ``Vary`` aren't removed.

.. function:: get_cache_key(request, key_prefix=None)

    Returns a cache key based on the request path. It can be used in the
    request phase because it pulls the list of headers to take into account
    from the global path registry and uses those to build a cache key to
    check against.

    If there is no headerlist stored, the page needs to be rebuilt, so this
    function returns ``None``.

.. function:: learn_cache_key(request, response, cache_timeout=None, key_prefix=None)

    Learns what headers to take into account for some request path from the
    response object. It stores those headers in a global path registry so that
    later access to that path will know what headers to take into account
    without building the response object itself. The headers are named in
    the ``Vary`` header of the response, but we want to prevent response
    generation.

    The list of headers to use for cache key generation is stored in the same
    cache as the pages themselves. If the cache ages some data out of the
    cache, this just means that we have to build the response once to get at
    the Vary header and so at the list of headers to use for the cache key.

``django.utils.dateparse``
==========================

.. module:: django.utils.dateparse
   :synopsis: Functions to parse strings to datetime objects.

The functions defined in this module share the following properties:

- They accept strings in ISO 8601 date/time formats (or some close
  alternatives) and return objects from the corresponding classes in Python's
  :mod:`datetime` module.
- They raise :exc:`ValueError` if their input is well formatted but isn't a
  valid date or time.
- They return ``None`` if it isn't well formatted at all.
- They accept up to picosecond resolution in input, but they truncate it to
  microseconds, since that's what Python supports.

.. function:: parse_date(value)

    Parses a string and returns a :class:`datetime.date`.

.. function:: parse_time(value)

    Parses a string and returns a :class:`datetime.time`.

    UTC offsets aren't supported; if ``value`` describes one, the result is
    ``None``.

.. function:: parse_datetime(value)

    Parses a string and returns a :class:`datetime.datetime`.

    UTC offsets are supported; if ``value`` describes one, the result's
    ``tzinfo`` attribute is a :class:`datetime.timezone` instance.

    .. versionchanged:: 2.2

        In older versions, the ``tzinfo`` attribute is a
        :class:`~django.utils.timezone.FixedOffset` instance.

.. function:: parse_duration(value)

    Parses a string and returns a :class:`datetime.timedelta`.

    Expects data in the format ``"DD HH:MM:SS.uuuuuu"`` or as specified by ISO
    8601 (e.g. ``P4DT1H15M20S`` which is equivalent to ``4 1:15:20``) or
    PostgreSQL's day-time interval format (e.g. ``3 days 04:05:06``).

``django.utils.decorators``
===========================

.. module:: django.utils.decorators
    :synopsis: Functions that help with creating decorators for views.

.. function:: method_decorator(decorator, name='')

    Converts a function decorator into a method decorator. It can be used to
    decorate methods or classes; in the latter case, ``name`` is the name
    of the method to be decorated and is required.

    ``decorator`` may also be a list or tuple of functions. They are wrapped
    in reverse order so that the call order is the order in which the functions
    appear in the list/tuple.

    See :ref:`decorating class based views <decorating-class-based-views>` for
    example usage.

.. function:: decorator_from_middleware(middleware_class)

    Given a middleware class, returns a view decorator. This lets you use
    middleware functionality on a per-view basis. The middleware is created
    with no params passed.

    It assumes middleware that's compatible with the old style of Django 1.9
    and earlier (having methods like ``process_request()``,
    ``process_exception()``, and ``process_response()``).

.. function:: decorator_from_middleware_with_args(middleware_class)

    Like ``decorator_from_middleware``, but returns a function
    that accepts the arguments to be passed to the middleware_class.
    For example, the :func:`~django.views.decorators.cache.cache_page`
    decorator is created from the ``CacheMiddleware`` like this::

         cache_page = decorator_from_middleware_with_args(CacheMiddleware)

         @cache_page(3600)
         def my_view(request):
             pass

``django.utils.encoding``
=========================

.. module:: django.utils.encoding
   :synopsis: A series of helper functions to manage character encoding.

.. function:: python_2_unicode_compatible

    A decorator that defines ``__unicode__`` and ``__str__`` methods under
    Python 2. Under Python 3 it does nothing.

    To support Python 2 and 3 with a single code base, define a ``__str__``
    method returning text (use ``six.text_type()`` if you're doing some
    casting) and apply this decorator to the class.

.. function:: smart_text(s, encoding='utf-8', strings_only=False, errors='strict')

    Returns a ``str`` object representing arbitrary object ``s``. Treats
    bytestrings using the ``encoding`` codec.

    If ``strings_only`` is ``True``, don't convert (some) non-string-like
    objects.

.. function:: is_protected_type(obj)

    Determine if the object instance is of a protected type.

    Objects of protected types are preserved as-is when passed to
    ``force_text(strings_only=True)``.

.. function:: force_text(s, encoding='utf-8', strings_only=False, errors='strict')

    Similar to ``smart_text``, except that lazy instances are resolved to
    strings, rather than kept as lazy objects.

    If ``strings_only`` is ``True``, don't convert (some) non-string-like
    objects.

.. function:: smart_bytes(s, encoding='utf-8', strings_only=False, errors='strict')

    Returns a bytestring version of arbitrary object ``s``, encoded as
    specified in ``encoding``.

    If ``strings_only`` is ``True``, don't convert (some) non-string-like
    objects.

.. function:: force_bytes(s, encoding='utf-8', strings_only=False, errors='strict')

    Similar to ``smart_bytes``, except that lazy instances are resolved to
    bytestrings, rather than kept as lazy objects.

    If ``strings_only`` is ``True``, don't convert (some) non-string-like
    objects.

.. function:: smart_str(s, encoding='utf-8', strings_only=False, errors='strict')

    Alias of :func:`smart_text`. This function returns a ``str`` or a lazy
    string.

    For instance, this is suitable for writing to :data:`sys.stdout`.

    Alias of :func:`smart_bytes` on Python 2 (in older versions of Django that
    support it).

.. function:: force_str(s, encoding='utf-8', strings_only=False, errors='strict')

    Alias of :func:`force_text`. This function always returns a ``str``.

    Alias of :func:`force_bytes` on Python 2 (in older versions of Django that
    support it).

.. function:: iri_to_uri(iri)

    Convert an Internationalized Resource Identifier (IRI) portion to a URI
    portion that is suitable for inclusion in a URL.

    This is the algorithm from section 3.1 of :rfc:`3987#section-3.1`, slightly
    simplified since the input is assumed to be a string rather than an
    arbitrary byte stream.

    Takes an IRI (string or UTF-8 bytes) and returns a string containing the
    encoded result.

.. function:: uri_to_iri(uri)

    Converts a Uniform Resource Identifier into an Internationalized Resource
    Identifier.

    This is an algorithm from section 3.2 of :rfc:`3987#section-3.2`.

    Takes a URI in ASCII bytes and returns a string containing the encoded
    result.

.. function:: filepath_to_uri(path)

    Convert a file system path to a URI portion that is suitable for inclusion
    in a URL. The path is assumed to be either UTF-8 bytes or string.

    This method will encode certain characters that would normally be
    recognized as special characters for URIs.  Note that this method does not
    encode the ' character, as it is a valid character within URIs. See
    ``encodeURIComponent()`` JavaScript function for more details.

    Returns an ASCII string containing the encoded result.

.. function:: escape_uri_path(path)

    Escapes the unsafe characters from the path portion of a Uniform Resource
    Identifier (URI).

``django.utils.feedgenerator``
==============================

.. module:: django.utils.feedgenerator
   :synopsis: Syndication feed generation library -- used for generating RSS, etc.

Sample usage::

    >>> from django.utils import feedgenerator
    >>> feed = feedgenerator.Rss201rev2Feed(
    ...     title="Poynter E-Media Tidbits",
    ...     link="http://www.poynter.org/column.asp?id=31",
    ...     description="A group Weblog by the sharpest minds in online media/journalism/publishing.",
    ...     language="en",
    ... )
    >>> feed.add_item(
    ...     title="Hello",
    ...     link="http://www.holovaty.com/test/",
    ...     description="Testing.",
    ... )
    >>> with open('test.rss', 'w') as fp:
    ...     feed.write(fp, 'utf-8')

For simplifying the selection of a generator use ``feedgenerator.DefaultFeed``
which is currently ``Rss201rev2Feed``

For definitions of the different versions of RSS, see:
https://web.archive.org/web/20110718035220/http://diveintomark.org/archives/2004/02/04/incompatible-rss

.. function:: get_tag_uri(url, date)

    Creates a TagURI.

    See https://web.archive.org/web/20110514113830/http://diveintomark.org/archives/2004/05/28/howto-atom-id

``SyndicationFeed``
-------------------

.. class:: SyndicationFeed

    Base class for all syndication feeds. Subclasses should provide write().

    .. method:: __init__(title, link, description, language=None, author_email=None, author_name=None, author_link=None, subtitle=None, categories=None, feed_url=None, feed_copyright=None, feed_guid=None, ttl=None, **kwargs)

        Initialize the feed with the given dictionary of metadata, which applies
        to the entire feed.

        Any extra keyword arguments you pass to ``__init__`` will be stored in
        ``self.feed``.

        All parameters should be strings, except ``categories``, which should
        be a sequence of strings.

    .. method:: add_item(title, link, description, author_email=None, author_name=None, author_link=None, pubdate=None, comments=None, unique_id=None, categories=(), item_copyright=None, ttl=None, updateddate=None, enclosures=None, **kwargs)

        Adds an item to the feed. All args are expected to be strings except
        ``pubdate`` and ``updateddate``, which are ``datetime.datetime``
        objects, and ``enclosures``, which is a list of ``Enclosure`` instances.

    .. method:: num_items()

    .. method:: root_attributes()

        Return extra attributes to place on the root (i.e. feed/channel)
        element. Called from ``write()``.

    .. method:: add_root_elements(handler)

        Add elements in the root (i.e. feed/channel) element.
        Called from ``write()``.

    .. method:: item_attributes(item)

        Return extra attributes to place on each item (i.e. item/entry)
        element.

    .. method:: add_item_elements(handler, item)

        Add elements on each item (i.e. item/entry) element.

    .. method:: write(outfile, encoding)

        Outputs the feed in the given encoding to ``outfile``, which is a
        file-like object. Subclasses should override this.

    .. method:: writeString(encoding)

        Returns the feed in the given encoding as a string.

    .. method:: latest_post_date()

        Returns the latest ``pubdate`` or ``updateddate`` for all items in the
        feed. If no items have either of these attributes this returns the
        current UTC date/time.

``Enclosure``
-------------

.. class:: Enclosure

    Represents an RSS enclosure

``RssFeed``
-----------

.. class:: RssFeed(SyndicationFeed)

``Rss201rev2Feed``
------------------

.. class:: Rss201rev2Feed(RssFeed)

    Spec: https://cyber.harvard.edu/rss/rss.html

``RssUserland091Feed``
----------------------

.. class:: RssUserland091Feed(RssFeed)

    Spec: http://backend.userland.com/rss091

``Atom1Feed``
-------------

.. class:: Atom1Feed(SyndicationFeed)

    Spec: https://tools.ietf.org/html/rfc4287

``django.utils.functional``
===========================

.. module:: django.utils.functional
    :synopsis: Functional programming tools.

.. class:: cached_property(func, name=None)

    The ``@cached_property`` decorator caches the result of a method with a
    single ``self`` argument as a property. The cached result will persist
    as long as the instance does, so if the instance is passed around and the
    function subsequently invoked, the cached result will be returned.

    Consider a typical case, where a view might need to call a model's method
    to perform some computation, before placing the model instance into the
    context, where the template might invoke the method once more::

        # the model
        class Person(models.Model):

            def friends(self):
                # expensive computation
                ...
                return friends

        # in the view:
        if person.friends():
            ...

    And in the template you would have:

    .. code-block:: html+django

        {% for friend in person.friends %}

    Here, ``friends()`` will be called twice. Since the instance ``person`` in
    the view and the template are the same, decorating the ``friends()`` method
    with ``@cached_property`` can avoid that::

        from django.utils.functional import cached_property

        class Person(models.Model):

            @cached_property
            def friends(self):
                ...

    Note that as the method is now a property, in Python code it will need to
    be accessed appropriately::

        # in the view:
        if person.friends:
            ...

    The cached value can be treated like an ordinary attribute of the instance::

        # clear it, requiring re-computation next time it's called
        del person.friends # or delattr(person, "friends")

        # set a value manually, that will persist on the instance until cleared
        person.friends = ["Huckleberry Finn", "Tom Sawyer"]

    As well as offering potential performance advantages, ``@cached_property``
    can ensure that an attribute's value does not change unexpectedly over the
    life of an instance. This could occur with a method whose computation is
    based on ``datetime.now()``, or simply if a change were saved to the
    database by some other process in the brief interval between subsequent
    invocations of a method on the same instance.

    You can use the ``name`` argument to make cached properties of other
    methods. For example, if you had an expensive ``get_friends()`` method and
    wanted to allow calling it without retrieving the cached value, you could
    write::

        friends = cached_property(get_friends, name='friends')

    While ``person.get_friends()`` will recompute the friends on each call, the
    value of the cached property will persist until you delete it as described
    above::

        x = person.friends         # calls first time
        y = person.get_friends()   # calls again
        z = person.friends         # does not call
        x is z                     # is True

    .. warning::

        ``cached_property`` doesn't work properly with a mangled__ name unless
        it's passed a ``name`` of the form ``_Class__attribute``::

            __friends = cached_property(get_friends, name='_Person__friends')

        __ https://docs.python.org/faq/programming.html#i-try-to-use-spam-and-i-get-an-error-about-someclassname-spam

.. function:: keep_lazy(func, *resultclasses)

    Django offers many utility functions (particularly in ``django.utils``)
    that take a string as their first argument and do something to that string.
    These functions are used by template filters as well as directly in other
    code.

    If you write your own similar functions and deal with translations, you'll
    face the problem of what to do when the first argument is a lazy
    translation object. You don't want to convert it to a string immediately,
    because you might be using this function outside of a view (and hence the
    current thread's locale setting will not be correct).

    For cases like this, use the ``django.utils.functional.keep_lazy()``
    decorator. It modifies the function so that *if* it's called with a lazy
    translation as one of its arguments, the function evaluation is delayed
    until it needs to be converted to a string.

    For example::

        from django.utils.functional import keep_lazy, keep_lazy_text

        def fancy_utility_function(s, ...):
            # Do some conversion on string 's'
            ...
        fancy_utility_function = keep_lazy(str)(fancy_utility_function)

        # Or more succinctly:
        @keep_lazy(str)
        def fancy_utility_function(s, ...):
            ...

    The ``keep_lazy()`` decorator takes a number of extra arguments (``*args``)
    specifying the type(s) that the original function can return. A common
    use case is to have functions that return text. For these, you can just
    pass the ``str`` type to ``keep_lazy`` (or even simpler, use the
    :func:`keep_lazy_text` decorator described in the next section).

    Using this decorator means you can write your function and assume that the
    input is a proper string, then add support for lazy translation objects at
    the end.

.. function:: keep_lazy_text(func)

    A shortcut for ``keep_lazy(str)(func)``.

    If you have a function that returns text and you want to be able to take
    lazy arguments while delaying their evaluation, simply use this decorator::

        from django.utils.functional import keep_lazy, keep_lazy_text

        # Our previous example was:
        @keep_lazy(str)
        def fancy_utility_function(s, ...):
            ...

        # Which can be rewritten as:
        @keep_lazy_text
        def fancy_utility_function(s, ...):
            ...

``django.utils.html``
=====================

.. module:: django.utils.html
   :synopsis: HTML helper functions

Usually you should build up HTML using Django's templates to make use of its
autoescape mechanism, using the utilities in :mod:`django.utils.safestring`
where appropriate. This module provides some additional low level utilities for
escaping HTML.

.. function:: escape(text)

    Returns the given text with ampersands, quotes and angle brackets encoded
    for use in HTML. The input is first coerced to a string and the output has
    :func:`~django.utils.safestring.mark_safe` applied.

.. function:: conditional_escape(text)

    Similar to ``escape()``, except that it doesn't operate on pre-escaped
    strings, so it will not double escape.

.. function:: format_html(format_string, *args, **kwargs)

    This is similar to :meth:`str.format`, except that it is appropriate for
    building up HTML fragments. All args and kwargs are passed through
    :func:`conditional_escape` before being passed to ``str.format()``.

    For the case of building up small HTML fragments, this function is to be
    preferred over string interpolation using ``%`` or ``str.format()``
    directly, because it applies escaping to all arguments - just like the
    template system applies escaping by default.

    So, instead of writing::

        mark_safe("%s <b>%s</b> %s" % (
            some_html,
            escape(some_text),
            escape(some_other_text),
        ))

    You should instead use::

        format_html("{} <b>{}</b> {}",
            mark_safe(some_html),
            some_text,
            some_other_text,
        )

    This has the advantage that you don't need to apply :func:`escape` to each
    argument and risk a bug and an XSS vulnerability if you forget one.

    Note that although this function uses ``str.format()`` to do the
    interpolation, some of the formatting options provided by ``str.format()``
    (e.g. number formatting) will not work, since all arguments are passed
    through :func:`conditional_escape` which (ultimately) calls
    :func:`~django.utils.encoding.force_text` on the values.

.. function:: format_html_join(sep, format_string, args_generator)

    A wrapper of :func:`format_html`, for the common case of a group of
    arguments that need to be formatted using the same format string, and then
    joined using ``sep``. ``sep`` is also passed through
    :func:`conditional_escape`.

    ``args_generator`` should be an iterator that returns the sequence of
    ``args`` that will be passed to :func:`format_html`. For example::

        format_html_join(
            '\n', "<li>{} {}</li>",
            ((u.first_name, u.last_name) for u in users)
        )

.. function:: strip_tags(value)

    Tries to remove anything that looks like an HTML tag from the string, that
    is anything contained within ``<>``.

    Absolutely NO guarantee is provided about the resulting string being
    HTML safe. So NEVER mark safe the result of a ``strip_tag`` call without
    escaping it first, for example with :func:`~django.utils.html.escape`.

    For example::

        strip_tags(value)

    If ``value`` is ``"<b>Joel</b> <button>is</button> a <span>slug</span>"``
    the return value will be ``"Joel is a slug"``.

    If you are looking for a more robust solution, take a look at the `bleach
    <https://pypi.org/project/bleach/>`_ Python library.

.. function:: html_safe()

    The ``__html__()`` method on a class helps non-Django templates detect
    classes whose output doesn't require HTML escaping.

    This decorator defines the ``__html__()`` method on the decorated class
    by wrapping ``__str__()`` in :meth:`~django.utils.safestring.mark_safe`.
    Ensure the ``__str__()`` method does indeed return text that doesn't
    require HTML escaping.

``django.utils.http``
=====================

.. module:: django.utils.http
   :synopsis: HTTP helper functions. (URL encoding, cookie handling, ...)

.. function:: urlencode(query, doseq=False)

    A version of Python's :func:`urllib.parse.urlencode` function that can
    operate on ``MultiValueDict`` and non-string values.

.. function:: cookie_date(epoch_seconds=None)

    .. deprecated:: 2.1

        Use :func:`http_date` instead, which follows the latest RFC.

    Formats the time to ensure compatibility with Netscape's cookie standard.

    Accepts a floating point number expressed in seconds since the epoch in
    UTC--such as that outputted by ``time.time()``. If set to ``None``,
    defaults to the current time.

    Outputs a string in the format ``Wdy, DD-Mon-YYYY HH:MM:SS GMT``.

.. function:: http_date(epoch_seconds=None)

    Formats the time to match the :rfc:`1123` date format as specified by HTTP
    :rfc:`7231#section-7.1.1.1`.

    Accepts a floating point number expressed in seconds since the epoch in
    UTC--such as that outputted by ``time.time()``. If set to ``None``,
    defaults to the current time.

    Outputs a string in the format ``Wdy, DD Mon YYYY HH:MM:SS GMT``.

.. function:: base36_to_int(s)

    Converts a base 36 string to an integer.

.. function:: int_to_base36(i)

    Converts a positive integer to a base 36 string.

.. function:: urlsafe_base64_encode(s)

    Encodes a bytestring in base64 for use in URLs, stripping any trailing
    equal signs.

.. function::  urlsafe_base64_decode(s)

    Decodes a base64 encoded string, adding back any trailing equal signs that
    might have been stripped.

``django.utils.module_loading``
===============================

.. module:: django.utils.module_loading
   :synopsis: Functions for working with Python modules.

Functions for working with Python modules.

.. function:: import_string(dotted_path)

    Imports a dotted module path and returns the attribute/class designated by
    the last name in the path. Raises ``ImportError`` if the import failed. For
    example::

        from django.utils.module_loading import import_string
        ValidationError = import_string('django.core.exceptions.ValidationError')

    is equivalent to::

        from django.core.exceptions import ValidationError

``django.utils.safestring``
===========================

.. module:: django.utils.safestring
   :synopsis: Functions and classes for working with strings that can be displayed safely without further escaping in HTML.

Functions and classes for working with "safe strings": strings that can be
displayed safely without further escaping in HTML. Marking something as a "safe
string" means that the producer of the string has already turned characters
that should not be interpreted by the HTML engine (e.g. '<') into the
appropriate entities.

.. class:: SafeString

    A ``str`` subclass that has been specifically marked as "safe"
    (requires no further escaping) for HTML output purposes. Alias of
    :class:`SafeText`.

.. class:: SafeText

    A ``str`` subclass that has been specifically marked as "safe" for HTML
    output purposes.

.. function:: mark_safe(s)

    Explicitly mark a string as safe for (HTML) output purposes. The returned
    object can be used everywhere a string is appropriate.

    Can be called multiple times on a single string.

    Can also be used as a decorator.

    For building up fragments of HTML, you should normally be using
    :func:`django.utils.html.format_html` instead.

    String marked safe will become unsafe again if modified. For example::

        >>> mystr = '<b>Hello World</b>   '
        >>> mystr = mark_safe(mystr)
        >>> type(mystr)
        <class 'django.utils.safestring.SafeText'>

        >>> mystr = mystr.strip()  # removing whitespace
        >>> type(mystr)
        <type 'str'>

``django.utils.text``
=====================

.. module:: django.utils.text
    :synopsis: Text manipulation.

.. function:: format_lazy(format_string, *args, **kwargs)

    A version of :meth:`str.format` for when ``format_string``, ``args``,
    and/or ``kwargs`` contain lazy objects. The first argument is the string to
    be formatted. For example::

        from django.utils.text import format_lazy
        from django.utils.translation import pgettext_lazy

        urlpatterns = [
            path(format_lazy('{person}/<int:pk>/', person=pgettext_lazy('URL', 'person')),
                 PersonDetailView.as_view()),
        ]

    This example allows translators to translate part of the URL. If "person"
    is translated to "persona", the regular expression will match
    ``persona/(?P<pk>\d+)/$``, e.g. ``persona/5/``.

.. function:: slugify(value, allow_unicode=False)

    Converts a string to a URL slug by:

    #. Converting to ASCII if ``allow_unicode`` is ``False`` (the default).
    #. Removing characters that aren't alphanumerics, underscores, hyphens, or
       whitespace.
    #. Removing leading and trailing whitespace.
    #. Converting to lowercase.
    #. Replacing any whitespace or repeated dashes with single dashes.

    For example::

        >>> slugify(' Joel is a slug ')
        'joel-is-a-slug'

    If you want to allow Unicode characters, pass ``allow_unicode=True``. For
    example::

        >>> slugify('你好 World', allow_unicode=True)
        '你好-world'

.. _time-zone-selection-functions:

``django.utils.timezone``
=========================

.. module:: django.utils.timezone
    :synopsis: Timezone support.

.. data:: utc

    :class:`~datetime.tzinfo` instance that represents UTC.

.. class:: FixedOffset(offset=None, name=None)

    A :class:`~datetime.tzinfo` subclass modeling a fixed offset from UTC.
    ``offset`` is an integer number of minutes east of UTC.

    .. deprecated:: 2.2

        Use :class:`datetime.timezone` instead.

.. function:: get_fixed_timezone(offset)

    Returns a :class:`~datetime.tzinfo` instance that represents a time zone
    with a fixed offset from UTC.

    ``offset`` is a :class:`datetime.timedelta` or an integer number of
    minutes. Use positive values for time zones east of UTC and negative
    values for west of UTC.

.. function:: get_default_timezone()

    Returns a :class:`~datetime.tzinfo` instance that represents the
    :ref:`default time zone <default-current-time-zone>`.

.. function:: get_default_timezone_name()

    Returns the name of the :ref:`default time zone
    <default-current-time-zone>`.

.. function:: get_current_timezone()

    Returns a :class:`~datetime.tzinfo` instance that represents the
    :ref:`current time zone <default-current-time-zone>`.

.. function:: get_current_timezone_name()

    Returns the name of the :ref:`current time zone
    <default-current-time-zone>`.

.. function:: activate(timezone)

    Sets the :ref:`current time zone <default-current-time-zone>`. The
    ``timezone`` argument must be an instance of a :class:`~datetime.tzinfo`
    subclass or a time zone name.

.. function:: deactivate()

    Unsets the :ref:`current time zone <default-current-time-zone>`.

.. function:: override(timezone)

    This is a Python context manager that sets the :ref:`current time zone
    <default-current-time-zone>` on entry with :func:`activate()`, and restores
    the previously active time zone on exit. If the ``timezone`` argument is
    ``None``, the :ref:`current time zone <default-current-time-zone>` is unset
    on entry with :func:`deactivate()` instead.

    ``override`` is also usable as a function decorator.

.. function:: localtime(value=None, timezone=None)

    Converts an aware :class:`~datetime.datetime` to a different time zone,
    by default the :ref:`current time zone <default-current-time-zone>`.

    When ``value`` is omitted, it defaults to :func:`now`.

    This function doesn't work on naive datetimes; use :func:`make_aware`
    instead.

.. function:: localdate(value=None, timezone=None)

    Uses :func:`localtime` to convert an aware :class:`~datetime.datetime` to a
    :meth:`~datetime.datetime.date` in a different time zone, by default the
    :ref:`current time zone <default-current-time-zone>`.

    When ``value`` is omitted, it defaults to :func:`now`.

    This function doesn't work on naive datetimes.

.. function:: now()

    Returns a :class:`~datetime.datetime` that represents the
    current point in time. Exactly what's returned depends on the value of
    :setting:`USE_TZ`:

    * If :setting:`USE_TZ` is ``False``, this will be a
      :ref:`naive <naive_vs_aware_datetimes>` datetime (i.e. a datetime
      without an associated timezone) that represents the current time
      in the system's local timezone.

    * If :setting:`USE_TZ` is ``True``, this will be an
      :ref:`aware <naive_vs_aware_datetimes>` datetime representing the
      current time in UTC. Note that :func:`now` will always return
      times in UTC regardless of the value of :setting:`TIME_ZONE`;
      you can use :func:`localtime` to get the time in the current time zone.

.. function:: is_aware(value)

    Returns ``True`` if ``value`` is aware, ``False`` if it is naive. This
    function assumes that ``value`` is a :class:`~datetime.datetime`.

.. function:: is_naive(value)

    Returns ``True`` if ``value`` is naive, ``False`` if it is aware. This
    function assumes that ``value`` is a :class:`~datetime.datetime`.

.. function:: make_aware(value, timezone=None, is_dst=None)

    Returns an aware :class:`~datetime.datetime` that represents the same
    point in time as ``value`` in ``timezone``, ``value`` being a naive
    :class:`~datetime.datetime`. If ``timezone`` is set to ``None``, it
    defaults to the :ref:`current time zone <default-current-time-zone>`.

    The ``pytz.AmbiguousTimeError`` exception is raised if you try to make
    ``value`` aware during a DST transition where the same time occurs twice
    (when reverting from DST). Setting ``is_dst`` to ``True`` or ``False`` will
    avoid the exception by choosing if the time is pre-transition or
    post-transition respectively.

    The ``pytz.NonExistentTimeError`` exception is raised if you try to make
    ``value`` aware during a DST transition such that the time never occurred
    (when entering into DST). Setting ``is_dst`` to ``True`` or ``False`` will
    avoid the exception by moving the hour backwards or forwards by 1
    respectively. For example, ``is_dst=True`` would change a nonexistent
    time of 2:30 to 1:30 and ``is_dst=False`` would change the time to 3:30.

.. function:: make_naive(value, timezone=None)

    Returns a naive :class:`~datetime.datetime` that represents in
    ``timezone``  the same point in time as ``value``, ``value`` being an
    aware :class:`~datetime.datetime`. If ``timezone`` is set to ``None``, it
    defaults to the :ref:`current time zone <default-current-time-zone>`.

``django.utils.translation``
============================

.. module:: django.utils.translation
   :synopsis: Internationalization support.

For a complete discussion on the usage of the following see the
:doc:`translation documentation </topics/i18n/translation>`.

The ``u`` prefix on the functions below comes from a difference in Python 2
between unicode and bytestrings. If your code doesn't support Python 2, use the
functions without the ``u``.

.. function:: gettext(message)
.. function:: ugettext(message)

    Translates ``message`` and returns it as a string.

.. function:: pgettext(context, message)

    Translates ``message`` given the ``context`` and returns it as a string.

    For more information, see :ref:`contextual-markers`.

.. function:: gettext_lazy(message)
.. function:: ugettext_lazy(message)
.. function:: pgettext_lazy(context, message)

    Same as the non-lazy versions above, but using lazy execution.

    See :ref:`lazy translations documentation <lazy-translations>`.

.. function:: gettext_noop(message)
.. function:: ugettext_noop(message)

    Marks strings for translation but doesn't translate them now. This can be
    used to store strings in global variables that should stay in the base
    language (because they might be used externally) and will be translated
    later.

.. function:: ngettext(singular, plural, number)
.. function:: ungettext(singular, plural, number)

    Translates ``singular`` and ``plural`` and returns the appropriate string
    based on ``number``.

.. function:: npgettext(context, singular, plural, number)

    Translates ``singular`` and ``plural`` and returns the appropriate string
    based on ``number`` and the ``context``.

.. function:: ngettext_lazy(singular, plural, number)
.. function:: ungettext_lazy(singular, plural, number)
.. function:: npgettext_lazy(context, singular, plural, number)

    Same as the non-lazy versions above, but using lazy execution.

    See :ref:`lazy translations documentation <lazy-translations>`.

.. function:: activate(language)

    Fetches the translation object for a given language and activates it as
    the current translation object for the current thread.

.. function:: deactivate()

    Deactivates the currently active translation object so that further _ calls
    will resolve against the default translation object, again.

.. function:: deactivate_all()

    Makes the active translation object a ``NullTranslations()`` instance.
    This is useful when we want delayed translations to appear as the original
    string for some reason.

.. function:: override(language, deactivate=False)

    A Python context manager that uses
    :func:`django.utils.translation.activate` to fetch the translation object
    for a given language, activates it as the translation object for the
    current thread and reactivates the previous active language on exit.
    Optionally, it can simply deactivate the temporary translation on exit with
    :func:`django.utils.translation.deactivate` if the ``deactivate`` argument
    is ``True``. If you pass ``None`` as the language argument, a
    ``NullTranslations()`` instance is activated within the context.

    ``override`` is also usable as a function decorator.

.. function:: check_for_language(lang_code)

    Checks whether there is a global language file for the given language
    code (e.g. 'fr', 'pt_BR'). This is used to decide whether a user-provided
    language is available.

.. function:: get_language()

    Returns the currently selected language code. Returns ``None`` if
    translations are temporarily deactivated (by :func:`deactivate_all()` or
    when ``None`` is passed to :func:`override()`).

.. function:: get_language_bidi()

    Returns selected language's BiDi layout:

    * ``False`` = left-to-right layout
    * ``True`` = right-to-left layout

.. function:: get_language_from_request(request, check_path=False)

    Analyzes the request to find what language the user wants the system to
    show. Only languages listed in settings.LANGUAGES are taken into account.
    If the user requests a sublanguage where we have a main language, we send
    out the main language.

    If ``check_path`` is ``True``, the function first checks the requested URL
    for whether its path begins with a language code listed in the
    :setting:`LANGUAGES` setting.

.. function:: get_supported_language_variant(lang_code, strict=False)

    .. versionadded:: 2.1

    Returns ``lang_code`` if it's in the :setting:`LANGUAGES` setting, possibly
    selecting a more generic variant. For example, ``'es'`` is returned if
    ``lang_code`` is ``'es-ar'`` and ``'es'`` is in :setting:`LANGUAGES` but
    ``'es-ar'`` isn't.

    If ``strict`` is ``False`` (the default), a country-specific variant may
    be returned when neither the language code nor its generic variant is found.
    For example, if only ``'es-co'`` is in :setting:`LANGUAGES`, that's
    returned for ``lang_code``\s like ``'es'`` and ``'es-ar'``. Those matches
    aren't returned if ``strict=True``.

    Raises :exc:`LookupError` if nothing is found.

.. function:: to_locale(language)

    Turns a language name (en-us) into a locale name (en_US).

.. function:: templatize(src)

    Turns a Django template into something that is understood by ``xgettext``.
    It does so by translating the Django translation tags into standard
    ``gettext`` function invocations.

.. data:: LANGUAGE_SESSION_KEY

    Session key under which the active language for the current session is
    stored.