Django documentation

Django Utils

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 internal release deprecation policy.

django.utils.cache

This module contains helper functions for controlling 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 2616 section 14.44.

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, internationalization middleware would need to distinguish caches by the Accept-language header.

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.
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).

patch_response_headers(response, cache_timeout=None)

Adds some useful headers to the given HttpResponse object:

  • ETag
  • Last-Modified
  • Expires
  • Cache-Control

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

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

add_never_cache_headers(response)

Adds headers to a response to indicate that a page should never be cached.

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.

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.

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.datastructures

class SortedDict
Deprecated in Django 1.7:

Deprecated since version 1.7: SortedDict is deprecated and will be removed in Django 1.9. Use collections.OrderedDict instead.

The django.utils.datastructures.SortedDict class is a dictionary that keeps its keys in the order in which they’re inserted.

Creating a new SortedDict

Creating a new SortedDict must be done in a way where ordering is guaranteed. For example:

SortedDict({'b': 1, 'a': 2, 'c': 3})

will not work. Passing in a basic Python dict could produce unreliable results. Instead do:

SortedDict([('b', 1), ('a', 2), ('c', 3)])

django.utils.dateparse

The functions defined in this module share the following properties:

  • They raise 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.
parse_date(value)

Parses a string and returns a datetime.date.

parse_time(value)

Parses a string and returns a datetime.time.

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

parse_datetime(value)

Parses a string and returns a datetime.datetime.

UTC offsets are supported; if value describes one, the result’s tzinfo attribute is a FixedOffset instance.

django.utils.decorators

method_decorator(decorator)

Converts a function decorator into a method decorator. See decorating class based views for example usage.

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.

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 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

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 and apply this decorator to the class.

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

Returns a text object representing sunicode on Python 2 and str on Python 3. Treats bytestrings using the encoding codec.

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

smart_unicode(s, encoding='utf-8', strings_only=False, errors='strict')

Historical name of smart_text(). Only available under Python 2.

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).

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.

force_unicode(s, encoding='utf-8', strings_only=False, errors='strict')

Historical name of force_text(). Only available under Python 2.

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

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

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

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.

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

Alias of smart_bytes() on Python 2 and smart_text() on Python 3. This function returns a str or a lazy string.

For instance, this is suitable for writing to sys.stdout on Python 2 and 3.

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

Alias of force_bytes() on Python 2 and force_text() on Python 3. This function always returns a str.

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. However, since we are assuming input is either UTF-8 or unicode already, we can simplify things a little from the full method.

Returns an ASCII string containing the encoded result.

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 or unicode.

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.

django.utils.feedgenerator

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: http://web.archive.org/web/20110718035220/http://diveintomark.org/archives/2004/02/04/incompatible-rss

get_tag_uri(url, date)

Creates a TagURI.

See http://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().

__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 Unicode objects, except categories, which should be a sequence of Unicode objects.

add_item(title, link, description[, author_email=None, author_name=None, author_link=None, pubdate=None, comments=None, unique_id=None, enclosure=None, categories=(), item_copyright=None, ttl=None, updateddate=None, **kwargs])

Adds an item to the feed. All args are expected to be Python unicode objects except pubdate and updateddate, which are datetime.datetime objects, and enclosure, which is an instance of the Enclosure class.

New in Django 1.7:

The optional updateddate argument was added.

num_items()
root_attributes()

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

add_root_elements(handler)

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

item_attributes(item)

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

add_item_elements(handler, item)

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

write(outfile, encoding)

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

writeString(encoding)

Returns the feed in the given encoding as a string.

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 date/time.

Enclosure

class Enclosure

Represents an RSS enclosure

RssFeed

class RssFeed(SyndicationFeed)

Rss201rev2Feed

class Rss201rev2Feed(RssFeed)

Spec: http://cyber.law.harvard.edu/rss/rss.html

RssUserland091Feed

class RssUserland091Feed(RssFeed)

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

Atom1Feed

class Atom1Feed(SyndicationFeed)

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

django.utils.functional

class cached_property(object)

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():

# in the template:
{% for friend in person.friends %}

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

from django.utils.functional import cached_property

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

Note that as the method is now a property, in Python code it will need to be invoked 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.

allow_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.allow_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 allow_lazy

def fancy_utility_function(s, ...):
    # Do some conversion on string 's'
    ...
# Replace unicode by str on Python 3
fancy_utility_function = allow_lazy(fancy_utility_function, unicode)

The allow_lazy() decorator takes, in addition to the function to decorate, a number of extra arguments (*args) specifying the type(s) that the original function can return. Usually, it’s enough to include unicode (or str on Python 3) here and ensure that your function returns only Unicode strings.

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.

django.utils.html

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

escape(text)

Returns the given text with ampersands, quotes and angle brackets encoded for use in HTML. The input is first passed through force_text() and the output has mark_safe() applied.

conditional_escape(text)

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

format_html(format_string, *args, **kwargs)

This is similar to str.format, except that it is appropriate for building up HTML fragments. All args and kwargs are passed through 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("{0} <b>{1}</b> {2}",
            mark_safe(some_html), some_text, some_other_text)

This has the advantage that you don’t need to apply 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 conditional_escape() which (ultimately) calls force_text() on the values.

format_html_join(sep, format_string, args_generator)

A wrapper of 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 conditional_escape().

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

format_html_join('\n', "<li>{0} {1}</li>", ((u.first_name, u.last_name)
                                            for u in users))
strip_tags(value)

Tries to remove anything that looks like an HTML tag from the string, that is anything contained within <>. Absolutely NO guaranty is provided about the resulting string being entirely HTML safe. So NEVER mark safe the result of a strip_tag call without escaping it first, for example with 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 Python library.

remove_tags(value, tags)

Removes a space-separated list of [X]HTML tag names from the output.

For example:

remove_tags(value, "b span")

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

Note that this filter is case-sensitive.

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

django.utils.http

urlquote(url, safe='/')

A version of Python’s urllib.quote() function that can operate on unicode strings. The url is first UTF-8 encoded before quoting. The returned string can safely be used as part of an argument to a subsequent iri_to_uri() call without double-quoting occurring. Employs lazy execution.

urlquote_plus(url, safe='')

A version of Python’s urllib.quote_plus() function that can operate on unicode strings. The url is first UTF-8 encoded before quoting. The returned string can safely be used as part of an argument to a subsequent iri_to_uri() call without double-quoting occurring. Employs lazy execution.

urlencode(query, doseq=0)

A version of Python’s urllib.urlencode() function that can operate on unicode strings. The parameters are first case to UTF-8 encoded strings and then encoded as per normal.

cookie_date(epoch_seconds=None)

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.

http_date(epoch_seconds=None)

Formats the time to match the RFC 1123 date format as specified by HTTP RFC 2616 section 3.3.1.

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.

base36_to_int(s)

Converts a base 36 string to an integer. On Python 2 the output is guaranteed to be an int and not a long.

int_to_base36(i)

Converts a positive integer to a base 36 string. On Python 2 i must be smaller than sys.maxint.

urlsafe_base64_encode(s)

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

urlsafe_base64_decode(s)

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

django.utils.module_loading

Functions for working with Python modules.

import_string(dotted_path)
New in Django 1.7.

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
ImproperlyConfigured = import_string('django.core.exceptions.ImproperlyConfigured')

is equivalent to:

from django.core.exceptions import ImproperlyConfigured
import_by_path(dotted_path, error_prefix='')
Deprecated in Django 1.7:

Deprecated since version 1.7: Use import_string() instead.

Imports a dotted module path and returns the attribute/class designated by the last name in the path. Raises ImproperlyConfigured if something goes wrong.

django.utils.safestring

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 SafeBytes

A bytes subclass that has been specifically marked as “safe” (requires no further escaping) for HTML output purposes.

class SafeString

A str subclass that has been specifically marked as “safe” (requires no further escaping) for HTML output purposes. This is SafeBytes on Python 2 and SafeText on Python 3.

class SafeText

A str (in Python 3) or unicode (in Python 2) subclass that has been specifically marked as “safe” for HTML output purposes.

class SafeUnicode

Historical name of SafeText. Only available under Python 2.

mark_safe(s)

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

Can be called multiple times on a single string.

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.SafeBytes'>

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

Explicitly mark a string as requiring HTML escaping upon output. Has no effect on SafeData subclasses.

Can be called multiple times on a single string (the resulting escaping is only applied once).

django.utils.text

slugify()

Converts to lowercase, removes non-word characters (alphanumerics and underscores) and converts spaces to hyphens. Also strips leading and trailing whitespace.

For example:

slugify(value)

If value is "Joel is a slug", the output will be "joel-is-a-slug".

django.utils.translation

For a complete discussion on the usage of the following see the translation documentation.

gettext(message)

Translates message and returns it in a UTF-8 bytestring

ugettext(message)

Translates message and returns it in a unicode string

pgettext(context, message)

Translates message given the context and returns it in a unicode string.

For more information, see Contextual markers.

gettext_lazy(message)
ugettext_lazy(message)
pgettext_lazy(context, message)

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

See lazy translations documentation.

gettext_noop(message)
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.

ngettext(singular, plural, number)

Translates singular and plural and returns the appropriate string based on number in a UTF-8 bytestring.

ungettext(singular, plural, number)

Translates singular and plural and returns the appropriate string based on number in a unicode string.

npgettext(context, singular, plural, number)

Translates singular and plural and returns the appropriate string based on number and the context in a unicode string.

ngettext_lazy(singular, plural, number)
ungettext_lazy(singular, plural, number)
npgettext_lazy(context, singular, plural, number)

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

See lazy translations documentation.

string_concat(*strings)

Lazy variant of string concatenation, needed for translations that are constructed from multiple parts.

activate(language)

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

deactivate()

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

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.

override(language, deactivate=False)

A Python context manager that uses 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 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.

get_language()

Returns the currently selected language code.

get_language_bidi()

Returns selected language’s BiDi layout:

  • False = left-to-right layout
  • True = right-to-left layout
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 LANGUAGES setting.

to_locale(language)

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

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.

LANGUAGE_SESSION_KEY

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

django.utils.timezone

utc

tzinfo instance that represents UTC.

get_fixed_timezone(offset)
New in Django 1.7.

Returns a tzinfo instance that represents a time zone with a fixed offset from UTC.

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

get_default_timezone()

Returns a tzinfo instance that represents the default time zone.

get_default_timezone_name()

Returns the name of the default time zone.

get_current_timezone()

Returns a tzinfo instance that represents the current time zone.

get_current_timezone_name()

Returns the name of the current time zone.

activate(timezone)

Sets the current time zone. The timezone argument must be an instance of a tzinfo subclass or, if pytz is available, a time zone name.

deactivate()

Unsets the current time zone.

override(timezone)

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

localtime(value, timezone=None)

Converts an aware datetime to a different time zone, by default the current time zone.

This function doesn’t work on naive datetimes; use make_aware() instead.

now()

Returns a datetime that represents the current point in time. Exactly what’s returned depends on the value of USE_TZ:

  • If USE_TZ is False, this will be be a naive datetime (i.e. a datetime without an associated timezone) that represents the current time in the system’s local timezone.
  • If USE_TZ is True, this will be an aware datetime representing the current time in UTC. Note that now() will always return times in UTC regardless of the value of TIME_ZONE; you can use localtime() to convert to a time in the current time zone.
is_aware(value)

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

is_naive(value)

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

make_aware(value, timezone)

Returns an aware datetime that represents the same point in time as value in timezone, value being a naive datetime.

This function can raise an exception if value doesn’t exist or is ambiguous because of DST transitions.

make_naive(value, timezone)

Returns an naive datetime that represents in timezone the same point in time as value, value being an aware datetime

django.utils.tzinfo

Deprecated in Django 1.7:

Deprecated since version 1.7: Use timezone instead.

class FixedOffset

Fixed offset in minutes east from UTC.

Deprecated in Django 1.7:

Deprecated since version 1.7: Use get_fixed_timezone() instead.

class LocalTimezone

Proxy timezone information from time module.

Deprecated in Django 1.7:

Deprecated since version 1.7: Use get_default_timezone() instead.

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