Document django.utils.functional (first draft for comments)

django/utils/functional.py

@@ -18,11 +18,10 @@ def _curried(*moreargs, **morekwargs):
 
 def memoize(func, cache, num_args):
     """
-    Wrap a function so that results for any argument tuple are stored in
-    'cache'. Note that the args to the function must be usable as dictionary
-    keys.
-
-    Only the first num_args are considered when creating the key.
+    Wraps a function 'func' so that its results are stored in a cache
+    dictionary 'cache'. A tuple of the function's first num_args arguments
+    will be used as the cache key. All of these arguments must be usable as
+    dictionary keys.
     """
     @wraps(func)
     def wrapper(*args):

docs/ref/urlresolvers.txt

@@ -4,8 +4,8 @@
 
 .. module:: django.core.urlresolvers
 
-reverse()
----------
+:func:`~reverse`
+----------------
 
 If you need to use something similar to the :ttag:`url` template tag in
 your code, Django provides the following function:
@@ -68,8 +68,10 @@ You can use ``kwargs`` instead of ``args``. For example::
     ``urllib.quote``) to the output of ``reverse()`` may produce undesirable
     results.
 
-reverse_lazy()
---------------
+.. function:: reverse_lazy
+
+:func:`reverse_lazy`
+--------------------
 
 A lazily evaluated version of `reverse()`_.
 

docs/ref/utils.txt

@@ -423,6 +423,15 @@ Atom1Feed
 .. module:: django.utils.functional
     :synopsis: Functional programming tools.
 
+It is important to note that these tools were by and large created to provide
+some useful utilities for use within the codebase of Django itself. They exist
+chiefly because they serve the needs of Django internals and are not all
+necessarily the best way to meet the needs of your own application. Other
+Python libraries may exist that provide the same functionality more robustly.
+
+Classes
+-------
+
 .. class:: cached_property(object)
 
     The ``@cached_property`` decorator caches the result of a method with a
@@ -481,17 +490,120 @@ Atom1Feed
     database by some other process in the brief interval between subsequent
     invocations of a method on the same instance.
 
+.. class:: LazyObject
+
+    ``LazyObject`` is the base class for lazy objects, including
+    :class:`~SimpleLazyObject` below. Subclass ``LazyObject`` for your object
+    if you need to intercept and manage its instantiation manually; otherwise,
+    subclass :class:`~SimpleLazyObject`.
+
+.. class:: SimpleLazyObject
+
+    A ``SimpleLazyObject`` resists instantiation until it's required.
+
+    ``SimpleLazyObject`` can be used as the base for a new class, as in the
+    case of ``django.utils.text.Truncator``.
+
+    It can also be used to create any object lazily::
+
+        lazy_object = SimpleLazyObject(lambda: some_function(*args))
+
+    ``SimpleLazyObject`` is for compound objects of arbitrary type. For Python
+    built-ins, or objects whose type is known, use
+    :func:`~lazy`.
+
+.. class:: lazy_property(fget=None, fset=None, fdel=None, doc=None)
+
+    ``lazy_property`` creates a property whose instantiation will be delayed.
+
+    A ``lazy_property`` can be created with arguments referring to functions
+    that will get, set, and delete the property's value.
+
+    For example::
+
+        class Person(models.Model):
+
+            # define the methods to get and set the property's value
+            def _get_whereabouts(self):
+                ...
+
+            def _set_whereabouts(self):
+                ...
+
+            # define the lazy_property
+            whereabouts = lazy_property(_get_whereabouts, _set_whereabouts)
+
+Functions
+---------
+
+.. function:: memoize(func, cache, num_args)
+
+    Wraps a function ``func``, so that its output is saved in the dictionary
+    ``cache``. ``memoize`` uses the function's first ``num_args`` arguments as
+    a key for the cache.
+
+    An example::
+
+        # an expensive function we want to cache
+        def solve(crises=(), when="today")
+            ...
+            return solutions
+
+        # create a cache dictionary
+        solution_cache = {}
+
+        # wrap solve(), using both arguments as the cache key
+        solve = memoize(solve, solution_cache, 2)
+
+    The first time the wapped ``solve()`` function is called with particular
+    values of the ``crises`` and ``when`` arguments, it will execute normally
+    and save the output in the ``solution_cache`` dictionary. On subsequent
+    occasions, it will hit the cache instead. If it's called with new values of
+    either ``crises`` or ``when``, it will execute and save the output in the
+    cache with a new key.
+
+    Any arguments that are to be used in this way must therefore be usable as
+    dictionary keys. If one of the arguments were itself a dictionary (and
+    therefore not possible as a dictionary key) it would need to be excluded.
+    In this example, the function takes the dictionary ``agencies`` as an
+    argument::
+
+        # an expensive function we want to cache
+        def solve(crises=(), when="today", agencies={})
+            ...
+            return solutions
+
+        # create a cache dictionary
+        solution_cache = {}
+
+        # wrap solve(), using only first 2 arguments as the cache key
+        solve = memoize(solve, solution_cache, 2)
+
+    Any arguments that don't affect the output of the function can also be
+    excluded.
+
+.. function:: lazy(func, *resultclasses)
+
+    ``lazy()`` returns a lazy version of any callable, that resists evaluation
+    until required . This is used by for example
+    :func:`django.core.urlresolvers.reverse_lazy`, a `lazily evaluated
+    <understanding_laziness>`_ version of
+    :func:`django.core.urlresolvers.reverse`.
+
+    The ``*resultclasses`` argument is used to cast the return values of the
+    orginal function into the correct type(s).
+
 .. function:: 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).
+    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
@@ -508,15 +620,26 @@ Atom1Feed
         # 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.
+    The ``allow_lazy()`` decorator takes, in addition to the function to
+    decorate, a number of extra arguments (``*resultclasses``) 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.
+    input is a proper string, then add support for lazy translation objects at
+    the end.
+
+.. function:: partition(predicate, values)
+
+    ``partition`` splits a collection of values into two sets, based on whether
+    the ``predicate`` evaluates as ``True`` or ``False`` when supplied with
+    that value.
+
+    For example::
+
+        >>> partition(lambda x: x > 3, range(5))
+        [0, 1, 2, 3], [4]
 
 ``django.utils.html``
 =====================

docs/topics/performance.txt

@@ -183,6 +183,8 @@ method to a property.
 Certain Django components also have their own caching functionality; these are
 discussed below in the sections related to those components.
 
+.. _understanding_laziness:
+
 Understanding laziness
 ======================
 
@@ -223,6 +225,11 @@ QuerySet <when-querysets-are-evaluated>`. Avoiding the premature evaluation of
 a ``QuerySet`` can save making an expensive and unnecessary trip to the
 database.
 
+Laziness-related utility functions
+^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+
+:meth:`~django.utils.functional.lazy` creates a lazy version of any callable.
+
 Django also offers an :meth:`~django.utils.functional.allow_lazy` decorator.
 This allows a function that has been called with a lazy argument to behave
 lazily itself, only being evaluated when it needs to be. Thus the lazy argument