Issue #16206: Backport dict documentation improvements from 3.2.

Improve the documentation of the dict constructor.  This change includes
replacing the single-line signature documentation with a more complete
multiple-line signature.

Doc/library/functions.rst

@@ -275,14 +275,17 @@ available.  They are listed here in alphabetical order.
 
 
 .. _func-dict:
-.. function:: dict([arg])
+.. function:: dict(**kwarg)
+              dict(mapping, **kwarg)
+              dict(iterable, **kwarg)
    :noindex:
 
-   Create a new data dictionary, optionally with items taken from *arg*.
-   The dictionary type is described in :ref:`typesmapping`.
+   Create a new dictionary.  The :class:`dict` object is the dictionary class.
+   See :class:`dict` and :ref:`typesmapping` for documentation about this
+   class.
 
-   For other containers see the built in :class:`list`, :class:`set`, and
-   :class:`tuple` classes, and the :mod:`collections` module.
+   For other containers see the built-in :class:`list`, :class:`set`, and
+   :class:`tuple` classes, as well as the :mod:`collections` module.
 
 
 .. function:: dir([object])

Doc/library/stdtypes.rst

@@ -1956,32 +1956,41 @@ Dictionaries can be created by placing a comma-separated list of ``key: value``
 pairs within braces, for example: ``{'jack': 4098, 'sjoerd': 4127}`` or ``{4098:
 'jack', 4127: 'sjoerd'}``, or by the :class:`dict` constructor.
 
-.. class:: dict([arg])
+.. class:: dict(**kwarg)
+           dict(mapping, **kwarg)
+           dict(iterable, **kwarg)
 
-   Return a new dictionary initialized from an optional positional argument or from
-   a set of keyword arguments. If no arguments are given, return a new empty
-   dictionary. If the positional argument *arg* is a mapping object, return a
-   dictionary mapping the same keys to the same values as does the mapping object.
-   Otherwise the positional argument must be a sequence, a container that supports
-   iteration, or an iterator object.  The elements of the argument must each also
-   be of one of those kinds, and each must in turn contain exactly two objects.
-   The first is used as a key in the new dictionary, and the second as the key's
-   value.  If a given key is seen more than once, the last value associated with it
-   is retained in the new dictionary.
+   Return a new dictionary initialized from an optional positional argument
+   and a possibly empty set of keyword arguments.
 
-   If keyword arguments are given, the keywords themselves with their associated
-   values are added as items to the dictionary. If a key is specified both in the
-   positional argument and as a keyword argument, the value associated with the
-   keyword is retained in the dictionary. For example, these all return a
-   dictionary equal to ``{"one": 1, "two": 2}``:
+   If no positional argument is given, an empty dictionary is created.
+   If a positional argument is given and it is a mapping object, a dictionary
+   is created with the same key-value pairs as the mapping object.  Otherwise,
+   the positional argument must be an :term:`iterator` object.  Each item in
+   the iterable must itself be an iterator with exactly two objects.  The
+   first object of each item becomes a key in the new dictionary, and the
+   second object the corresponding value.  If a key occurs more than once, the
+   last value for that key becomes the corresponding value in the new
+   dictionary.
 
-   * ``dict(one=1, two=2)``
-   * ``dict({'one': 1, 'two': 2})``
-   * ``dict(zip(('one', 'two'), (1, 2)))``
-   * ``dict([['two', 2], ['one', 1]])``
+   If keyword arguments are given, the keyword arguments and their values are
+   added to the dictionary created from the positional argument.  If a key
+   being added is already present, the value from the keyword argument
+   replaces the value from the positional argument.
 
-   The first example only works for keys that are valid Python
-   identifiers; the others work with any valid keys.
+   To illustrate, the following examples all return a dictionary equal to
+   ``{"one": 1, "two": 2}``::
+
+      >>> a = dict(one=1, two=2)
+      >>> b = dict({'one': 1, 'two': 2})
+      >>> c = dict(zip(('one', 'two'), (1, 2)))
+      >>> d = dict([['two', 2], ['one', 1]])
+      >>> e = {"one": 1, "two": 2}
+      >>> a == b == c == d == e
+      True
+
+   Providing keyword arguments as in the first example only works for keys that
+   are valid Python identifiers.  Otherwise, any valid keys can be used.
 
    .. versionadded:: 2.2