bpo-16011: clarify that 'in' always returns a boolean value (GH-152) (GH-883)

(cherry picked from commit 0ae7c8b)

Author: Mariatta

Doc/reference/expressions.rst

@@ -1249,37 +1249,30 @@ Python does not enforce these consistency rules.
 Membership test operations
 --------------------------
 
-The operators :keyword:`in` and :keyword:`not in` test for collection
-membership.  ``x in s`` evaluates to true if *x* is a member of the collection
-*s*, and false otherwise.  ``x not in s`` returns the negation of ``x in s``.
-The collection membership test has traditionally been bound to sequences; an
-object is a member of a collection if the collection is a sequence and contains
-an element equal to that object.  However, it make sense for many other object
-types to support membership tests without being a sequence.  In particular,
-dictionaries (for keys) and sets support membership testing.
-
-For the list and tuple types, ``x in y`` is true if and only if there exists an
-index *i* such that either ``x is y[i]`` or ``x == y[i]`` is true.
-
-For the Unicode and string types, ``x in y`` is true if and only if *x* is a
-substring of *y*.  An equivalent test is ``y.find(x) != -1``.  Note, *x* and *y*
-need not be the same type; consequently, ``u'ab' in 'abc'`` will return
-``True``. Empty strings are always considered to be a substring of any other
-string, so ``"" in "abc"`` will return ``True``.
-
-.. versionchanged:: 2.3
-   Previously, *x* was required to be a string of length ``1``.
+The operators :keyword:`in` and :keyword:`not in` test for membership.  ``x in
+s`` evaluates to ``True`` if *x* is a member of *s*, and ``False`` otherwise.
+``x not in s`` returns the negation of ``x in s``.  All built-in sequences and
+set types support this as well as dictionary, for which :keyword:`in` tests
+whether the dictionary has a given key. For container types such as list, tuple,
+set, frozenset, dict, or collections.deque, the expression ``x in y`` is equivalent
+to ``any(x is e or x == e for e in y)``.
+
+For the string and bytes types, ``x in y`` is ``True`` if and only if *x* is a
+substring of *y*.  An equivalent test is ``y.find(x) != -1``.  Empty strings are
+always considered to be a substring of any other string, so ``"" in "abc"`` will
+return ``True``.
 
 For user-defined classes which define the :meth:`__contains__` method, ``x in
-y`` is true if and only if ``y.__contains__(x)`` is true.
+y`` returns ``True`` if ``y.__contains__(x)`` returns a true value, and
+``False`` otherwise.
 
 For user-defined classes which do not define :meth:`__contains__` but do define
-:meth:`__iter__`, ``x in y`` is true if some value ``z`` with ``x == z`` is
+:meth:`__iter__`, ``x in y`` is ``True`` if some value ``z`` with ``x == z`` is
 produced while iterating over ``y``.  If an exception is raised during the
 iteration, it is as if :keyword:`in` raised that exception.
 
 Lastly, the old-style iteration protocol is tried: if a class defines
-:meth:`__getitem__`, ``x in y`` is true if and only if there is a non-negative
+:meth:`__getitem__`, ``x in y`` is ``True`` if and only if there is a non-negative
 integer index *i* such that ``x == y[i]``, and all lower integer indices do not
 raise :exc:`IndexError` exception. (If any other exception is raised, it is as
 if :keyword:`in` raised that exception).