bpo-27385: Clarify docstring for groupby() (#3738)

Doc/library/itertools.rst

@@ -53,7 +53,7 @@ Iterator                        Arguments                       Results
 :func:`compress`                data, selectors                 (d[0] if s[0]), (d[1] if s[1]), ...                 ``compress('ABCDEF', [1,0,1,0,1,1]) --> A C E F``
 :func:`dropwhile`               pred, seq                       seq[n], seq[n+1], starting when pred fails          ``dropwhile(lambda x: x<5, [1,4,6,4,1]) --> 6 4 1``
 :func:`filterfalse`             pred, seq                       elements of seq where pred(elem) is false           ``filterfalse(lambda x: x%2, range(10)) --> 0 2 4 6 8``
-:func:`groupby`                 iterable[, keyfunc]             sub-iterators grouped by value of keyfunc(v)
+:func:`groupby`                 iterable[, key]                 sub-iterators grouped by value of key(v)
 :func:`islice`                  seq, [start,] stop [, step]     elements from seq[start:stop:step]                  ``islice('ABCDEFG', 2, None) --> C D E F G``
 :func:`starmap`                 func, seq                       func(\*seq[0]), func(\*seq[1]), ...                 ``starmap(pow, [(2,5), (3,2), (10,3)]) --> 32 9 1000``
 :func:`takewhile`               pred, seq                       seq[0], seq[1], until pred fails                    ``takewhile(lambda x: x<5, [1,4,6,4,1]) --> 1 4``

Modules/itertoolsmodule.c

@@ -176,8 +176,9 @@ static PyMethodDef groupby_methods[] = {
 };
 
 PyDoc_STRVAR(groupby_doc,
-"groupby(iterable[, keyfunc]) -> create an iterator which returns\n\
-(key, sub-iterator) grouped by each value of key(value).\n");
+"groupby(iterable, key=None) -> make an iterator that returns consecutive\n\
+keys and groups from the iterable.  If the key function is not specified or\n\
+is None, the element itself is used for grouping.\n");
 
 static PyTypeObject groupby_type = {
     PyVarObject_HEAD_INIT(NULL, 0)