Document default fmtspec. Fix inaccurate note. Document exceptions.

diff --git a/Doc/library/functions.rst b/Doc/library/functions.rst
index 3020128ce5f1db0f94f7abd3d863eb4529b9f9a7..8b7eef5f2377365c82fe4de86576655aa1fbdcf9 100644 (file)
--- a/Doc/library/functions.rst
+++ b/Doc/library/functions.rst
@@ -498,11 +498,14 @@ are always available.  They are listed here in alphabetical order.
    of the *value* argument, however there is a standard formatting syntax that
    is used by most built-in types: :ref:`formatspec`.
 
-   .. note::
-
-      ``format(value, format_spec)`` merely calls
-      ``value.__format__(format_spec)``.
-
+   The default *format_spec* is an empty string which usually gives the same
+   effect as calling ``str(value)``.
+
+   A call to ``format(value, format_spec)`` is translated to
+   ``type(value).__format__(format_spec)`` which bypasses the instance
+   dictionary when searching for the value's :meth:`__format__` method.  A
+   :exc:`TypeError` exception is raised if the method is not found or if either
+   the *format_spec* or the return value are not strings.
 
 .. function:: frozenset([iterable])
    :noindex: