pythongh-88287: Add BufferingFormatter documentation. (pythonGH-96608)

(cherry picked from commit d5e0786)

Co-authored-by: Vinay Sajip <[email protected]>

Doc/library/logging.rst

@@ -652,6 +652,35 @@ The useful mapping keys in a :class:`LogRecord` are given in the section on
       :func:`traceback.print_stack`, but with the last newline removed) as a
       string. This default implementation just returns the input value.
 
+.. class:: BufferingFormatter(linefmt=None)
+
+   A base formatter class suitable for subclassing when you want to format a
+   number of records. You can pass a :class:`Formatter` instance which you want
+   to use to format each line (that corresponds to a single record). If not
+   specified, the default formatter (which just outputs the event message) is
+   used as the line formatter.
+
+   .. method:: formatHeader(records)
+
+      Return a header for a list of *records*. The base implementation just
+      returns the empty string. You will need to override this method if you
+      want specific behaviour, e.g. to show the count of records, a title or a
+      separator line.
+
+   .. method:: formatFooter(records)
+
+      Return a footer for a list of *records*. The base implementation just
+      returns the empty string. You will need to override this method if you
+      want specific behaviour, e.g. to show the count of records or a separator
+      line.
+
+   .. method:: format(records)
+
+      Return formatted text for a list of *records*. The base implementation
+      just returns the empty string if there are no records; otherwise, it
+      returns the concatenation of the header, each record formatted with the
+      line formatter, and the footer.
+
 .. _filter:
 
 Filter Objects