Add an experimental mechanism to support extending the pprint formatting.

Partly responds to SF bug #505152.

Doc/lib/libpprint.tex

@@ -173,3 +173,26 @@ this returns false.
 \begin{methoddesc}{isrecursive}{object}
 Determine if the object requires a recursive representation.
 \end{methoddesc}
+
+This method is provided as a hook to allow subclasses to modify the
+way objects are converted to strings.  The default implementation uses
+the internals of the \function{saferepr()} implementation.
+
+\begin{methoddesc}{format}{object, context, maxlevels, level}
+Returns three values: the formatted version of \var{object} as a
+string, a flag indicating whether the result is readable, and a flag
+indicating whether recursion was detected.  The first argument is the
+object to be presented.  The second is a dictionary which contains the
+\function{id()} of objects that are part of the current presentation
+context (direct and indirect containers for \var{object} that are
+affecting the presentation) as the keys; if an object needs to be
+presented which is already represented in \var{context}, the third
+return value should be true.  Recursive calls to the \method{format()}
+method should add additionaly entries for containers to this
+dictionary.  The fourth argument, \var{maxlevels}, gives the requested
+limit to recursion; this will be \code{0} if there is no requested
+limit.  This argument should be passed unmodified to recursive calls.
+The fourth argument, \var{level} gives the current level; recursive
+calls should be passed a value less than that of the current call.
+\versionadded{2.3}
+\end{methoddesc}

Lib/pprint.py

@@ -115,15 +115,11 @@ class PrettyPrinter:
         return sio.getvalue()
 
     def isrecursive(self, object):
-        self.__recursive = 0
-        self.__repr(object, {}, 0)
-        return self.__recursive
+        return self.format(object, {}, 0)[2]
 
     def isreadable(self, object):
-        self.__recursive = 0
-        self.__readable = 1
-        self.__repr(object, {}, 0)
-        return self.__readable and not self.__recursive
+        s, readable, recursive = self.format(object, {}, 0)
+        return readable and not recursive
 
     def __format(self, object, stream, indent, allowance, context, level):
         level = level + 1
@@ -196,14 +192,22 @@ class PrettyPrinter:
         write(rep)
 
     def __repr(self, object, context, level):
-        repr, readable, recursive = _safe_repr(object, context,
-                                               self.__depth, level)
+        repr, readable, recursive = self.format(object, context.copy(),
+                                                self.__depth, level)
         if not readable:
             self.__readable = 0
         if recursive:
             self.__recursive = 1
         return repr
 
+    def format(self, object, context, maxlevels, level):
+        """Format object for a specific context, returning a string
+        and flags indicating whether the representation is 'readable'
+        and whether the object represents a recursive construct.
+        """
+        return _safe_repr(object, context, maxlevels, level)
+
+
 # Return triple (repr_string, isreadable, isrecursive).
 
 def _safe_repr(object, context, maxlevels, level):

Lib/test/test_pprint.py

@@ -96,6 +96,27 @@ class QueryTestCase(unittest.TestCase):
  'write_io_runtime_us': 43690}"""
         self.assertEqual(pprint.pformat(o), exp)
 
+    def test_subclassing(self):
+        o = {'names with spaces': 'should be presented using repr()',
+             'others.should.not.be': 'like.this'}
+        exp = """\
+{'names with spaces': 'should be presented using repr()',
+ others.should.not.be: like.this}"""
+        self.assertEqual(DottedPrettyPrinter().pformat(o), exp)
+
+
+class DottedPrettyPrinter(pprint.PrettyPrinter):
+    def format(self, object, context, maxlevels, level):
+        if isinstance(object, str):
+            if ' ' in object:
+                return `object`, 1, 0
+            else:
+                return object, 0, 0
+        else:
+            return pprint.PrettyPrinter.format(
+                self, object, context, maxlevels, level)
+
+
 def test_main():
     test_support.run_unittest(QueryTestCase)