Now that internal dialect type is immutable, and the dialect registry

only contains instances of the dialect type, we can refer directly to the
dialect instances rather than creating new ones. In other words, if the
dialect comes from the registry, and we apply no further modifications,
the reader/writer can use the dialect object directly.

Modules/_csv.c

@@ -301,11 +301,12 @@ static char *dialect_kws[] = {
 	NULL
 };
 
-static int
-dialect_init(DialectObj * self, PyObject * args, PyObject * kwargs)
+static PyObject *
+dialect_new(PyTypeObject *type, PyObject *args, PyObject *kwargs)
 {
-	int ret = -1;
-        PyObject *dialect = NULL;
+	DialectObj *self;
+	PyObject *ret = NULL;
+	PyObject *dialect = NULL;
 	PyObject *delimiter = NULL;
 	PyObject *doublequote = NULL;
 	PyObject *escapechar = NULL;
@@ -326,7 +327,35 @@ dialect_init(DialectObj * self, PyObject * args, PyObject * kwargs)
 					 &quoting,
 					 &skipinitialspace,
 					 &strict))
-                return -1;
+		return NULL;
+
+	if (dialect != NULL) {
+		if (IS_BASESTRING(dialect)) {
+			dialect = get_dialect_from_registry(dialect);
+			if (dialect == NULL)
+				return NULL;
+		}
+		else
+			Py_INCREF(dialect);
+		/* Can we reuse this instance? */
+		if (PyObject_TypeCheck(dialect, &Dialect_Type) &&
+		    delimiter == 0 &&
+		    doublequote == 0 &&
+		    escapechar == 0 &&
+		    lineterminator == 0 &&
+		    quotechar == 0 &&
+		    quoting == 0 &&
+		    skipinitialspace == 0 &&
+		    strict == 0)
+			return dialect;
+	}
+
+	self = (DialectObj *)type->tp_alloc(type, 0);
+	if (self == NULL) {
+		Py_XDECREF(dialect);
+		return NULL;
+	}
+	self->lineterminator = NULL;
 
 	Py_XINCREF(delimiter);
 	Py_XINCREF(doublequote);
@@ -337,16 +366,9 @@ dialect_init(DialectObj * self, PyObject * args, PyObject * kwargs)
 	Py_XINCREF(skipinitialspace);
 	Py_XINCREF(strict);
 	if (dialect != NULL) {
-		if (IS_BASESTRING(dialect)) {
-			dialect = get_dialect_from_registry(dialect);
-			if (dialect == NULL)
-				goto err;
-		} else
-			Py_INCREF(dialect);
 #define DIALECT_GETATTR(v, n) \
 		if (v == NULL) \
 			v = PyObject_GetAttrString(dialect, n)
-
 		DIALECT_GETATTR(delimiter, "delimiter");
 		DIALECT_GETATTR(doublequote, "doublequote");
 		DIALECT_GETATTR(escapechar, "escapechar");
@@ -356,7 +378,6 @@ dialect_init(DialectObj * self, PyObject * args, PyObject * kwargs)
 		DIALECT_GETATTR(skipinitialspace, "skipinitialspace");
 		DIALECT_GETATTR(strict, "strict");
 		PyErr_Clear();
-		Py_DECREF(dialect);
 	}
 
 	/* check types and convert to C values */
@@ -387,12 +408,13 @@ dialect_init(DialectObj * self, PyObject * args, PyObject * kwargs)
 		goto err;
 	}
 	if (self->lineterminator == 0) {
-                PyErr_SetString(PyExc_TypeError, "lineterminator must be set");
+		PyErr_SetString(PyExc_TypeError, "lineterminator must be set");
 		goto err;
 	}
 
-	ret = 0;
+	ret = (PyObject *)self;
 err:
+	Py_XDECREF(dialect);
 	Py_XDECREF(delimiter);
 	Py_XDECREF(doublequote);
 	Py_XDECREF(escapechar);
@@ -401,18 +423,7 @@ err:
 	Py_XDECREF(quoting);
 	Py_XDECREF(skipinitialspace);
 	Py_XDECREF(strict);
-        return ret;
-}
-
-static PyObject *
-dialect_new(PyTypeObject *type, PyObject *args, PyObject *kwargs)
-{
-        DialectObj *self;
-        self = (DialectObj *)type->tp_alloc(type, 0);
-        if (self != NULL) {
-                self->lineterminator = NULL;
-        }
-        return (PyObject *)self;
+	return ret;
 }
 
 
@@ -459,8 +470,8 @@ static PyTypeObject Dialect_Type = {
 	0,					/* tp_descr_get */
 	0,					/* tp_descr_set */
 	0,					/* tp_dictoffset */
-	(initproc)dialect_init,			/* tp_init */
-	PyType_GenericAlloc,	                /* tp_alloc */
+	0,					/* tp_init */
+	0,					/* tp_alloc */
 	dialect_new,			        /* tp_new */
 	0,                           		/* tp_free */
 };