Enhance issubclass() and PyObject_IsSubclass() so that a tuple is

doerwalter · doerwalter · commit d9a6ad3bebc2 · 2002-12-12T16:41:44.000Z
supported as the second argument. This has the same meaning as
for isinstance(), i.e. issubclass(X, (A, B)) is equivalent
to issubclass(X, A) or issubclass(X, B). Compared to isinstance(),
this patch does not search the tuple recursively for classes, i.e.
any entry in the tuple that is not a class, will result in a
TypeError.

This closes SF patch #649608.
diff --git a/Doc/api/abstract.tex b/Doc/api/abstract.tex
@@ -205,10 +205,15 @@ \section{Object Protocol \label{object}}
                                             PyObject *cls}
   Returns \code{1} if the class \var{derived} is identical to or
   derived from the class \var{cls}, otherwise returns \code{0}.  In
-  case of an error, returns \code{-1}.  If either \var{derived} or
-  \var{cls} is not an actual class object, this function uses the
-  generic algorithm described above.
+  case of an error, returns \code{-1}. If \var{cls}
+  is a tuple, the check will be done against every entry in \var{cls}.
+  The result will be \code{1} when at least one of the checks returns
+  \code{1}, otherwise it will be \code{0}. If either \var{derived} or
+  \var{cls} is not an actual class object (or tuple), this function
+  uses the generic algorithm described above.
   \versionadded{2.1}
+  \versionchanged[Older versions of Python did not support a tuple
+                  as the second argument]{2.3}
 \end{cfuncdesc}
 
 
diff --git a/Doc/lib/libfuncs.tex b/Doc/lib/libfuncs.tex
@@ -550,11 +550,13 @@ \section{Built-in Functions \label{built-in-funcs}}
   \versionchanged[Support for a tuple of type information was added]{2.2}
 \end{funcdesc}
 
-\begin{funcdesc}{issubclass}{class1, class2}
-  Return true if \var{class1} is a subclass (direct or indirect) of
-  \var{class2}.  A class is considered a subclass of itself.  If
-  either argument is not a class object, a \exception{TypeError}
-  exception is raised.
+\begin{funcdesc}{issubclass}{class, classinfo}
+  Return true if \var{class} is a subclass (direct or indirect) of
+  \var{classinfo}.  A class is considered a subclass of itself.
+  \var{classinfo} may be a tuple of class objects, in which case every
+  entry in \var{classinfo} will be checked. In any other case, a
+  \exception{TypeError} exception is raised.
+  \versionchanged[Support for a tuple of type information was added]{2.3}
 \end{funcdesc}
 
 \begin{funcdesc}{iter}{o\optional{, sentinel}}
diff --git a/Lib/test/test_isinstance.py b/Lib/test/test_isinstance.py
@@ -218,6 +218,15 @@ def test_subclass_abstract(self):
         self.assertEqual(False, issubclass(AbstractChild, Super))
         self.assertEqual(False, issubclass(AbstractChild, Child))
 
+    def test_subclass_tuple(self):
+        # test with a tuple as the second argument classes
+        self.assertEqual(True, issubclass(Child, (Child,)))
+        self.assertEqual(True, issubclass(Child, (Super,)))
+        self.assertEqual(False, issubclass(Super, (Child,)))
+        self.assertEqual(True, issubclass(Super, (Child, Super)))
+        self.assertEqual(False, issubclass(Child, ()))
+        self.assertRaises(TypeError, issubclass, Child, ((Child,),))
+
 
 
 
diff --git a/Misc/NEWS b/Misc/NEWS
@@ -84,6 +84,10 @@ Type/class unification and new-style classes
 Core and builtins
 -----------------
 
+- issubclass now supports a tuple as the second argument, just like
+  isinstance does. ``issubclass(X, (A, B))`` is equivalent to
+  ``issubclass(X, A) or issubclass(X, B)``.
+
 - Thanks to Armin Rigo, the last known way to provoke a system crash
   by cleverly arranging for a comparison function to mutate a list
   during a list.sort() operation has been fixed.  The effect of
diff --git a/Objects/abstract.c b/Objects/abstract.c
@@ -1914,6 +1914,15 @@ abstract_issubclass(PyObject *derived, PyObject *cls)
 	if (derived == cls)
 		return 1;
 
+	if (PyTuple_Check(cls)) {
+		/* Not a general sequence -- that opens up the road to
+		   recursion and stack overflow. */
+		n = PyTuple_GET_SIZE(cls);
+		for (i = 0; i < n; i++) {
+			if (derived == PyTuple_GET_ITEM(cls, i))
+				return 1;
+		}
+	}
 	bases = abstract_get_bases(derived);
 	if (bases == NULL) {
 		if (PyErr_Occurred())
@@ -1932,6 +1941,20 @@ abstract_issubclass(PyObject *derived, PyObject *cls)
 	return r;
 }
 
+static int
+check_class(PyObject *cls, const char *error)
+{
+	PyObject *bases = abstract_get_bases(cls);
+	if (bases == NULL) {
+		/* Do not mask errors. */
+		if (!PyErr_Occurred())
+			PyErr_SetString(PyExc_TypeError, error);
+		return 0;
+	}
+	Py_DECREF(bases);
+	return -1;
+}
+
 int
 PyObject_IsInstance(PyObject *inst, PyObject *cls)
 {
@@ -1962,16 +1985,10 @@ PyObject_IsInstance(PyObject *inst, PyObject *cls)
 		return retval;
 	}
 	else {
-		PyObject *cls_bases = abstract_get_bases(cls);
-		if (cls_bases == NULL) {
-			/* Do not mask errors. */
-			if (!PyErr_Occurred())
-				PyErr_SetString(PyExc_TypeError,
-				"isinstance() arg 2 must be a class, type,"
-				" or tuple of classes and types");
+		if (!check_class(cls,
+			"isinstance() arg 2 must be a class, type,"
+			" or tuple of classes and types"))
 			return -1;
-		}
-		Py_DECREF(cls_bases);
 		if (__class__ == NULL) {
 			__class__ = PyString_FromString("__class__");
 			if (__class__ == NULL)
@@ -1997,28 +2014,25 @@ PyObject_IsSubclass(PyObject *derived, PyObject *cls)
 	int retval;
 
 	if (!PyClass_Check(derived) || !PyClass_Check(cls)) {
-		PyObject *derived_bases;
-		PyObject *cls_bases;
-
-		derived_bases = abstract_get_bases(derived);
-		if (derived_bases == NULL) {
-			/* Do not mask errors */
-			if (!PyErr_Occurred())
-				PyErr_SetString(PyExc_TypeError,
-					"issubclass() arg 1 must be a class");
+		if (!check_class(derived, "issubclass() arg 1 must be a class"))
 			return -1;
+
+		if (PyTuple_Check(cls)) {
+			int i;
+			int n = PyTuple_GET_SIZE(cls);
+			for (i = 0; i < n; ++i) {
+				if (!check_class(PyTuple_GET_ITEM(cls, i),
+						"issubclass() arg 2 must be a class"
+						" or tuple of classes"))
+					return -1;
+			}
 		}
-		Py_DECREF(derived_bases);
-
-		cls_bases = abstract_get_bases(cls);
-		if (cls_bases == NULL) {
-			/* Do not mask errors */
-			if (!PyErr_Occurred())
-				PyErr_SetString(PyExc_TypeError,
-					"issubclass() arg 2 must be a class");
-			return -1;
+		else {
+			if (!check_class(cls,
+					"issubclass() arg 2 must be a class"
+					" or tuple of classes"))
+				return -1;
 		}
-		Py_DECREF(cls_bases);
 
 		retval = abstract_issubclass(derived, cls);
 	}
diff --git a/Objects/classobject.c b/Objects/classobject.c
@@ -487,6 +487,13 @@ PyClass_IsSubclass(PyObject *class, PyObject *base)
 	PyClassObject *cp;
 	if (class == base)
 		return 1;
+	if (PyTuple_Check(base)) {
+		n = PyTuple_GET_SIZE(base);
+		for (i = 0; i < n; i++) {
+			if (class == PyTuple_GET_ITEM(base, i))
+				return 1;
+		}
+	}
 	if (class == NULL || !PyClass_Check(class))
 		return 0;
 	cp = (PyClassObject *)class;
diff --git a/Python/bltinmodule.c b/Python/bltinmodule.c
@@ -1586,7 +1586,9 @@ builtin_issubclass(PyObject *self, PyObject *args)
 PyDoc_STRVAR(issubclass_doc,
 "issubclass(C, B) -> bool\n\
 \n\
-Return whether class C is a subclass (i.e., a derived class) of class B.");
+Return whether class C is a subclass (i.e., a derived class) of class B.\n\
+When using a tuple as the second argument issubclass(X, (A, B, ...)),\n\
+is a shortcut for issubclass(X, A) or issubclass(X, B) or ... (etc.).");
 
 
 static PyObject*
