bpo-33018: Improve issubclass() error checking and message. (GH-5944)

This improves error message for situations when a non-class is
checked w.r.t. an abstract base class.

Lib/_py_abc.py

@@ -107,6 +107,8 @@ class ABCMeta(type):
 
     def __subclasscheck__(cls, subclass):
         """Override for issubclass(subclass, cls)."""
+        if not isinstance(subclass, type):
+            raise TypeError('issubclass() arg 1 must be a class')
         # Check cache
         if subclass in cls._abc_cache:
             return True

Misc/ACKS

@@ -202,6 +202,7 @@ Dillon Brock
 Richard Brodie
 Michael Broghton
 Ammar Brohi
+Josh Bronson
 Daniel Brotsky
 Jean Brouwers
 Gary S. Brown

Misc/NEWS.d/next/Core and Builtins/2018-03-22-23-09-06.bpo-33018.0ncEJV.rst

+Improve consistency of errors raised by ``issubclass()`` when called with a
+non-class and an abstract base class as the first and second arguments,
+respectively. Patch by Josh Bronson.

Modules/_abc.c

@@ -569,6 +569,11 @@ _abc__abc_subclasscheck_impl(PyObject *module, PyObject *self,
                              PyObject *subclass)
 /*[clinic end generated code: output=b56c9e4a530e3894 input=1d947243409d10b8]*/
 {
+    if (!PyType_Check(subclass)) {
+        PyErr_SetString(PyExc_TypeError, "issubclass() arg 1 must be a class");
+        return NULL;
+    }
+
     PyObject *ok, *mro = NULL, *subclasses = NULL, *result = NULL;
     Py_ssize_t pos;
     int incache;