From ce0bc6e6af34f4256de053dc7bdbb2120a276cd6 Mon Sep 17 00:00:00 2001
From: jab <jab@users.noreply.github.com>
Date: Wed, 28 Feb 2018 23:55:11 +0000
Subject: [PATCH 1/2] Improve issubclass() error checking and message.

---
 Lib/_py_abc.py | 2 ++
 Modules/_abc.c | 5 +++++
 2 files changed, 7 insertions(+)

diff --git a/Lib/_py_abc.py b/Lib/_py_abc.py
index 6f42ef32fa6961..3c3aa8e3d61bee 100644
--- a/Lib/_py_abc.py
+++ b/Lib/_py_abc.py
@@ -107,6 +107,8 @@ def __instancecheck__(cls, instance):
 
     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
diff --git a/Modules/_abc.c b/Modules/_abc.c
index 504e23d9a74d3e..f413b2a1cfdffd 100644
--- a/Modules/_abc.c
+++ b/Modules/_abc.c
@@ -568,6 +568,11 @@ _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, *subclasses = NULL, *result = NULL;
     Py_ssize_t pos;
     int incache;

From 175a18059580fcabc02886a881a3a20a2409c1a8 Mon Sep 17 00:00:00 2001
From: jab <jab@users.noreply.github.com>
Date: Thu, 22 Mar 2018 23:14:33 +1300
Subject: [PATCH 2/2] add NEWS item

---
 Misc/ACKS                                                      | 1 +
 .../Core and Builtins/2018-03-22-23-09-06.bpo-33018.0ncEJV.rst | 3 +++
 2 files changed, 4 insertions(+)
 create mode 100644 Misc/NEWS.d/next/Core and Builtins/2018-03-22-23-09-06.bpo-33018.0ncEJV.rst

diff --git a/Misc/ACKS b/Misc/ACKS
index d752d8a35434ae..05932a8d8991ca 100644
--- a/Misc/ACKS
+++ b/Misc/ACKS
@@ -202,6 +202,7 @@ Dillon Brock
 Richard Brodie
 Michael Broghton
 Ammar Brohi
+Josh Bronson
 Daniel Brotsky
 Jean Brouwers
 Gary S. Brown
diff --git a/Misc/NEWS.d/next/Core and Builtins/2018-03-22-23-09-06.bpo-33018.0ncEJV.rst b/Misc/NEWS.d/next/Core and Builtins/2018-03-22-23-09-06.bpo-33018.0ncEJV.rst
new file mode 100644
index 00000000000000..e799e9834aa111
--- /dev/null
+++ b/Misc/NEWS.d/next/Core and Builtins/2018-03-22-23-09-06.bpo-33018.0ncEJV.rst	
@@ -0,0 +1,3 @@
+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.