[3.6] bpo-29822: make inspect.isabstract() work during __init_subclass__ (#1979)

At the time when an abstract base class' __init_subclass__ runs,
ABCMeta.__new__ has not yet finished running, so in the presence of
__init_subclass__, inspect.isabstract() can no longer depend only on
TPFLAGS_IS_ABSTRACT.
(cherry picked from commit fcfe80ec2592fed8b3941c79056a8737abef7d3b)

3 files changed, 49 insertions, 1 deletions

diff --git a/Lib/inspect.py b/Lib/inspect.py
index a2dcb88..2894672 100644
--- a/Lib/inspect.py
+++ b/Lib/inspect.py
@@ -31,6 +31,7 @@ Here are some of the useful functions provided by this module:
 __author__ = ('Ka-Ping Yee <ping@lfw.org>',
               'Yury Selivanov <yselivanov@sprymix.com>')
 
+import abc
 import ast
 import dis
 import collections.abc
@@ -291,7 +292,27 @@ def isroutine(object):
 
 def isabstract(object):
     """Return true if the object is an abstract base class (ABC)."""
-    return bool(isinstance(object, type) and object.__flags__ & TPFLAGS_IS_ABSTRACT)
+    if not isinstance(object, type):
+        return False
+    if object.__flags__ & TPFLAGS_IS_ABSTRACT:
+        return True
+    if not issubclass(type(object), abc.ABCMeta):
+        return False
+    if hasattr(object, '__abstractmethods__'):
+        # It looks like ABCMeta.__new__ has finished running;
+        # TPFLAGS_IS_ABSTRACT should have been accurate.
+        return False
+    # It looks like ABCMeta.__new__ has not finished running yet; we're
+    # probably in __init_subclass__. We'll look for abstractmethods manually.
+    for name, value in object.__dict__.items():
+        if getattr(value, "__isabstractmethod__", False):
+            return True
+    for base in object.__bases__:
+        for name in getattr(base, "__abstractmethods__", ()):
+            value = getattr(object, name, None)
+            if getattr(value, "__isabstractmethod__", False):
+                return True
+    return False
 
 def getmembers(object, predicate=None):
     """Return all members of an object as (name, value) pairs sorted by name.
diff --git a/Lib/test/test_inspect.py b/Lib/test/test_inspect.py
index cfea281..37e4511 100644
--- a/Lib/test/test_inspect.py
+++ b/Lib/test/test_inspect.py
@@ -232,6 +232,30 @@ class TestPredicates(IsTestBase):
         self.assertFalse(inspect.isabstract(int))
         self.assertFalse(inspect.isabstract(5))
 
+    def test_isabstract_during_init_subclass(self):
+        from abc import ABCMeta, abstractmethod
+        isabstract_checks = []
+        class AbstractChecker(metaclass=ABCMeta):
+            def __init_subclass__(cls):
+                isabstract_checks.append(inspect.isabstract(cls))
+        class AbstractClassExample(AbstractChecker):
+            @abstractmethod
+            def foo(self):
+                pass
+        class ClassExample(AbstractClassExample):
+            def foo(self):
+                pass
+        self.assertEqual(isabstract_checks, [True, False])
+
+        isabstract_checks.clear()
+        class AbstractChild(AbstractClassExample):
+            pass
+        class AbstractGrandchild(AbstractChild):
+            pass
+        class ConcreteGrandchild(ClassExample):
+            pass
+        self.assertEqual(isabstract_checks, [True, True, False])
+
 
 class TestInterpreterStack(IsTestBase):
     def __init__(self, *args, **kwargs):
diff --git a/Misc/NEWS b/Misc/NEWS
index 515960d..b819c6d 100644
--- a/Misc/NEWS
+++ b/Misc/NEWS
@@ -45,6 +45,9 @@ Core and Builtins
 Library
 -------
 
+- bpo-29822: inspect.isabstract() now works during __init_subclass__.  Patch
+  by Nate Soares.
+
 - bpo-29581: ABCMeta.__new__ now accepts ``**kwargs``, allowing abstract base
   classes to use keyword parameters in __init_subclass__. Patch by Nate Soares.