Issue #1785: Fix inspect and pydoc with misbehaving descriptors.

Also fixes issue #13581: `help(type)` wouldn't display anything.

Author: pitrou

Lib/inspect.py

@@ -247,12 +247,23 @@ def isabstract(object):
 def getmembers(object, predicate=None):
     """Return all members of an object as (name, value) pairs sorted by name.
     Optionally, only return members that satisfy a given predicate."""
+    if isclass(object):
+        mro = (object,) + getmro(object)
+    else:
+        mro = ()
     results = []
     for key in dir(object):
-        try:
-            value = getattr(object, key)
-        except AttributeError:
-            continue
+        # First try to get the value via __dict__. Some descriptors don't
+        # like calling their __get__ (see bug #1785).
+        for base in mro:
+            if key in base.__dict__:
+                value = base.__dict__[key]
+                break
+        else:
+            try:
+                value = getattr(object, key)
+            except AttributeError:
+                continue
         if not predicate or predicate(value):
             results.append((key, value))
     results.sort()
@@ -288,30 +299,21 @@ def classify_class_attrs(cls):
     names = dir(cls)
     result = []
     for name in names:
-        # Get the object associated with the name.
+        # Get the object associated with the name, and where it was defined.
         # Getting an obj from the __dict__ sometimes reveals more than
         # using getattr.  Static and class methods are dramatic examples.
-        if name in cls.__dict__:
-            obj = cls.__dict__[name]
+        # Furthermore, some objects may raise an Exception when fetched with
+        # getattr(). This is the case with some descriptors (bug #1785).
+        # Thus, we only use getattr() as a last resort.
+        homecls = None
+        for base in (cls,) + mro:
+            if name in base.__dict__:
+                obj = base.__dict__[name]
+                homecls = base
+                break
         else:
             obj = getattr(cls, name)
-
-        # Figure out where it was defined.
-        homecls = getattr(obj, "__objclass__", None)
-        if homecls is None:
-            # search the dicts.
-            for base in mro:
-                if name in base.__dict__:
-                    homecls = base
-                    break
-
-        # Get the object again, in order to get it from the defining
-        # __dict__ instead of via getattr (if possible).
-        if homecls is not None and name in homecls.__dict__:
-            obj = homecls.__dict__[name]
-
-        # Also get the object via getattr.
-        obj_via_getattr = getattr(cls, name)
+            homecls = getattr(obj, "__objclass__", homecls)
 
         # Classify the object.
         if isinstance(obj, staticmethod):
@@ -320,11 +322,18 @@ def classify_class_attrs(cls):
             kind = "class method"
         elif isinstance(obj, property):
             kind = "property"
-        elif (ismethod(obj_via_getattr) or
-              ismethoddescriptor(obj_via_getattr)):
+        elif ismethoddescriptor(obj):
             kind = "method"
-        else:
+        elif isdatadescriptor(obj):
             kind = "data"
+        else:
+            obj_via_getattr = getattr(cls, name)
+            if (ismethod(obj_via_getattr) or
+                ismethoddescriptor(obj_via_getattr)):
+                kind = "method"
+            else:
+                kind = "data"
+            obj = obj_via_getattr
 
         result.append(Attribute(name, kind, homecls, obj))
 

Lib/pydoc.py

@@ -740,8 +740,15 @@ def spill(msg, attrs, predicate):
                 hr.maybe()
                 push(msg)
                 for name, kind, homecls, value in ok:
-                    push(self.document(getattr(object, name), name, mod,
-                                       funcs, classes, mdict, object))
+                    try:
+                        value = getattr(object, name)
+                    except Exception:
+                        # Some descriptors may meet a failure in their __get__.
+                        # (bug #1785)
+                        push(self._docdescriptor(name, value, mod))
+                    else:
+                        push(self.document(value, name, mod,
+                                        funcs, classes, mdict, object))
                     push('\n')
             return attrs
 
@@ -781,7 +788,12 @@ def spilldata(msg, attrs, predicate):
         mdict = {}
         for key, kind, homecls, value in attrs:
             mdict[key] = anchor = '#' + name + '-' + key
-            value = getattr(object, key)
+            try:
+                value = getattr(object, name)
+            except Exception:
+                # Some descriptors may meet a failure in their __get__.
+                # (bug #1785)
+                pass
             try:
                 # The value may not be hashable (e.g., a data attr with
                 # a dict or list value).
@@ -1161,8 +1173,15 @@ def spill(msg, attrs, predicate):
                 hr.maybe()
                 push(msg)
                 for name, kind, homecls, value in ok:
-                    push(self.document(getattr(object, name),
-                                       name, mod, object))
+                    try:
+                        value = getattr(object, name)
+                    except Exception:
+                        # Some descriptors may meet a failure in their __get__.
+                        # (bug #1785)
+                        push(self._docdescriptor(name, value, mod))
+                    else:
+                        push(self.document(value,
+                                        name, mod, object))
             return attrs
 
         def spilldescriptors(msg, attrs, predicate):

Lib/test/test_inspect.py

@@ -404,10 +404,37 @@ def test_findsource_code_in_linecache(self):
         self.assertEqual(inspect.findsource(co), (lines,0))
         self.assertEqual(inspect.getsource(co), lines[0])
 
+
+class _BrokenDataDescriptor(object):
+    """
+    A broken data descriptor. See bug #1785.
+    """
+    def __get__(*args):
+        raise AssertionError("should not __get__ data descriptors")
+
+    def __set__(*args):
+        raise RuntimeError
+
+    def __getattr__(*args):
+        raise AssertionError("should not __getattr__ data descriptors")
+
+
+class _BrokenMethodDescriptor(object):
+    """
+    A broken method descriptor. See bug #1785.
+    """
+    def __get__(*args):
+        raise AssertionError("should not __get__ method descriptors")
+
+    def __getattr__(*args):
+        raise AssertionError("should not __getattr__ method descriptors")
+
+
 # Helper for testing classify_class_attrs.
 def attrs_wo_objs(cls):
     return [t[:3] for t in inspect.classify_class_attrs(cls)]
 
+
 class TestClassesAndFunctions(unittest.TestCase):
     def test_classic_mro(self):
         # Test classic-class method resolution order.
@@ -494,13 +521,18 @@ def m1(self): pass
 
             datablob = '1'
 
+            dd = _BrokenDataDescriptor()
+            md = _BrokenMethodDescriptor()
+
         attrs = attrs_wo_objs(A)
         self.assertIn(('s', 'static method', A), attrs, 'missing static method')
         self.assertIn(('c', 'class method', A), attrs, 'missing class method')
         self.assertIn(('p', 'property', A), attrs, 'missing property')
         self.assertIn(('m', 'method', A), attrs, 'missing plain method')
         self.assertIn(('m1', 'method', A), attrs, 'missing plain method')
         self.assertIn(('datablob', 'data', A), attrs, 'missing data')
+        self.assertIn(('md', 'method', A), attrs, 'missing method descriptor')
+        self.assertIn(('dd', 'data', A), attrs, 'missing data descriptor')
 
         class B(A):
             def m(self): pass
@@ -512,6 +544,8 @@ def m(self): pass
         self.assertIn(('m', 'method', B), attrs, 'missing plain method')
         self.assertIn(('m1', 'method', A), attrs, 'missing plain method')
         self.assertIn(('datablob', 'data', A), attrs, 'missing data')
+        self.assertIn(('md', 'method', A), attrs, 'missing method descriptor')
+        self.assertIn(('dd', 'data', A), attrs, 'missing data descriptor')
 
 
         class C(A):
@@ -525,6 +559,8 @@ def c(self): pass
         self.assertIn(('m', 'method', C), attrs, 'missing plain method')
         self.assertIn(('m1', 'method', A), attrs, 'missing plain method')
         self.assertIn(('datablob', 'data', A), attrs, 'missing data')
+        self.assertIn(('md', 'method', A), attrs, 'missing method descriptor')
+        self.assertIn(('dd', 'data', A), attrs, 'missing data descriptor')
 
         class D(B, C):
             def m1(self): pass
@@ -539,6 +575,8 @@ def m1(self): pass
         self.assertIn(('m', 'method', B), attrs, 'missing plain method')
         self.assertIn(('m1', 'method', D), attrs, 'missing plain method')
         self.assertIn(('datablob', 'data', A), attrs, 'missing data')
+        self.assertIn(('md', 'method', A), attrs, 'missing method descriptor')
+        self.assertIn(('dd', 'data', A), attrs, 'missing data descriptor')
 
 
     def test_classify_oldstyle(self):
@@ -554,6 +592,64 @@ def test_classify_newstyle(self):
         """
         self._classify_test(True)
 
+    def test_classify_builtin_types(self):
+        # Simple sanity check that all built-in types can have their
+        # attributes classified.
+        for name in dir(__builtin__):
+            builtin = getattr(__builtin__, name)
+            if isinstance(builtin, type):
+                inspect.classify_class_attrs(builtin)
+
+    def test_getmembers_descriptors(self):
+        # Old-style classes
+        class A:
+            dd = _BrokenDataDescriptor()
+            md = _BrokenMethodDescriptor()
+
+        self.assertEqual(inspect.getmembers(A, inspect.ismethoddescriptor),
+            [('md', A.__dict__['md'])])
+        self.assertEqual(inspect.getmembers(A, inspect.isdatadescriptor),
+            [('dd', A.__dict__['dd'])])
+
+        class B(A):
+            pass
+
+        self.assertEqual(inspect.getmembers(B, inspect.ismethoddescriptor),
+            [('md', A.__dict__['md'])])
+        self.assertEqual(inspect.getmembers(B, inspect.isdatadescriptor),
+            [('dd', A.__dict__['dd'])])
+
+        # New-style classes
+        class A(object):
+            dd = _BrokenDataDescriptor()
+            md = _BrokenMethodDescriptor()
+
+        def pred_wrapper(pred):
+            # A quick'n'dirty way to discard standard attributes of new-style
+            # classes.
+            class Empty(object):
+                pass
+            def wrapped(x):
+                if hasattr(x, '__name__') and hasattr(Empty, x.__name__):
+                    return False
+                return pred(x)
+            return wrapped
+
+        ismethoddescriptor = pred_wrapper(inspect.ismethoddescriptor)
+        isdatadescriptor = pred_wrapper(inspect.isdatadescriptor)
+
+        self.assertEqual(inspect.getmembers(A, ismethoddescriptor),
+            [('md', A.__dict__['md'])])
+        self.assertEqual(inspect.getmembers(A, isdatadescriptor),
+            [('dd', A.__dict__['dd'])])
+
+        class B(A):
+            pass
+
+        self.assertEqual(inspect.getmembers(B, ismethoddescriptor),
+            [('md', A.__dict__['md'])])
+        self.assertEqual(inspect.getmembers(B, isdatadescriptor),
+            [('dd', A.__dict__['dd'])])
 
 
 class TestGetcallargsFunctions(unittest.TestCase):

Misc/NEWS

@@ -86,6 +86,8 @@ Core and Builtins
 Library
 -------
 
+- Issue #1785: Fix inspect and pydoc with misbehaving descriptors.
+
 - Issue #7502: Fix equality comparison for DocTestCase instances.  Patch by
   Cédric Krier.