ENH: improve duck typing inside iscomplexobj

Both `iscomplexobj` and `isrealobj` now try to refer to the argument's
`dtype` attribute if it exists. This significantly extends the list of
types for which `iscomplexobj` returns correct results (including e.g.
scipy sparse matrices and pandas objects).

Extended the tests of the `iscomplexobj` routine for the following cases:
* simple scalars
* standard lists (test internal auto-conversion to numpy arrays)
* "Duck typing" for objects that define a dtype attribute (either
  referring to one of the existing numpy dtypes, or a custom dtype, as
  pandas does)

This fixes #7924

2 files changed, 45 insertions, 2 deletions

diff --git a/numpy/lib/tests/test_type_check.py b/numpy/lib/tests/test_type_check.py
index f7430c27d..93a4da97a 100644
--- a/numpy/lib/tests/test_type_check.py
+++ b/numpy/lib/tests/test_type_check.py
@@ -148,6 +148,41 @@ class TestIscomplexobj(TestCase):
         z = np.array([-1j, 0, -1])
         assert_(iscomplexobj(z))
 
+    def test_scalar(self):
+        assert_(not iscomplexobj(1.0))
+        assert_(iscomplexobj(1+0j))
+
+    def test_list(self):
+        assert_(iscomplexobj([3, 1+0j, True]))
+        assert_(not iscomplexobj([3, 1, True]))
+
+    def test_duck(self):
+        class DummyComplexArray:
+            @property
+            def dtype(self):
+                return np.dtype(complex)
+        dummy = DummyComplexArray()
+        assert_(iscomplexobj(dummy))
+
+    def test_pandas_duck(self):
+        # This tests a custom np.dtype duck-typed class, such as used by pandas
+        # (pandas.core.dtypes)
+        class PdComplex(np.complex128):
+            pass
+        class PdDtype(object):
+            name = 'category'
+            names = None
+            type = PdComplex
+            kind = 'c'
+            str = '<c16'
+            base = np.dtype('complex128')
+        class DummyPd:
+            @property
+            def dtype(self):
+                return PdDtype
+        dummy = DummyPd()
+        assert_(iscomplexobj(dummy))
+
 
 class TestIsrealobj(TestCase):
     def test_basic(self):
diff --git a/numpy/lib/type_check.py b/numpy/lib/type_check.py
index d6e0704ad..f620d49d5 100644
--- a/numpy/lib/type_check.py
+++ b/numpy/lib/type_check.py
@@ -266,7 +266,15 @@ def iscomplexobj(x):
     True
 
     """
-    return issubclass(asarray(x).dtype.type, _nx.complexfloating)
+    try:
+        dtype = x.dtype
+    except AttributeError:
+        dtype = asarray(x).dtype
+    try:
+        return issubclass(dtype.type, _nx.complexfloating)
+    except AttributeError:
+        return False
+
 
 def isrealobj(x):
     """
@@ -300,7 +308,7 @@ def isrealobj(x):
     False
 
     """
-    return not issubclass(asarray(x).dtype.type, _nx.complexfloating)
+    return not iscomplexobj(x)
 
 #-----------------------------------------------------------------------------