BUG: Make iscomplexobj compatible with custom dtypes again

This change makes iscomplexobj compatible with custom array types
using custom dtypes, that are not fully compatible to Numpys dtypes,
which can nevertheless be coerced to a numpy array with asarray
again, as has been the behaviour before PR #7936.

Fixes #8601

2 files changed, 12 insertions, 5 deletions

diff --git a/numpy/lib/tests/test_type_check.py b/numpy/lib/tests/test_type_check.py
index 93a4da97a..4523e3f24 100644
--- a/numpy/lib/tests/test_type_check.py
+++ b/numpy/lib/tests/test_type_check.py
@@ -183,6 +183,15 @@ class TestIscomplexobj(TestCase):
         dummy = DummyPd()
         assert_(iscomplexobj(dummy))
 
+    def test_custom_dtype_duck(self):
+        class MyArray(list):
+            @property
+            def dtype(self):
+                return complex
+
+        a = MyArray([1+0j, 2+0j, 3+0j])
+        assert_(iscomplexobj(a))
+
 
 class TestIsrealobj(TestCase):
     def test_basic(self):
diff --git a/numpy/lib/type_check.py b/numpy/lib/type_check.py
index 1658f160c..3bbee0258 100644
--- a/numpy/lib/type_check.py
+++ b/numpy/lib/type_check.py
@@ -268,12 +268,10 @@ def iscomplexobj(x):
     """
     try:
         dtype = x.dtype
+        type_ = dtype.type
     except AttributeError:
-        dtype = asarray(x).dtype
-    try:
-        return issubclass(dtype.type, _nx.complexfloating)
-    except AttributeError:
-        return False
+        type_ = asarray(x).dtype.type
+    return issubclass(type_, _nx.complexfloating)
 
 
 def isrealobj(x):