BUG: recfunctions.join_by fails for colliding values with different dtypes

Fixes #9338

2 files changed, 25 insertions, 11 deletions

diff --git a/numpy/lib/recfunctions.py b/numpy/lib/recfunctions.py
index 08faeee0e..e42421786 100644
--- a/numpy/lib/recfunctions.py
+++ b/numpy/lib/recfunctions.py
@@ -963,27 +963,28 @@ def join_by(key, r1, r2, jointype='inner', r1postfix='1', r2postfix='2',
     ndtype = [list(_) for _ in r1k.dtype.descr]
     # Add the other fields
     ndtype.extend(list(_) for _ in r1.dtype.descr if _[0] not in key)
-    # Find the new list of names (it may be different from r1names)
-    names = list(_[0] for _ in ndtype)
+
     for desc in r2.dtype.descr:
         desc = list(desc)
-        name = desc[0]
         # Have we seen the current name already ?
-        if name in names:
-            nameidx = ndtype.index(desc)
+        name = desc[0]
+        names = list(_[0] for _ in ndtype)
+        try:
+            nameidx = names.index(name)
+        except ValueError:
+            #... we haven't: just add the description to the current list
+            ndtype.append(desc)
+        else:
             current = ndtype[nameidx]
-            # The current field is part of the key: take the largest dtype
             if name in key:
+                # The current field is part of the key: take the largest dtype
                 current[-1] = max(desc[1], current[-1])
-            # The current field is not part of the key: add the suffixes
             else:
+                # The current field is not part of the key: add the suffixes,
+                # and place the new field adjacent to the old one
                 current[0] += r1postfix
                 desc[0] += r2postfix
                 ndtype.insert(nameidx + 1, desc)
-        #... we haven't: just add the description to the current list
-        else:
-            names.extend(desc[0])
-            ndtype.append(desc)
     # Revert the elements to tuples
     ndtype = [tuple(_) for _ in ndtype]
     # Find the largest nb of common fields :
diff --git a/numpy/lib/tests/test_recfunctions.py b/numpy/lib/tests/test_recfunctions.py
index e9cfa4993..a5d15cb24 100644
--- a/numpy/lib/tests/test_recfunctions.py
+++ b/numpy/lib/tests/test_recfunctions.py
@@ -656,6 +656,19 @@ class TestJoinBy(TestCase):
         b = np.ones(3, dtype=[('c', 'u1'), ('b', 'f4'), ('a', 'i4')])
         assert_raises(ValueError, join_by, ['a', 'b', 'b'], a, b)
 
+    def test_same_name_different_dtypes(self):
+        # gh-9338
+        a_dtype = np.dtype([('key', 'S10'), ('value', '<f4')])
+        b_dtype = np.dtype([('key', 'S10'), ('value', '<f8')])
+        expected_dtype = np.dtype([
+            ('key', '|S10'), ('value1', '<f4'), ('value2', '<f8')])
+
+        a = np.array([('Sarah',  8.0), ('John', 6.0)], dtype=a_dtype)
+        b = np.array([('Sarah', 10.0), ('John', 7.0)], dtype=b_dtype)
+        res = join_by('key', a, b)
+
+        assert_equal(res.dtype, expected_dtype)
+
 
 class TestJoinBy2(TestCase):
     @classmethod