BUG: imprecise logic leading to malformed Series in Series.apply, close #1183

Author: wesm

pandas/core/series.py

@@ -1872,8 +1872,10 @@ def apply(self, func):
         """
         try:
             result = func(self)
-            if not isinstance(result, Series):
+            if isinstance(result, np.ndarray):
                 result = Series(result, index=self.index, name=self.name)
+            else:
+                raise ValueError('Must yield array')
             return result
         except Exception:
             mapped = lib.map_infer(self.values, func)

pandas/tests/test_series.py

@@ -2069,6 +2069,19 @@ def test_apply(self):
         result = self.ts.apply(lambda x: x.values * 2)
         assert_series_equal(result, self.ts * 2)
 
+    def test_apply_same_length_inference_bug(self):
+        s = Series([1, 2])
+        f = lambda x: (x, x + 1)
+
+        result = s.apply(f)
+        expected = s.map(f)
+        assert_series_equal(result, expected)
+
+        s = Series([1, 2, 3])
+        result = s.apply(f)
+        expected = s.map(f)
+        assert_series_equal(result, expected)
+
     def test_align(self):
         def _check_align(a, b, how='left', fill=None):
             aa, ab = a.align(b, join=how, fill_value=fill)