Fixing shift() for ExtensionArray

pandas/core/arrays/base.py

@@ -449,10 +449,12 @@ def shift(self, periods=1):
         """
         # Note: this implementation assumes that `self.dtype.na_value` can be
         # stored in an instance of your ExtensionArray with `self.dtype`.
-        if periods == 0:
+        if len == 0 or periods == 0:
             return self.copy()
-        empty = self._from_sequence([self.dtype.na_value] * abs(periods),
-                                    dtype=self.dtype)
+        empty = self._from_sequence(
+            [self.dtype.na_value] * min(abs(periods), len(self)),
+            dtype=self.dtype
+        )
         if periods > 0:
             a = empty
             b = self[:-periods]

pandas/tests/extension/base/interface.py

@@ -1,4 +1,5 @@
 import numpy as np
+import pytest
 
 from pandas.compat import StringIO
 
@@ -86,3 +87,16 @@ def test_isna_extension_array(self, data_missing):
             assert not na.all()
 
             assert na.dtype._is_boolean
+
+    @pytest.mark.parametrize('periods, indices', [
+        [-4, [-1, -1]],
+        [-1, [1, -1]],
+        [0, [0, 1]],
+        [1, [-1, 0]],
+        [4, [-1, -1]]
+    ])
+    def test_shift(self, data, periods, indices):
+        subset = data[:2]
+        result = subset.shift(periods)
+        expected = subset.take(indices, allow_fill=True)
+        self.assert_extension_array_equal(result, expected)