BUG: don't assume series is length > 0

Author: hexgnu

doc/source/whatsnew/v0.23.0.txt

@@ -557,3 +557,4 @@ Other
 ^^^^^
 
 - Improved error message when attempting to use a Python keyword as an identifier in a ``numexpr`` backed query (:issue:`18221`)
+- Bug in `Series.memory_usage` which assumes series will always have more than one element (:issue:`19368`)

pandas/_libs/lib.pyx

@@ -67,7 +67,8 @@ def memory_usage_of_objects(ndarray[object, ndim=1] arr):
     cdef int64_t s = 0
 
     n = len(arr)
-    for i from 0 <= i < n:
+    # Hrm why was this 0 <= i < n
+    for i from 0 < i < n:
         s += arr[i].__sizeof__()
     return s
 

pandas/tests/sparse/series/test_series.py

@@ -971,6 +971,17 @@ def test_combine_first(self):
         tm.assert_sp_series_equal(result, result2)
         tm.assert_sp_series_equal(result, expected)
 
+    @pytest.mark.parametrize('deep,fill_values', [([True, False],
+                                                   [0, 1, np.nan, None])])
+    def test_memory_usage_deep(self, deep, fill_values):
+        for fv in fill_values:
+            sparse_series = SparseSeries(fill_values, fill_value=fv)
+            dense_series = Series(fill_values)
+            sparse_usage = sparse_series.memory_usage(deep=deep)
+            dense_usage = dense_series.memory_usage(deep=deep)
+
+            assert sparse_usage < dense_usage
+
 
 class TestSparseHandlingMultiIndexes(object):