Rebase BUG: Ignore division by 0 when merging empty dataframes (pandas-dev#17776)

Author: yeemey

pandas/core/reshape/merge.py

@@ -1514,6 +1514,7 @@ def _sort_labels(uniques, left, right):
 
 
 def _get_join_keys(llab, rlab, shape, sort):
+    np.seterr(divide='ignore')
 
     # how many levels can be done without overflow
     pred = lambda i: not is_int64_overflow_possible(shape[:i])
@@ -1525,9 +1526,11 @@ def _get_join_keys(llab, rlab, shape, sort):
     rkey = stride * rlab[0].astype('i8', subok=False, copy=False)
 
     for i in range(1, nlev):
-        stride //= shape[i]
+        with np.errstate(divide='ignore'):
+            stride //= shape[i]
         lkey += llab[i] * stride
         rkey += rlab[i] * stride
+    np.seterr(divide='warn')
 
     if nlev == len(shape):  # all done!
         return lkey, rkey

pandas/tests/reshape/test_merge.py

@@ -861,6 +861,14 @@ def test_validation(self):
         result = merge(left, right, on=['a', 'b'], validate='1:1')
         assert_frame_equal(result, expected_multi)
 
+    def test_merge_two_empty_df_no_division_error(self):
+        # GH17776, PR #17846
+        import warnings
+        a = pd.DataFrame({'a': [], 'b': [], 'c': []})
+        with warnings.catch_warnings(record=True) as record:
+            merge(a, a, on=('a', 'b'))
+            assert len(record) == 0
+
 
 def _check_merge(x, y):
     for how in ['inner', 'left', 'outer']: