DOC/ERR: better error message on no common merge keys (pandas-dev#19427)

Author: swyoon

doc/source/whatsnew/v0.23.0.txt

@@ -667,6 +667,7 @@ Reshaping
 - Bug in :func:`DataFrame.stack`, :func:`DataFrame.unstack`, :func:`Series.unstack` which were not returning subclasses (:issue:`15563`)
 - Bug in timezone comparisons, manifesting as a conversion of the index to UTC in ``.concat()`` (:issue:`18523`)
 - Bug in :func:`concat` when concatting sparse and dense series it returns only a ``SparseDataFrame``. Should be a ``DataFrame``. (:issue:`18914`, :issue:`18686`, and :issue:`16874`)
+- Improved error message for :func:`DataFrame.merge` when there is no common merge key (:issue:`19427`)
 -
 
 

pandas/core/frame.py

@@ -233,7 +233,7 @@
 --------
 merge_ordered
 merge_asof
-
+DataFrame.join
 """
 
 # -----------------------------------------------------------------------

pandas/core/reshape/merge.py

@@ -1021,7 +1021,12 @@ def _validate_specification(self):
  common_cols = self.left.columns.intersection(
  self.right.columns)
  if len(common_cols) == 0:
- raise MergeError('No common columns to perform merge on')
+ raise MergeError(
+ 'No common columns to perform merge on. '
+ 'Merge options: left_on={lon}, right_on={ron}, '
+ 'left_index={lidx}, right_index={ridx}'
+ .format(lon=self.left_on, ron=self.right_on,
+ lidx=self.left_index, ridx=self.right_index))
  if not common_cols.is_unique:
  raise MergeError("Data columns not unique: {common!r}"
  .format(common=common_cols))

pandas/tests/reshape/merge/test_merge.py

@@ -270,6 +270,14 @@ def test_no_overlap_more_informative_error(self):
  df2 = DataFrame({'y': ['b', 'c']}, index=[dt, dt])
  pytest.raises(MergeError, merge, df1, df2)
 
+ msg = ('No common columns to perform merge on. '
+ 'Merge options: left_on={lon}, right_on={ron}, '
+ 'left_index={lidx}, right_index={ridx}'
+ .format(lon=None, ron=None, lidx=False, ridx=False))
+
+ with tm.assert_raises_regex(MergeError, msg):
+ merge(df1, df2)
+
  def test_merge_non_unique_indexes(self):
 
  dt = datetime(2012, 5, 1)