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DOC/ERR: better error message on no common merge keys #19427

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Merged
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Feb 6, 2018
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DOC/ERR: better error message on no common merge keys #19427

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1 change: 1 addition & 0 deletions doc/source/whatsnew/v0.23.0.txt
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Original file line number Diff line number Diff line change
Expand Up @@ -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`)
-


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2 changes: 1 addition & 1 deletion pandas/core/frame.py
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Expand Up @@ -233,7 +233,7 @@
--------
merge_ordered
merge_asof

DataFrame.join
"""

# -----------------------------------------------------------------------
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7 changes: 6 additions & 1 deletion pandas/core/reshape/merge.py
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Expand Up @@ -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))
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8 changes: 8 additions & 0 deletions pandas/tests/reshape/merge/test_merge.py
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Expand Up @@ -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)
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