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Backport PR #31211 on branch 1.0.x (BUG: Fixed upcast dtype for datetime64 in merge) #31240

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Merged
jorisvandenbossche merged 1 commit into from
Jan 23, 2020
Merged

Backport PR #31211 on branch 1.0.x (BUG: Fixed upcast dtype for datetime64 in merge) #31240

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2 changes: 1 addition & 1 deletion pandas/core/internals/concat.py
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Original file line number Diff line number Diff line change
Expand Up @@ -350,7 +350,7 @@ def _get_empty_dtype_and_na(join_units):
dtype = upcast_classes["datetimetz"]
return dtype[0], tslibs.NaT
elif "datetime" in upcast_classes:
return np.dtype("M8[ns]"), tslibs.iNaT
return np.dtype("M8[ns]"), np.datetime64("NaT", "ns")
elif "timedelta" in upcast_classes:
return np.dtype("m8[ns]"), np.timedelta64("NaT", "ns")
else: # pragma
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17 changes: 17 additions & 0 deletions pandas/tests/reshape/merge/test_merge.py
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Expand Up @@ -2153,3 +2153,20 @@ def test_merge_multiindex_columns():
expected["id"] = ""

tm.assert_frame_equal(result, expected)


def test_merge_datetime_upcast_dtype():
# https://github.com/pandas-dev/pandas/issues/31208
df1 = pd.DataFrame({"x": ["a", "b", "c"], "y": ["1", "2", "4"]})
df2 = pd.DataFrame(
{"y": ["1", "2", "3"], "z": pd.to_datetime(["2000", "2001", "2002"])}
)
result = pd.merge(df1, df2, how="left", on="y")
expected = pd.DataFrame(
{
"x": ["a", "b", "c"],
"y": ["1", "2", "4"],
"z": pd.to_datetime(["2000", "2001", "NaT"]),
}
)
tm.assert_frame_equal(result, expected)