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ENH: try to preserve the dtype on combine_first for the case where the two DataFrame objects have the same columns #39051

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Merged
merged 13 commits into from
Jan 15, 2021
Merged

ENH: try to preserve the dtype on combine_first for the case where the two DataFrame objects have the same columns #39051

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0c1d126
ENH: add argument to preserve dtypes of common columns in combine_first
danielhrisca Jan 11, 2021
1a5fe0f
fix black code style
danielhrisca Jan 11, 2021
24f6ffc
fix misspelled word in docstring
danielhrisca Jan 11, 2021
d0f9ed3
update tests and remove preserve_dtypes argument from combine_first
danielhrisca Jan 11, 2021
198eaa4
fix isort and flake8 errors
danielhrisca Jan 11, 2021
f209590
updates according to erview
danielhrisca Jan 12, 2021
5e252d0
update whatsnew with example code
danielhrisca Jan 12, 2021
7c67e3c
wrong header in documentation
danielhrisca Jan 12, 2021
47d0911
fix black code style
danielhrisca Jan 12, 2021
1b5691c
update whatsnew with example code as requested in the review
danielhrisca Jan 13, 2021
ba49f9c
remove redundant check
danielhrisca Jan 13, 2021
a2d4e38
further fix and polish the whatsnew entry
danielhrisca Jan 13, 2021
f937928
fix single letter variable names
danielhrisca Jan 15, 2021
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fix isort and flake8 errors
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@danielhrisca
danielhrisca committed Jan 11, 2021
commit 198eaa4ad3b23810dbb5a0f78934e1365c66a983
8 changes: 7 additions & 1 deletion pandas/core/frame.py
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Expand Up @@ -6488,14 +6488,20 @@ def combiner(x, y):

for col in self.columns.intersection(other.columns):
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this should be a simple list-comprehension

try:
# if the column has different dtype in the
# DataFrame objects then add the common dtype
# to the columns dtype conversion dict
if combined.dtypes[col] != self.dtypes[col]:
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use is_dtype_equal here

dtypes[col] = find_common_type(
[self.dtypes[col], other.dtypes[col]]
)
except TypeError:
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we do not want to do multiple try/excepts ever as these tend to hide errors.
in fact you should not need here at all. find_common_type will always succeed (it could of course be object).

# numpy dtype was compared with pandas dtype
try:
# just try to apply the initial column dtype
combined[col] = combined[col].astype(self.dtypes[col])
except:
except ValueError:
# could not apply the initial dtype, so skip
pass

if dtypes:
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3 changes: 2 additions & 1 deletion pandas/tests/frame/methods/test_combine_first.py
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Expand Up @@ -3,10 +3,11 @@
import numpy as np
import pytest

from pandas.core.dtypes.cast import find_common_type

import pandas as pd
from pandas import DataFrame, Index, MultiIndex, Series
import pandas._testing as tm
from pandas.core.dtypes.cast import find_common_type


class TestDataFrameCombineFirst:
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