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ENH Warn about panel.to_frame() discarding NaN GH7879 #7970

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ENH Warn about panel.to_frame() discarding NaN GH7879 #7970

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9 changes: 9 additions & 0 deletions pandas/core/panel.py
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Original file line number Diff line number Diff line change
Expand Up @@ -8,6 +8,7 @@
from pandas import compat
import sys
import numpy as np
import warnings
from pandas.core.common import (PandasError, _try_sort, _default_index,
_infer_dtype_from_scalar, notnull)
from pandas.core.categorical import Categorical
Expand Down Expand Up @@ -846,6 +847,7 @@ def to_frame(self, filter_observations=True):
filter_observations : boolean, default True
Drop (major, minor) pairs without a complete set of observations
across all the items
Default will be changed to False in future versions

Returns
-------
Expand All @@ -858,6 +860,13 @@ def to_frame(self, filter_observations=True):
mask = com.notnull(self.values).all(axis=0)
# size = mask.sum()
selector = mask.ravel()
if not selector.all():
warnings.warn("Panel to_frame method discards entries with "
"NaN/None in the data by default, use "
"filter_observations = False to save them. "
"This will be default behaviour "
"in future versions.",
FutureWarning)
else:
# size = N * K
selector = slice(None, None)
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6 changes: 5 additions & 1 deletion pandas/tests/test_panel.py
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Expand Up @@ -1496,7 +1496,11 @@ def test_to_frame_multi_major(self):
assert_frame_equal(result, expected)

wp.iloc[0, 0].iloc[0] = np.nan # BUG on setting. GH #5773
result = wp.to_frame()

with tm.assert_produces_warning(FutureWarning):
setattr(panelm, '__warningregistry__', {})
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what does this setattr line do?

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Resets warnings in case it's already been raised somewhere. So I can for sure catch this warning.

result = wp.to_frame()

assert_frame_equal(result, expected[1:])

idx = MultiIndex.from_tuples([(1, 'two'), (1, 'one'), (2, 'one'),
Expand Down