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feat: df.to_pandas_batches() returns one empty DataFrame if df is empty #1878

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Merged
merged 2 commits into from
Jul 7, 2025
Merged

feat: df.to_pandas_batches() returns one empty DataFrame if df is empty #1878

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feat: `df.to_pandas_batches()` returns one empty DataFrame if `df` is…
tswast Jul 1, 2025
a256bbe
Merge branch 'main' into b428918844-to_pandas_batches
tswast Jul 7, 2025
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16 changes: 16 additions & 0 deletions bigframes/core/blocks.py
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Original file line number Diff line number Diff line change
Expand Up @@ -620,15 +620,31 @@ def to_pandas_batches(
ordered=True,
use_explicit_destination=allow_large_results,
)

total_batches = 0
for df in execute_result.to_pandas_batches(
page_size=page_size, max_results=max_results
):
total_batches += 1
self._copy_index_to_pandas(df)
if squeeze:
yield df.squeeze(axis=1)
else:
yield df

# To reduce the number of edge cases to consider when working with the
# results of this, always return at least one DataFrame. See:
# b/428918844.
if total_batches == 0:
df = pd.DataFrame(
{
col: pd.Series([], dtype=self.expr.get_column_type(col))
for col in itertools.chain(self.value_columns, self.index_columns)
}
)
self._copy_index_to_pandas(df)
yield df

def _copy_index_to_pandas(self, df: pd.DataFrame):
"""Set the index on pandas DataFrame to match this block.

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22 changes: 22 additions & 0 deletions tests/system/small/test_dataframe_io.py
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Expand Up @@ -347,6 +347,28 @@ def test_to_pandas_batches_w_correct_dtypes(scalars_df_default_index):
pd.testing.assert_series_equal(actual, expected)


def test_to_pandas_batches_w_empty_dataframe(session):
"""Verify to_pandas_batches() APIs returns at least one DataFrame.

See b/428918844 for additional context.
"""
empty = bpd.DataFrame(
{
"idx1": [],
"idx2": [],
"col1": pandas.Series([], dtype="string[pyarrow]"),
"col2": pandas.Series([], dtype="Int64"),
},
session=session,
).set_index(["idx1", "idx2"], drop=True)

results = list(empty.to_pandas_batches())
assert len(results) == 1
assert list(results[0].index.names) == ["idx1", "idx2"]
assert list(results[0].columns) == ["col1", "col2"]
pandas.testing.assert_series_equal(results[0].dtypes, empty.dtypes)


@pytest.mark.parametrize("allow_large_results", (True, False))
def test_to_pandas_batches_w_page_size_and_max_results(session, allow_large_results):
"""Verify to_pandas_batches() APIs returns the expected page size.
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