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[Python] Cursor.fetch_polars() returns a struct Series instead of a DataFrame with polars 2

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Assessment

Difficulty
2/5
Estimated time
1-3 hours
Newbie friendliness
72/100
Issue type
Bug
Clarity
Clearly specified
Activity status
Active
Tech stack
python
Domain
database

Research direction

_RowIterator.fetch_polars() in the Python driver manager is the entry point: it wraps polars.from_arrow(self.fetch_arrow()) in a typing.cast, which hides that polars 2 returns a polars.Series. Start by running the sqlite repro in the report and confirming the Series/struct output, then apply the unnest (or build a polars.DataFrame from the stream) and check the result is a DataFrame with columns a and b under both polars 1.x and 2.x, including an empty result.

Written by the indexing model from the issue text.

Description

Describe the bug

With polars 2.0, adbc_driver_manager.dbapi.Cursor.fetch_polars() returns a polars.Series with a single struct column instead of a polars.DataFrame.

_RowIterator.fetch_polars() calls polars.from_arrow(self.fetch_arrow()), which passes in an ArrowArrayStreamHandle. polars 2 now turns an Arrow C stream given to from_arrow into a struct Series. polars 1.x returned a DataFrame. The typing.cast(polars.DataFrame, ...) around the call hides this from type checkers.

Reproduce
import adbc_driver_sqlite.dbapi as d

with d.connect() as c, c.cursor() as cur:
    cur.execute("SELECT 1 AS a, 'x' AS b")
    r = cur.fetch_polars()
    print(type(r).__name__, r.dtype, r.shape)

Output with polars 2.0.0:

Series Struct({'a': Int64, 'b': String}) (1,)

Expected: a DataFrame with columns a and b. That is what polars 1.x returns.

Versions
  • adbc-driver-manager 1.12.0
  • polars 2.0.0
  • pyarrow 26.0.0
  • Python 3.12, Linux
Possible fix

One option is to unnest when polars hands back a Series:

result = polars.from_arrow(self.fetch_arrow())
if isinstance(result, polars.Series):
    result = result.struct.unnest()

Another is to build the frame with polars.DataFrame(...) from the stream. That needs checking against both polars 1.x and 2.x. We've applied the unnest workaround downstream in adbc-poolhouse 1.6.4, and it keeps the frame's schema, including for empty results.

Dominant language
C#
Stars
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Forks
233
Avg merge
1d 8h
Merged PRs (30d)
91

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