Not assigning cursor from `DuckDBPyConnection.cursor()` causes `Connection Error` when fetching data

[provinzkraut]

What happens?

When obtaining a cursor from DuckDBPyConnection.cursor(), and not assigning it to a variable, that cursor cannot be used to fetch any data, and will raise a Connection Error.

It seems that not assigning the cursor causes it to be garbage collected immediately, which should not be the case, there's still a reference being held to it.

To Reproduce

This raises _duckdb.ConnectionException: Connection Error: Connection has already been closed:

import duckdb

conn = duckdb.connect()
conn.cursor().sql("SELECT 1 as foo").fetchall()

while this works

import duckdb

conn = duckdb.connect()

cur = conn.cursor()
cur.sql("SELECT 1 as foo").fetchall()

OS:

macOS

DuckDB Package Version:

1.4.3

Python Version:

3.13.7

Full Name:

Janek Nouvertné

Affiliation:

nope

What is the latest build you tested with? If possible, we recommend testing with the latest nightly build.

I have not tested with any build

Did you include all relevant data sets for reproducing the issue?

Yes

Did you include all code required to reproduce the issue?

• Yes, I have

Did you include all relevant configuration to reproduce the issue?

• Yes, I have

[evertlammerts]

That's a bug indeed, in the relational API.

To work around, use execute():

import duckdb

conn = duckdb.connect()
conn.cursor().execute("SELECT 1 as foo").fetchall()

[provinzkraut]

I'm using the .pl method is only available on the relational api, but using the cursor as a context manager also works:

import duckdb

conn = duckdb.connect()

with conn.cursor() as cur:
    cur.sql("SELECT 1 as foo").fetchall()