fix polars.read_csv inadvertently closing BytesIO input buffers (#2704)

py-polars/polars/io.py

@@ -108,7 +108,7 @@ def managed_file(file: Any) -> Iterator[Any]:
     if isinstance(file, StringIO):
         return BytesIO(file.read().encode("utf8"))
     if isinstance(file, BytesIO):
-        return file
+        return managed_file(file)
     if isinstance(file, Path):
         return managed_file(str(file))
     if isinstance(file, str):

py-polars/tests/test_io.py

@@ -242,6 +242,18 @@ def test_read_csv_columns_argument(
     assert df.columns == col_out
 
 
+def test_read_csv_buffer_ownership() -> None:
+    buf = io.BytesIO(b"\xf0\x9f\x98\x80,5.55,333\n\xf0\x9f\x98\x86,-5.0,666")
+    df = pl.read_csv(
+        buf,
+        has_header=False,
+        new_columns=["emoji", "flt", "int"],
+    )
+    # confirm that read_csv succeeded, and didn't close the input buffer (#2696)
+    assert df.shape == (2, 3)
+    assert not buf.closed
+
+
 def test_column_rename_and_dtype_overwrite() -> None:
     csv = """
 a,b,c