fix(python): fix issue with invalid Mapping objects used as schema …

…being silently ignored (pola-rs#12027)

py-polars/polars/lazyframe/frame.py

@@ -549,6 +549,7 @@ def _scan_ndjson(
         else:
             sources = [normalize_filepath(source) for source in source]
             source = None  # type: ignore[assignment]
+
         self = cls.__new__(cls)
         self._ldf = PyLazyFrame.new_from_ndjson(
             source,
@@ -566,13 +567,13 @@ def _scan_ndjson(
     @classmethod
     def _scan_python_function(
         cls,
-        schema: pa.schema | dict[str, PolarsDataType],
+        schema: pa.schema | Mapping[str, PolarsDataType],
         scan_fn: Any,
         *,
         pyarrow: bool = False,
     ) -> Self:
         self = cls.__new__(cls)
-        if isinstance(schema, dict):
+        if isinstance(schema, Mapping):
             self._ldf = PyLazyFrame.scan_from_python_function_pl_schema(
                 list(schema.items()), scan_fn, pyarrow
             )

py-polars/polars/utils/_construction.py

@@ -734,7 +734,7 @@ def _unpack_schema(
         )
 
     # determine column names from schema
-    if isinstance(schema, dict):
+    if isinstance(schema, Mapping):
         column_names: list[str] = list(schema)
         # coerce schema to list[str | tuple[str, PolarsDataType | PythonDataType | None]
         schema = list(schema.items())
@@ -849,7 +849,7 @@ def dict_to_pydf(
     nan_to_null: bool = False,
 ) -> PyDataFrame:
     """Construct a PyDataFrame from a dictionary of sequences."""
-    if isinstance(schema, dict) and data:
+    if isinstance(schema, Mapping) and data:
         if not all((col in schema) for col in data):
             raise ValueError(
                 "the given column-schema names do not match the data dictionary"

py-polars/tests/unit/test_schema.py

@@ -1,14 +1,41 @@
 from __future__ import annotations
 
+from collections import OrderedDict
 from datetime import date, timedelta
-from typing import Any
+from typing import Any, Iterator, Mapping
 
 import pytest
 
 import polars as pl
 from polars.testing import assert_frame_equal
 
 
+class CustomSchema(Mapping[str, Any]):
+    """Dummy schema object for testing compatibility with Mapping."""
+
+    _entries: dict[str, Any]
+
+    def __init__(self, **named_entries: Any) -> None:
+        self._items = OrderedDict(named_entries.items())
+
+    def __getitem__(self, key: str) -> Any:
+        return self._items[key]
+
+    def __len__(self) -> int:
+        return len(self._items)
+
+    def __iter__(self) -> Iterator[str]:
+        yield from self._items
+
+
+def test_custom_schema() -> None:
+    df = pl.DataFrame(schema=CustomSchema(bool=pl.Boolean, misc=pl.UInt8))
+    assert df.schema == OrderedDict([("bool", pl.Boolean), ("misc", pl.UInt8)])
+
+    with pytest.raises(ValueError):
+        pl.DataFrame(schema=CustomSchema(bool="boolean", misc="unsigned int"))
+
+
 def test_schema_on_agg() -> None:
     df = pl.DataFrame({"a": ["x", "x", "y", "n"], "b": [1, 2, 3, 4]})