timsaucer · Oct 8, 2026 · Oct 7, 2026 · timsaucer · Oct 7, 2026 · timsaucer
diff --git a/.github/workflows/build.yml b/.github/workflows/build.yml
@@ -81,6 +81,10 @@ jobs:
           uv run --no-project ruff check --output-format=github python/
           uv run --no-project ruff format --check python/
 
+      - name: Run ty
+        run: |
+          uv run --no-project ty check --output-format github
+
       - name: Run codespell
         run: |
           uv run --no-project codespell --toml pyproject.toml

diff --git a/.pre-commit-config.yaml b/.pre-commit-config.yaml
@@ -30,6 +30,13 @@ repos:
           - id: ruff-format
       - repo: local
         hooks:
+          - id: ty
+            name: ty
+            description: Type check python/datafusion with ty.
+            entry: uv run --no-project ty check
+            pass_filenames: false
+            types: [file, python]
+            language: system
           - id: rust-fmt
             name: Rust fmt
             description: Run cargo fmt on files included in the commit. rustfmt should be installed before-hand.

diff --git a/pyproject.toml b/pyproject.toml
@@ -181,6 +181,16 @@ extend-allowed-calls = ["datafusion.lit", "lit"]
 
 # CI and pre-commit invoke codespell with different paths, so we have a little
 # redundancy here, and we intentionally drop python in the path.
+[tool.ty.src]
+# Only the published package is type checked for now. Tests and examples
+# can be added once the package itself is clean.
+include = ["python/datafusion"]
+
+[tool.ty.analysis]
+# `_internal` is the compiled PyO3 extension and ships no `.pyi` stubs.
+# pandas and polars are optional and only imported under TYPE_CHECKING.
+allowed-unresolved-imports = ["datafusion._internal", "pandas", "polars"]
+
 [tool.codespell]
 skip = [
   "*/tests/test_functions.py",
@@ -219,6 +229,8 @@ dev = [
   "pyyaml>=6.0.3",
   "ruff>=0.15.1",
   "toml>=0.10.2",
+  # Pinned exactly: ty is pre-1.0 and new releases can add diagnostics.
+  "ty==0.0.84",
 ]
 # Release tooling only. Kept out of `dev` because pygithub pulls in
 # cryptography, which ships no free-threaded wheel and fails to build

diff --git a/python/datafusion/__init__.py b/python/datafusion/__init__.py
@@ -57,13 +57,9 @@
 
 from __future__ import annotations
 
+import importlib.metadata as importlib_metadata
 from typing import Any
 
-try:
-    import importlib.metadata as importlib_metadata
-except ImportError:
-    import importlib_metadata  # type: ignore[import]
-
 # Public submodules
 from . import functions, ipc, object_store, substrait, unparser
 

diff --git a/python/datafusion/catalog.py b/python/datafusion/catalog.py
@@ -19,6 +19,7 @@
 
 from __future__ import annotations
 
+import sys
 from abc import ABC, abstractmethod
 from typing import TYPE_CHECKING, Any, Protocol
 
@@ -31,10 +32,10 @@
     from datafusion.context import TableProviderExportable
     from datafusion.expr import CreateExternalTable
 
-try:
-    from warnings import deprecated  # Python 3.13+
-except ImportError:
-    from typing_extensions import deprecated  # Python 3.12
+if sys.version_info >= (3, 13):
+    from warnings import deprecated
+else:
+    from typing_extensions import deprecated
 
 
 __all__ = [

diff --git a/python/datafusion/context.py b/python/datafusion/context.py
@@ -44,14 +44,15 @@
 
 from __future__ import annotations
 
+import sys
 import uuid
 import warnings
 from typing import TYPE_CHECKING, Any, Protocol
 
-try:
-    from warnings import deprecated  # Python 3.13+
-except ImportError:
-    from typing_extensions import deprecated  # Python 3.12
+if sys.version_info >= (3, 13):
+    from warnings import deprecated
+else:
+    from typing_extensions import deprecated
 
 
 from urllib.parse import urlparse
@@ -85,7 +86,6 @@
 
 if TYPE_CHECKING:
     import pathlib
-    import sys
     from collections.abc import Iterable, Sequence
 
     import pandas as pd
@@ -1292,7 +1292,7 @@ def register_csv(
                 delimiter=delimiter,
                 schema_infer_max_records=schema_infer_max_records,
                 file_extension=file_extension,
-                file_compression_type=file_compression_type,
+                file_compression_type=file_compression_type or "",
             )
         )
 
@@ -2196,7 +2196,7 @@ def read_csv(
                 schema_infer_max_records=schema_infer_max_records,
                 file_extension=file_extension,
                 table_partition_cols=table_partition_cols,
-                file_compression_type=file_compression_type,
+                file_compression_type=file_compression_type or "",
             )
         )
 

diff --git a/python/datafusion/dataframe.py b/python/datafusion/dataframe.py
@@ -44,19 +44,21 @@
 
 from __future__ import annotations
 
+import sys
 import warnings
 from collections.abc import AsyncIterator, Iterable, Iterator, Sequence
 from typing import (
     TYPE_CHECKING,
     Any,
     Literal,
+    cast,
     overload,
 )
 
-try:
-    from warnings import deprecated  # Python 3.13+
-except ImportError:
-    from typing_extensions import deprecated  # Python 3.12
+if sys.version_info >= (3, 13):
+    from warnings import deprecated
+else:
+    from typing_extensions import deprecated
 
 from datafusion._internal import DataFrame as DataFrameInternal
 from datafusion._internal import DataFrameWriteOptions as DataFrameWriteOptionsInternal
@@ -1158,8 +1160,8 @@ def join(
             if left_on is not None or right_on is not None:
                 error_msg = "`left_on` or `right_on` should not provided with `on`"
                 raise ValueError(error_msg)
-            left_on = on
-            right_on = on
+            # The legacy ``(left, right)`` tuple form was consumed above.
+            left_on = right_on = cast("str | Sequence[str]", on)
         elif left_on is not None or right_on is not None:
             if left_on is None or right_on is None:
                 error_msg = "`left_on` and `right_on` should both be provided."
@@ -1344,10 +1346,11 @@ def repartition_by_hash(self, *exprs: Expr | str, num: int) -> DataFrame:
         Returns:
             Repartitioned DataFrame.
         """
-        exprs = [self.parse_sql_expr(e) if isinstance(e, str) else e for e in exprs]
-        exprs = expr_list_to_raw_expr_list(exprs)
+        raw_exprs = expr_list_to_raw_expr_list(
+            [self.parse_sql_expr(e) if isinstance(e, str) else e for e in exprs]
+        )
 
-        return DataFrame(self.df.repartition_by_hash(*exprs, num=num))
+        return DataFrame(self.df.repartition_by_hash(*raw_exprs, num=num))
 
     def union(self, other: DataFrame, distinct: bool = False) -> DataFrame:
         """Calculate the union of two :py:class:`DataFrame`.
@@ -1833,10 +1836,9 @@ def unnest_columns(
             >>> df.unnest_columns("a", recursions=[("a", "a", 1)]).to_pydict()
             {'a': [1, 2, 3], 'b': ['x', 'x', 'y']}
         """
-        columns = list(columns)
         return DataFrame(
             self.df.unnest_columns(
-                columns, preserve_nulls=preserve_nulls, recursions=recursions
+                list(columns), preserve_nulls=preserve_nulls, recursions=recursions
             )
         )
 

diff --git a/python/datafusion/expr.py b/python/datafusion/expr.py
@@ -46,13 +46,14 @@
 
 from __future__ import annotations
 
+import sys
 from collections.abc import Callable, Iterable, Sequence
-from typing import TYPE_CHECKING, Any, ClassVar
+from typing import TYPE_CHECKING, Any, ClassVar, overload
 
-try:
-    from warnings import deprecated  # Python 3.13+
-except ImportError:
-    from typing_extensions import deprecated  # Python 3.12
+if sys.version_info >= (3, 13):
+    from warnings import deprecated
+else:
+    from typing_extensions import deprecated
 
 import pyarrow as pa
 
@@ -114,7 +115,7 @@ def _create_external_table_location(self: Any) -> str:
     return locations[0] if locations else ""
 
 
-CreateExternalTable.location = _create_external_table_location
+CreateExternalTable.location = _create_external_table_location  # ty: ignore[deprecated]
 
 CreateFunction = expr_internal.CreateFunction
 CreateFunctionBody = expr_internal.CreateFunctionBody
@@ -410,8 +411,18 @@ def _to_raw_expr(value: Expr | str) -> expr_internal.Expr:
     raise TypeError(error)
 
 
+@overload
+def expr_list_to_raw_expr_list(expr_list: None) -> None: ...
+
+
+@overload
+def expr_list_to_raw_expr_list(
+    expr_list: Sequence[Expr | str] | Expr | str,
+) -> list[expr_internal.Expr]: ...
+
+
 def expr_list_to_raw_expr_list(
-    expr_list: list[Expr] | Expr | None,
+    expr_list: Sequence[Expr | str] | Expr | str | None,
 ) -> list[expr_internal.Expr] | None:
     """Convert a sequence of expressions or column names to raw expressions."""
     if isinstance(expr_list, Expr | str):
@@ -428,6 +439,16 @@ def sort_or_default(e: Expr | SortExpr) -> expr_internal.SortExpr:
     return SortExpr(e, ascending=True, nulls_first=False).raw_sort
 
 
+@overload
+def sort_list_to_raw_sort_list(sort_list: None) -> None: ...
+
+
+@overload
+def sort_list_to_raw_sort_list(
+    sort_list: Sequence[SortKey] | SortKey,
+) -> list[expr_internal.SortExpr]: ...
+
+
 def sort_list_to_raw_sort_list(
     sort_list: Sequence[SortKey] | SortKey | None,
 ) -> list[expr_internal.SortExpr] | None:
@@ -771,7 +792,7 @@ def __getitem__(self, key: str | int) -> Expr:
             return Expr(functions_internal.array_slice(self.expr, start, stop, step))
         return Expr(self.expr.__getitem__(key))
 
-    def __eq__(self, rhs: object) -> Expr:
+    def __eq__(self, rhs: object) -> Expr:  # ty: ignore[invalid-method-override]
         """Equal to.
 
         Accepts either an expression or any valid PyArrow scalar literal value.
@@ -782,7 +803,7 @@ def __eq__(self, rhs: object) -> Expr:
             rhs = Expr.literal(rhs)
         return Expr(self.expr.__eq__(rhs.expr))
 
-    def __ne__(self, rhs: object) -> Expr:
+    def __ne__(self, rhs: object) -> Expr:  # ty: ignore[invalid-method-override]
         """Not equal to.
 
         Accepts either an expression or any valid PyArrow scalar literal value.

diff --git a/python/datafusion/functions/__init__.py b/python/datafusion/functions/__init__.py
@@ -871,8 +871,8 @@ def concat(*args: Expr) -> Expr:
         >>> result.collect_column("c")[0].as_py()
         'hello world'
     """
-    args = [arg.expr for arg in args]
-    return Expr(f.concat(args))
+    raw_args = [arg.expr for arg in args]
+    return Expr(f.concat(raw_args))
 
 
 def concat_ws(separator: str, *args: Expr) -> Expr:
@@ -888,8 +888,8 @@ def concat_ws(separator: str, *args: Expr) -> Expr:
         >>> result.collect_column("c")[0].as_py()
         'hello-world'
     """
-    args = [arg.expr for arg in args]
-    return Expr(f.concat_ws(separator, args))
+    raw_args = [arg.expr for arg in args]
+    return Expr(f.concat_ws(separator, raw_args))
 
 
 def order_by(expr: Expr, ascending: bool = True, nulls_first: bool = False) -> SortExpr:
@@ -1272,8 +1272,8 @@ def coalesce(*args: Expr) -> Expr:
         >>> result.collect_column("c")[0].as_py()
         2
     """
-    args = [arg.expr for arg in args]
-    return Expr(f.coalesce(*args))
+    raw_args = [arg.expr for arg in args]
+    return Expr(f.coalesce(*raw_args))
 
 
 def cos(arg: Expr) -> Expr:
@@ -3105,8 +3105,8 @@ def make_array(*args: Expr) -> Expr:
         >>> result.collect_column("arr")[0].as_py()
         [1, 2, 3]
     """
-    args = [arg.expr for arg in args]
-    return Expr(f.make_array(args))
+    raw_args = [arg.expr for arg in args]
+    return Expr(f.make_array(raw_args))
 
 
 def make_list(*args: Expr) -> Expr:
@@ -3240,8 +3240,8 @@ def struct(*args: Expr) -> Expr:
         >>> result.collect_column("s")[0].as_py() == {"c0": 1, "c1": 2}
         True
     """
-    args = [arg.expr for arg in args]
-    return Expr(f.struct(*args))
+    raw_args = [arg.expr for arg in args]
+    return Expr(f.struct(*raw_args))
 
 
 def named_struct(name_pairs: list[tuple[str, Expr]]) -> Expr:
@@ -3762,8 +3762,8 @@ def array_concat(*args: Expr) -> Expr:
         >>> result.collect_column("result")[0].as_py()
         [1, 2, 3, 4]
     """
-    args = [arg.expr for arg in args]
-    return Expr(f.array_concat(args))
+    raw_args = [arg.expr for arg in args]
+    return Expr(f.array_concat(raw_args))
 
 
 def array_cat(*args: Expr) -> Expr:

diff --git a/python/datafusion/functions/spark.py b/python/datafusion/functions/spark.py
@@ -56,14 +56,14 @@ def _filter_raw(filter: Expr | None) -> Any:
     return filter.expr if filter is not None else None
 
 
-def _coerce_i32(value: Expr | int | None) -> Expr | None:
-    """Coerce a native ``int`` to an int32 literal, passing ``Expr``/``None`` through.
+def _coerce_i32(value: Expr | int) -> Expr:
+    """Coerce a native ``int`` to an int32 literal, passing ``Expr`` through.
 
     Several Spark datetime and interval builders require 32-bit integer
     inputs, so a bare ``int`` must become an int32 literal rather than the
     int64 default that :meth:`Expr.literal` would produce.
     """
-    if value is None or isinstance(value, Expr):
+    if isinstance(value, Expr):
         return value
     return Expr.literal(pa.scalar(value, type=pa.int32()))
 

diff --git a/python/datafusion/plan.py b/python/datafusion/plan.py
@@ -143,7 +143,7 @@ def to_proto(self) -> bytes:
         )
         return self.to_bytes()
 
-    def __eq__(self, other: LogicalPlan) -> bool:
+    def __eq__(self, other: object) -> bool:
         """Test equality."""
         if not isinstance(other, LogicalPlan):
             return False