feat(python): validate Enum categories (pola-rs#13356)

py-polars/polars/datatypes/classes.py

@@ -538,16 +538,31 @@ class Enum(DataType):
 
     categories: list[str]
 
-    def __init__(self, categories: list[str]):
+    def __init__(self, categories: Iterable[str]):
         """
         A fixed set categorical encoding of a set of strings.
 
         Parameters
         ----------
         categories
-            Categories in the dataset.
+            Valid categories in the dataset.
 
         """
+        if not isinstance(categories, list):
+            categories = list(categories)
+
+        seen: set[str] = set()
+        for cat in categories:
+            if cat in seen:
+                raise ValueError(
+                    f"Enum categories must be unique; found duplicate {cat!r}"
+                )
+            if not isinstance(cat, str):
+                raise TypeError(
+                    f"Enum categories must be strings; found {cat!r} ({type(cat).__name__})"
+                )
+            seen.add(cat)
+
         self.categories = categories
 
     def __eq__(self, other: PolarsDataType) -> bool:  # type: ignore[override]

py-polars/tests/unit/datatypes/test_enum.py

@@ -1,6 +1,7 @@
 import operator
+from datetime import date
 from textwrap import dedent
-from typing import Callable
+from typing import Any, Callable
 
 import pytest
 
@@ -15,6 +16,13 @@ def test_enum_creation() -> None:
     assert s.len() == 3
     assert s.dtype == pl.Enum(categories=["a", "b"])
 
+    # from iterables
+    e = pl.Enum(f"x{i}" for i in range(5))
+    assert e.categories == ["x0", "x1", "x2", "x3", "x4"]
+
+    e = pl.Enum("abcde")
+    assert e.categories == ["a", "b", "c", "d", "e"]
+
 
 def test_enum_non_existent() -> None:
     with pytest.raises(
@@ -303,3 +311,17 @@ def test_different_enum_comparison_order() -> None:
             match="can only compare categoricals of the same type",
         ):
             df_enum.filter(op(pl.col("a_cat"), pl.col("b_cat")))
+
+
+@pytest.mark.parametrize(
+    "categories",
+    [[None], [date.today()], [-10, 10], ["x", "y", None]],
+)
+def test_valid_enum_category_types(categories: Any) -> None:
+    with pytest.raises(TypeError, match="Enum categories"):
+        pl.Enum(categories)
+
+
+def test_enum_categories_unique() -> None:
+    with pytest.raises(ValueError, match="must be unique; found duplicate 'a'"):
+        pl.Enum(["a", "a", "b", "b", "b", "c"])