feat(python): Add Expr.arg_true

py-polars/docs/source/reference/expressions/modify_select.rst

@@ -9,6 +9,7 @@ Manipulation/selection
     Expr.append
     Expr.arg_sort
     Expr.argsort
+    Expr.arg_true
     Expr.backward_fill
     Expr.cast
     Expr.ceil

py-polars/polars/internals/expr/expr.py

@@ -1,5 +1,6 @@
 from __future__ import annotations
 
+import contextlib
 import math
 import os
 import random
@@ -40,6 +41,9 @@
     sphinx_accessor,
 )
 
+with contextlib.suppress(ImportError):  # Module not available when building docs
+    from polars.polars import arg_where as py_arg_where
+
 if TYPE_CHECKING:
     import sys
 
@@ -418,6 +422,37 @@ def all(self) -> Self:
         """
         return self._from_pyexpr(self._pyexpr.all())
 
+    def arg_true(self) -> Self:
+        """
+        Return indices where expression evaluates `True`.
+
+        .. warning::
+            Modifies number of rows returned, so will fail in combination with other
+            expressions. Use as only expression in `select` / `with_columns`.
+
+        Examples
+        --------
+        >>> df = pl.DataFrame({"a": [1, 1, 2, 1]})
+        >>> df.select((pl.col("a") == 1).arg_true())
+        shape: (3, 1)
+        ┌─────┐
+        │ a   │
+        │ --- │
+        │ u32 │
+        ╞═════╡
+        │ 0   │
+        │ 1   │
+        │ 3   │
+        └─────┘
+
+        See Also
+        --------
+        Series.arg_true : Return indices where Series is True
+        pl.arg_where
+
+        """
+        return self._from_pyexpr(py_arg_where(self._pyexpr))
+
     def sqrt(self) -> Self:
         """
         Compute the square root of the elements.

py-polars/polars/internals/lazy_functions.py

@@ -2912,6 +2912,10 @@ def arg_where(
         3
     ]
 
+    See Also
+    --------
+    Series.arg_true : Return indices where Series is True
+
     """
     if eager:
         if not isinstance(condition, pli.Series):

py-polars/tests/unit/test_exprs.py

@@ -701,3 +701,10 @@ def test_exclude_invalid_input(input: tuple[Any, ...]) -> None:
     df = pl.DataFrame(schema=["a", "b", "c"])
     with pytest.raises(TypeError):
         df.select(pl.all().exclude(*input))
+
+
+def test_arg_true() -> None:
+    df = pl.DataFrame({"a": [1, 1, 2, 1]})
+    res = df.select((pl.col("a") == 1).arg_true())
+    expected = pl.DataFrame([pl.Series("a", [0, 1, 3], dtype=pl.UInt32)])
+    assert_frame_equal(res, expected)