fix: Allow exprs in .filter constraints (#2114)

* fix: allow exprs in filter constraints

* refactor(suggestion): Tidy-up constraints

---------

Co-authored-by: dangotbanned <[email protected]>

Author: FBruzzesi

narwhals/dataframe.py

@@ -202,21 +202,22 @@ def filter(
             and isinstance(predicates[0], list)
             and all(isinstance(x, bool) for x in predicates[0])
         ):
+            from narwhals.functions import col
+
             flat_predicates = flatten(predicates)
             check_expressions_preserve_length(*flat_predicates, function_name="filter")
-            compliant_predicates, _kinds = self._flatten_and_extract(*flat_predicates)
             plx = self.__narwhals_namespace__()
+            compliant_predicates, _kinds = self._flatten_and_extract(*flat_predicates)
+            compliant_constraints = (
+                (col(name) == v)._to_compliant_expr(plx)
+                for name, v in constraints.items()
+            )
             predicate = plx.all_horizontal(
-                *chain(
-                    compliant_predicates,
-                    (plx.col(name) == v for name, v in constraints.items()),
-                )
+                *chain(compliant_predicates, compliant_constraints)
             )
         else:
             predicate = predicates[0]
-        return self._from_compliant_dataframe(
-            self._compliant_frame.filter(predicate),
-        )
+        return self._from_compliant_dataframe(self._compliant_frame.filter(predicate))
 
     def sort(
         self: Self,

tests/frame/filter_test.py

@@ -54,3 +54,17 @@ def test_filter_raise_on_shape_mismatch(constructor: Constructor) -> None:
     df = nw.from_native(constructor(data))
     with pytest.raises((LengthChangingExprError, ShapeError)):
         df.filter(nw.col("b").unique() > 2).lazy().collect()
+
+
+def test_filter_with_constrains(constructor: Constructor) -> None:
+    data = {"a": [1, 3, 2], "b": [4, 4, 6]}
+    df = nw.from_native(constructor(data))
+    result_scalar = df.filter(a=3)
+    expected_scalar = {"a": [3], "b": [4]}
+
+    assert_equal_data(result_scalar, expected_scalar)
+
+    result_expr = df.filter(a=nw.col("b") // 3)
+    expected_expr = {"a": [1, 2], "b": [4, 6]}
+
+    assert_equal_data(result_expr, expected_expr)