warn when checking LazyFrame column membership

Author: VedantMadane

py-polars/src/polars/lazyframe/frame.py

@@ -740,6 +740,12 @@ def __le__(self, other: Any) -> NoReturn:
         self._comparison_error("<=")
 
     def __contains__(self, key: str) -> bool:
+        issue_warning(
+            "Determining column membership of a LazyFrame requires resolving its schema,"
+            " which is a potentially expensive operation. Use `LazyFrame.collect_schema()`"
+            " to inspect the schema without this warning.",
+            category=PerformanceWarning,
+        )
         return key in self.collect_schema()
 
     def __copy__(self) -> LazyFrame:

py-polars/tests/unit/lazyframe/test_lazyframe.py

@@ -1447,6 +1447,8 @@ def test_lf_properties() -> None:
         assert lf.dtypes == [pl.Int64, pl.Float64, pl.String]
     with pytest.warns(PerformanceWarning):
         assert lf.width == 3
+    with pytest.warns(PerformanceWarning):
+        assert "foo" in lf
 
 
 def test_lf_unnest() -> None: