Add lazy() method to LazyFrame (#4077)

py-polars/polars/internals/lazy_frame.py

@@ -692,6 +692,18 @@ def fetch(
         )
         return self._dataframe_class._from_pydf(ldf.fetch(n_rows))
 
+    def lazy(self: LDF) -> LDF:
+        """
+        Returns lazy representation, i.e. itself.
+
+        Useful for writing code that expects either a :class:`DataFrame` or :class:`LazyFrame`.
+
+        Returns
+        -------
+        LazyFrame
+        """
+        return self
+
     @property
     def columns(self) -> list[str]:
         """

py-polars/tests/test_lazy.py

@@ -1352,3 +1352,13 @@ def test_explode_inner_lists_3985() -> None:
         .agg(pl.col("categories"))
         .with_column(pl.col("categories").arr.eval(pl.element().explode()))
     ).collect().to_dict(False) == {"id": [1], "categories": [["a", "b", "a", "c"]]}
+
+
+def test_lazy_method() -> None:
+    """
+    We want to support `.lazy()` on a Lazy DataFrame as to allow more generic user code.
+    """
+    df = pl.DataFrame({"a": [1, 1, 2, 2, 3, 3], "b": [1, 2, 3, 4, 5, 6]})
+    lazy_df = df.lazy()
+
+    assert lazy_df.lazy() == lazy_df