feat(python): Add Expr.struct.unnest() as alias for Expr.struct.field…

…("*") (#19212)

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

@@ -10,6 +10,7 @@ The following methods are available under the `expr.struct` attribute.
    :template: autosummary/accessor_method.rst
 
     Expr.struct.field
+    Expr.struct.unnest
     Expr.struct.json_encode
     Expr.struct.rename_fields
     Expr.struct.with_fields

py-polars/polars/expr/struct.py

@@ -152,6 +152,43 @@ def field(self, name: str | list[str], *more_names: str) -> Expr:
 
         return wrap_expr(self._pyexpr.struct_field_by_name(name))
 
+    def unnest(self) -> Expr:
+        """
+        Expand the struct into its individual fields.
+
+        Alias for `Expr.struct.field("*")`.
+
+        >>> df = pl.DataFrame(
+        ...     {
+        ...         "aaa": [1, 2],
+        ...         "bbb": ["ab", "cd"],
+        ...         "ccc": [True, None],
+        ...         "ddd": [[1, 2], [3]],
+        ...     }
+        ... ).select(pl.struct("aaa", "bbb", "ccc", "ddd").alias("struct_col"))
+        >>> df
+        shape: (2, 1)
+        ┌──────────────────────┐
+        │ struct_col           │
+        │ ---                  │
+        │ struct[4]            │
+        ╞══════════════════════╡
+        │ {1,"ab",true,[1, 2]} │
+        │ {2,"cd",null,[3]}    │
+        └──────────────────────┘
+        >>> df.select(pl.col("struct_col").struct.unnest())
+        shape: (2, 4)
+        ┌─────┬─────┬──────┬───────────┐
+        │ aaa ┆ bbb ┆ ccc  ┆ ddd       │
+        │ --- ┆ --- ┆ ---  ┆ ---       │
+        │ i64 ┆ str ┆ bool ┆ list[i64] │
+        ╞═════╪═════╪══════╪═══════════╡
+        │ 1   ┆ ab  ┆ true ┆ [1, 2]    │
+        │ 2   ┆ cd  ┆ null ┆ [3]       │
+        └─────┴─────┴──────┴───────────┘
+        """
+        return self.field("*")
+
     def rename_fields(self, names: Sequence[str]) -> Expr:
         """
         Rename the fields of the struct.

py-polars/tests/unit/test_expansion.py

@@ -138,6 +138,20 @@ def test_struct_field_expand_star() -> None:
     assert_frame_equal(struct_df.select(pl.col("struct_col").struct.field("*")), df)
 
 
+def test_struct_unnest() -> None:
+    """Same as test_struct_field_expand_star but using the unnest alias."""
+    df = pl.DataFrame(
+        {
+            "aaa": [1, 2],
+            "bbb": ["ab", "cd"],
+            "ccc": [True, None],
+            "ddd": [[1, 2], [3]],
+        }
+    )
+    struct_df = df.select(pl.struct(["aaa", "bbb", "ccc", "ddd"]).alias("struct_col"))
+    assert_frame_equal(struct_df.select(pl.col("struct_col").struct.unnest()), df)
+
+
 def test_struct_field_expand_rewrite() -> None:
     df = pl.DataFrame({"A": [1], "B": [2]})
     assert df.select(