Add missing scalar functions

crates/core/src/functions.rs

@@ -695,8 +695,29 @@ expr_fn_vec!(named_struct);
 expr_fn!(from_unixtime, unixtime);
 expr_fn!(arrow_typeof, arg_1);
 expr_fn!(arrow_cast, arg_1 datatype);
+expr_fn_vec!(arrow_metadata);
+expr_fn!(union_tag, arg1);
 expr_fn!(random);
 
+#[pyfunction]
+fn get_field(expr: PyExpr, name: PyExpr) -> PyExpr {
+    functions::core::get_field()
+        .call(vec![expr.into(), name.into()])
+        .into()
+}
+
+#[pyfunction]
+fn union_extract(union_expr: PyExpr, field_name: PyExpr) -> PyExpr {
+    functions::core::union_extract()
+        .call(vec![union_expr.into(), field_name.into()])
+        .into()
+}
+
+#[pyfunction]
+fn version() -> PyExpr {
+    functions::core::version().call(vec![]).into()
+}
+
 // Array Functions
 array_fn!(array_append, array element);
 array_fn!(array_to_string, array delimiter);
@@ -1014,6 +1035,7 @@ pub(crate) fn init_module(m: &Bound<'_, PyModule>) -> PyResult<()> {
     m.add_wrapped(wrap_pyfunction!(array_agg))?;
     m.add_wrapped(wrap_pyfunction!(arrow_typeof))?;
     m.add_wrapped(wrap_pyfunction!(arrow_cast))?;
+    m.add_wrapped(wrap_pyfunction!(arrow_metadata))?;
     m.add_wrapped(wrap_pyfunction!(ascii))?;
     m.add_wrapped(wrap_pyfunction!(asin))?;
     m.add_wrapped(wrap_pyfunction!(asinh))?;
@@ -1142,6 +1164,10 @@ pub(crate) fn init_module(m: &Bound<'_, PyModule>) -> PyResult<()> {
     m.add_wrapped(wrap_pyfunction!(trim))?;
     m.add_wrapped(wrap_pyfunction!(trunc))?;
     m.add_wrapped(wrap_pyfunction!(upper))?;
+    m.add_wrapped(wrap_pyfunction!(get_field))?;
+    m.add_wrapped(wrap_pyfunction!(union_extract))?;
+    m.add_wrapped(wrap_pyfunction!(union_tag))?;
+    m.add_wrapped(wrap_pyfunction!(version))?;
     m.add_wrapped(wrap_pyfunction!(self::uuid))?; // Use self to avoid name collision
     m.add_wrapped(wrap_pyfunction!(var_pop))?;
     m.add_wrapped(wrap_pyfunction!(var_sample))?;

python/datafusion/functions.py

@@ -98,6 +98,7 @@
     "arrays_overlap",
     "arrays_zip",
     "arrow_cast",
+    "arrow_metadata",
     "arrow_typeof",
     "ascii",
     "asin",
@@ -163,6 +164,7 @@
     "gcd",
     "gen_series",
     "generate_series",
+    "get_field",
     "greatest",
     "ifnull",
     "in_list",
@@ -280,6 +282,7 @@
     "reverse",
     "right",
     "round",
+    "row",
     "row_number",
     "rpad",
     "rtrim",
@@ -322,12 +325,15 @@
     "translate",
     "trim",
     "trunc",
+    "union_extract",
+    "union_tag",
     "upper",
     "uuid",
     "var",
     "var_pop",
     "var_samp",
     "var_sample",
+    "version",
     "when",
     # Window Functions
     "window",
@@ -2628,22 +2634,184 @@ def arrow_typeof(arg: Expr) -> Expr:
     return Expr(f.arrow_typeof(arg.expr))
 
 
-def arrow_cast(expr: Expr, data_type: Expr) -> Expr:
+def arrow_cast(expr: Expr, data_type: Expr | str | pa.DataType) -> Expr:
     """Casts an expression to a specified data type.
 
+    The ``data_type`` can be a string, a ``pyarrow.DataType``, or an
+    ``Expr``. For simple types, :py:meth:`Expr.cast()
+    <datafusion.expr.Expr.cast>` is more concise
+    (e.g., ``col("a").cast(pa.float64())``). Use ``arrow_cast`` when
+    you want to specify the target type as a string using DataFusion's
+    type syntax, which can be more readable for complex types like
+    ``"Timestamp(Nanosecond, None)"``.
+
     Examples:
         >>> ctx = dfn.SessionContext()
         >>> df = ctx.from_pydict({"a": [1]})
-        >>> data_type = dfn.string_literal("Float64")
         >>> result = df.select(
-        ...     dfn.functions.arrow_cast(dfn.col("a"), data_type).alias("c")
+        ...     dfn.functions.arrow_cast(dfn.col("a"), "Float64").alias("c")
+        ... )
+        >>> result.collect_column("c")[0].as_py()
+        1.0
+
+        >>> import pyarrow as pa
+        >>> result = df.select(
+        ...     dfn.functions.arrow_cast(
+        ...         dfn.col("a"), data_type=pa.float64()
+        ...     ).alias("c")
         ... )
         >>> result.collect_column("c")[0].as_py()
         1.0
     """
+    if isinstance(data_type, pa.DataType):
+        return expr.cast(data_type)
+    if isinstance(data_type, str):
+        data_type = Expr.string_literal(data_type)
     return Expr(f.arrow_cast(expr.expr, data_type.expr))
 
 
+def arrow_metadata(expr: Expr, key: Expr | str | None = None) -> Expr:
+    """Returns the metadata of the input expression.
+
+    If called with one argument, returns a Map of all metadata key-value pairs.
+    If called with two arguments, returns the value for the specified metadata key.
+
+    Examples:
+        >>> import pyarrow as pa
+        >>> field = pa.field("val", pa.int64(), metadata={"k": "v"})
+        >>> schema = pa.schema([field])
+        >>> batch = pa.RecordBatch.from_arrays([pa.array([1])], schema=schema)
+        >>> ctx = dfn.SessionContext()
+        >>> df = ctx.create_dataframe([[batch]])
+        >>> result = df.select(
+        ...     dfn.functions.arrow_metadata(dfn.col("val")).alias("meta")
+        ... )
+        >>> ("k", "v") in result.collect_column("meta")[0].as_py()
+        True
+
+        >>> result = df.select(
+        ...     dfn.functions.arrow_metadata(
+        ...         dfn.col("val"), key="k"
+        ...     ).alias("meta_val")
+        ... )
+        >>> result.collect_column("meta_val")[0].as_py()
+        'v'
+    """
+    if key is None:
+        return Expr(f.arrow_metadata(expr.expr))
+    if isinstance(key, str):
+        key = Expr.string_literal(key)
+    return Expr(f.arrow_metadata(expr.expr, key.expr))
+
+
+def get_field(expr: Expr, name: Expr | str) -> Expr:
+    """Extracts a field from a struct or map by name.
+
+    When the field name is a static string, the bracket operator
+    ``expr["field"]`` is a convenient shorthand. Use ``get_field``
+    when the field name is a dynamic expression.
+
+    Examples:
+        >>> ctx = dfn.SessionContext()
+        >>> df = ctx.from_pydict({"a": [1], "b": [2]})
+        >>> df = df.with_column(
+        ...     "s",
+        ...     dfn.functions.named_struct(
+        ...         [("x", dfn.col("a")), ("y", dfn.col("b"))]
+        ...     ),
+        ... )
+        >>> result = df.select(
+        ...     dfn.functions.get_field(dfn.col("s"), "x").alias("x_val")
+        ... )
+        >>> result.collect_column("x_val")[0].as_py()
+        1
+
+        Equivalent using bracket syntax:
+
+        >>> result = df.select(
+        ...     dfn.col("s")["x"].alias("x_val")
+        ... )
+        >>> result.collect_column("x_val")[0].as_py()
+        1
+    """
+    if isinstance(name, str):
+        name = Expr.string_literal(name)
+    return Expr(f.get_field(expr.expr, name.expr))
+
+
+def union_extract(union_expr: Expr, field_name: Expr | str) -> Expr:
+    """Extracts a value from a union type by field name.
+
+    Returns the value of the named field if it is the currently selected
+    variant, otherwise returns NULL.
+
+    Examples:
+        >>> import pyarrow as pa
+        >>> ctx = dfn.SessionContext()
+        >>> types = pa.array([0, 1, 0], type=pa.int8())
+        >>> offsets = pa.array([0, 0, 1], type=pa.int32())
+        >>> arr = pa.UnionArray.from_dense(
+        ...     types, offsets, [pa.array([1, 2]), pa.array(["hi"])],
+        ...     ["int", "str"], [0, 1],
+        ... )
+        >>> batch = pa.RecordBatch.from_arrays([arr], names=["u"])
+        >>> df = ctx.create_dataframe([[batch]])
+        >>> result = df.select(
+        ...     dfn.functions.union_extract(dfn.col("u"), "int").alias("val")
+        ... )
+        >>> result.collect_column("val").to_pylist()
+        [1, None, 2]
+    """
+    if isinstance(field_name, str):
+        field_name = Expr.string_literal(field_name)
+    return Expr(f.union_extract(union_expr.expr, field_name.expr))
+
+
+def union_tag(union_expr: Expr) -> Expr:
+    """Returns the tag (active field name) of a union type.
+
+    Examples:
+        >>> import pyarrow as pa
+        >>> ctx = dfn.SessionContext()
+        >>> types = pa.array([0, 1, 0], type=pa.int8())
+        >>> offsets = pa.array([0, 0, 1], type=pa.int32())
+        >>> arr = pa.UnionArray.from_dense(
+        ...     types, offsets, [pa.array([1, 2]), pa.array(["hi"])],
+        ...     ["int", "str"], [0, 1],
+        ... )
+        >>> batch = pa.RecordBatch.from_arrays([arr], names=["u"])
+        >>> df = ctx.create_dataframe([[batch]])
+        >>> result = df.select(
+        ...     dfn.functions.union_tag(dfn.col("u")).alias("tag")
+        ... )
+        >>> result.collect_column("tag").to_pylist()
+        ['int', 'str', 'int']
+    """
+    return Expr(f.union_tag(union_expr.expr))
+
+
+def version() -> Expr:
+    """Returns the DataFusion version string.
+
+    Examples:
+        >>> ctx = dfn.SessionContext()
+        >>> df = ctx.empty_table()
+        >>> result = df.select(dfn.functions.version().alias("v"))
+        >>> "Apache DataFusion" in result.collect_column("v")[0].as_py()
+        True
+    """
+    return Expr(f.version())
+
+
+def row(*args: Expr) -> Expr:
+    """Returns a struct with the given arguments.
+
+    See Also:
+        This is an alias for :py:func:`struct`.
+    """
+    return struct(*args)
+
+
 def random() -> Expr:
     """Returns a random value in the range ``0.0 <= x < 1.0``.