feat: Add an is_literal method to expression meta namespace

crates/polars-plan/src/dsl/meta.rs

@@ -83,9 +83,21 @@ impl MetaNameSpace {
             | Expr::IndexColumn(_)
             | Expr::Selector(_)
             | Expr::Wildcard => true,
-            Expr::Alias(_, _) | Expr::KeepName(_) | Expr::RenameAlias { .. } if allow_aliasing => {
-                true
-            },
+            Expr::Alias(_, _) | Expr::KeepName(_) | Expr::RenameAlias { .. } => allow_aliasing,
+            _ => false,
+        })
+    }
+
+    /// Indicate if this expression represents a literal value (optionally aliased).
+    pub fn is_literal(&self, allow_aliasing: bool) -> bool {
+        self.0.into_iter().all(|e| match e {
+            Expr::Literal(_) => true,
+            Expr::Alias(_, _) => allow_aliasing,
+            Expr::Cast {
+                expr,
+                dtype: DataType::Datetime(_, _),
+                options: CastOptions::Strict,
+            } if matches!(&**expr, Expr::Literal(LiteralValue::DateTime(_, _, _))) => true,
             _ => false,
         })
     }

crates/polars-python/src/expr/meta.rs

@@ -58,6 +58,10 @@ impl PyExpr {
             .is_column_selection(allow_aliasing)
     }
 
+    fn meta_is_literal(&self, allow_aliasing: bool) -> bool {
+        self.inner.clone().meta().is_literal(allow_aliasing)
+    }
+
     fn _meta_selector_add(&self, other: PyExpr) -> PyResult<PyExpr> {
         let out = self
             .inner

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

@@ -13,6 +13,7 @@ The following methods are available under the `expr.meta` attribute.
     Expr.meta.has_multiple_outputs
     Expr.meta.is_column
     Expr.meta.is_column_selection
+    Expr.meta.is_literal
     Expr.meta.is_regex_projection
     Expr.meta.ne
     Expr.meta.output_name

py-polars/polars/expr/meta.py

@@ -108,15 +108,15 @@ def is_column_selection(self, *, allow_aliasing: bool = False) -> bool:
         """
         Indicate if this expression only selects columns (optionally with aliasing).
 
-        This can include bare columns, column matches by regex or dtype, selectors
+        This can include bare columns, columns matched by regex or dtype, selectors
         and exclude ops, and (optionally) column/expression aliasing.
 
         .. versionadded:: 0.20.30
 
         Parameters
         ----------
         allow_aliasing
-            If False (default), any aliasing is not considered pure column selection.
+            If False (default), any aliasing is not considered to be column selection.
             Set True to allow for column selection that also includes aliasing.
 
         Examples
@@ -142,6 +142,33 @@ def is_column_selection(self, *, allow_aliasing: bool = False) -> bool:
         """
         return self._pyexpr.meta_is_column_selection(allow_aliasing)
 
+    def is_literal(self, *, allow_aliasing: bool = False) -> bool:
+        """
+        Indicate if this expression is a literal value (optionally aliased).
+
+        .. versionadded:: 1.14
+
+        Parameters
+        ----------
+        allow_aliasing
+            If False (default), only a bare literal will match.
+            Set True to also allow for aliased literals.
+
+        Examples
+        --------
+        >>> from datetime import datetime
+        >>> e = pl.lit(123)
+        >>> e.meta.is_literal()
+        True
+        >>> e = pl.lit(987.654321).alias("foo")
+        >>> e.meta.is_literal()
+        False
+        >>> e = pl.lit(datetime.now()).alias("bar")
+        >>> e.meta.is_literal(allow_aliasing=True)
+        True
+        """
+        return self._pyexpr.meta_is_literal(allow_aliasing)
+
     @overload
     def output_name(self, *, raise_if_undetermined: Literal[True] = True) -> str: ...
 

py-polars/tests/unit/operations/namespaces/test_meta.py

@@ -1,6 +1,7 @@
 from __future__ import annotations
 
-from typing import TYPE_CHECKING
+from datetime import date, datetime, time, timedelta
+from typing import TYPE_CHECKING, Any
 
 import pytest
 
@@ -123,6 +124,34 @@ def test_is_column_selection(
         assert not expr.meta.is_column_selection()
 
 
+@pytest.mark.parametrize(
+    "value",
+    [
+        None,
+        1234,
+        567.89,
+        float("inf"),
+        date.today(),
+        datetime.now(),
+        time(10, 30, 45),
+        timedelta(hours=-24),
+        ["x", "y", "z"],
+        pl.Series([None, None]),
+        [[10, 20], [30, 40]],
+        "this is the way",
+    ],
+)
+def test_is_literal(value: Any) -> None:
+    e = pl.lit(value)
+    assert e.meta.is_literal()
+
+    e = pl.lit(value).alias("foo")
+    assert not e.meta.is_literal()
+
+    e = pl.lit(value).alias("foo")
+    assert e.meta.is_literal(allow_aliasing=True)
+
+
 def test_meta_is_regex_projection() -> None:
     e = pl.col("^.*$").name.suffix("_foo")
     assert e.meta.is_regex_projection()