Added ability for constant literals in DataChain.mutate(...) (#869)

Added ability for constant literals in `DataChain.mutate(...)`

src/datachain/lib/dc.py

@@ -25,6 +25,7 @@
 from sqlalchemy.sql.sqltypes import NullType
 
 from datachain.dataset import DatasetRecord
+from datachain.func import literal
 from datachain.func.base import Function
 from datachain.func.func import Func
 from datachain.lib.convert.python_to_sql import python_to_sql
@@ -1129,8 +1130,12 @@ def mutate(self, **kwargs) -> "Self":
         )
         ```
         """
+        primitives = (bool, str, int, float)
+
         for col_name, expr in kwargs.items():
-            if not isinstance(expr, (Column, Func)) and isinstance(expr.type, NullType):
+            if not isinstance(expr, (*primitives, Column, Func)) and isinstance(
+                expr.type, NullType
+            ):
                 raise DataChainColumnError(
                     col_name, f"Cannot infer type with expression {expr}"
                 )
@@ -1145,6 +1150,11 @@ def mutate(self, **kwargs) -> "Self":
             elif isinstance(value, Func):
                 # adding new signal
                 mutated[name] = value.get_column(schema)
+            elif isinstance(value, primitives):
+                # adding simple python constant primitives like str, int, float, bool
+                val = literal(value)
+                val.type = python_to_sql(type(value))()
+                mutated[name] = val  # type: ignore[assignment]
             else:
                 # adding new signal
                 mutated[name] = value

tests/unit/test_func.py

@@ -425,6 +425,12 @@ def test_lt_mutate(dc):
     assert list(res) == [0, 0, 0, 0, 0]
 
 
+@pytest.mark.parametrize("value", [1, 0.5, "a", True])
+def test_mutate_with_literal(dc, value):
+    res = dc.mutate(test=value).collect("test")
+    assert list(res) == [value] * 5
+
+
 def test_le():
     rnd1, rnd2 = rand(), rand()