feat: allow from_dict to not have native_namespace arg if all inputs are already narwhals Series (#760)

Author: MarcoGorelli

narwhals/functions.py

@@ -52,7 +52,7 @@ def from_dict(
     data: dict[str, Any],
     schema: dict[str, DType] | Schema | None = None,
     *,
-    native_namespace: ModuleType,
+    native_namespace: ModuleType | None = None,
 ) -> DataFrame[Any]:
     """
     Instantiate DataFrame from dictionary.
@@ -64,7 +64,8 @@ def from_dict(
     Arguments:
         data: Dictionary to create DataFrame from.
         schema: The DataFrame schema as Schema or dict of {name: type}.
-        native_namespace: The native library to use for DataFrame creation.
+        native_namespace: The native library to use for DataFrame creation. Only
+            necessary if inputs are not Narwhals Series.
 
     Examples:
         >>> import pandas as pd
@@ -97,6 +98,21 @@ def from_dict(
         │ 2   ┆ 4   │
         └─────┴─────┘
     """
+    from narwhals.series import Series
+    from narwhals.translate import to_native
+
+    if not data:
+        msg = "from_dict cannot be called with empty dictionary"
+        raise ValueError(msg)
+    if native_namespace is None:
+        for val in data.values():
+            if isinstance(val, Series):
+                native_namespace = val.__native_namespace__()
+                break
+        else:
+            msg = "Calling `from_dict` without `native_namespace` is only supported if all input values are already Narwhals Series"
+            raise TypeError(msg)
+        data = {key: to_native(value, strict=False) for key, value in data.items()}
     implementation = Implementation.from_native_namespace(native_namespace)
 
     if implementation is Implementation.POLARS:

narwhals/stable/v1.py

@@ -1473,7 +1473,7 @@ def from_dict(
     data: dict[str, Any],
     schema: dict[str, DType] | Schema | None = None,
     *,
-    native_namespace: ModuleType,
+    native_namespace: ModuleType | None = None,
 ) -> DataFrame[Any]:
     """
     Instantiate DataFrame from dictionary.
@@ -1485,7 +1485,8 @@ def from_dict(
     Arguments:
         data: Dictionary to create DataFrame from.
         schema: The DataFrame schema as Schema or dict of {name: type}.
-        native_namespace: The native library to use for DataFrame creation.
+        native_namespace: The native library to use for DataFrame creation. Only
+            necessary if inputs are not Narwhals Series.
 
     Examples:
         >>> import pandas as pd

tests/from_dict_test.py

@@ -29,3 +29,27 @@ def test_from_dict_schema(constructor: Any, request: Any) -> None:
         schema=schema,  # type: ignore[arg-type]
     )
     assert result.collect_schema() == schema
+
+
+def test_from_dict_without_namespace(constructor: Any) -> None:
+    df = nw.from_native(constructor({"a": [1, 2, 3], "b": [4, 5, 6]})).lazy().collect()
+    result = nw.from_dict({"c": df["a"], "d": df["b"]})
+    compare_dicts(result, {"c": [1, 2, 3], "d": [4, 5, 6]})
+
+
+def test_from_dict_without_namespace_invalid(constructor: Any) -> None:
+    df = nw.from_native(constructor({"a": [1, 2, 3], "b": [4, 5, 6]})).lazy().collect()
+    with pytest.raises(TypeError, match="namespace"):
+        nw.from_dict({"c": nw.to_native(df["a"]), "d": nw.to_native(df["b"])})
+
+
+def test_from_dict_one_native_one_narwhals(constructor: Any) -> None:
+    df = nw.from_native(constructor({"a": [1, 2, 3], "b": [4, 5, 6]})).lazy().collect()
+    result = nw.from_dict({"c": nw.to_native(df["a"]), "d": df["b"]})
+    expected = {"c": [1, 2, 3], "d": [4, 5, 6]}
+    compare_dicts(result, expected)
+
+
+def test_from_dict_empty() -> None:
+    with pytest.raises(ValueError, match="empty"):
+        nw.from_dict({})