feat: Pass Slots to {% fill %} with 'body' kwarg (#1203)

CHANGELOG.md

@@ -574,6 +574,30 @@
     - If `Slot` was created from string via `Slot("...")`, `Slot.contents` will contain that string.
     - If `Slot` was created from a function, `Slot.contents` will contain that function.
 
+- `{% fill %}` tag now accepts `body` kwarg to pass a Slot instance to fill.
+
+    First pass a [`Slot`](../api#django_components.Slot) instance to the template
+    with the [`get_template_data()`](../api#django_components.Component.get_template_data)
+    method:
+
+    ```python
+    from django_components import component, Slot
+
+    class Table(Component):
+      def get_template_data(self, args, kwargs, slots, context):
+        return {
+            "my_slot": Slot(lambda ctx: "Hello, world!"),
+        }
+    ```
+
+    Then pass the slot to the `{% fill %}` tag:
+
+    ```django
+    {% component "table" %}
+      {% fill "pagination" body=my_slot / %}
+    {% endcomponent %}
+    ```
+
 - Component caching can now take slots into account, by setting `Component.Cache.include_slots` to `True`.
 
     ```py

src/django_components/slots.py

@@ -47,6 +47,7 @@ SLOT_REQUIRED_FLAG = "required"
 SLOT_DEFAULT_FLAG = "default"
 FILL_DATA_KWARG = "data"
 FILL_FALLBACK_KWARG = "fallback"
+FILL_BODY_KWARG = "body"
 
 
 # Public types
@@ -954,6 +955,46 @@ class FillNode(BaseNode):
       {% endfill %}
     {% endcomponent %}
     ```
+
+    ### Passing slot fill from Python
+
+    You can pass a slot fill from Python to a component by setting the `body` kwarg
+    on the `{% fill %}` tag.
+
+    First pass a [`Slot`](../api#django_components.Slot) instance to the template
+    with the [`get_template_data()`](../api#django_components.Component.get_template_data)
+    method:
+
+    ```python
+    from django_components import component, Slot
+
+    class Table(Component):
+      def get_template_data(self, args, kwargs, slots, context):
+        return {
+            "my_slot": Slot(lambda ctx: "Hello, world!"),
+        }
+    ```
+
+    Then pass the slot to the `{% fill %}` tag:
+
+    ```django
+    {% component "table" %}
+      {% fill "pagination" body=my_slot / %}
+    {% endcomponent %}
+    ```
+
+    !!! warning
+
+        If you define both the `body` kwarg and the `{% fill %}` tag's body,
+        an error will be raised.
+
+        ```django
+        {% component "table" %}
+          {% fill "pagination" body=my_slot %}
+            ...
+          {% endfill %}
+        {% endcomponent %}
+        ```
     """
 
     tag = "fill"
@@ -967,6 +1008,7 @@ class FillNode(BaseNode):
         *,
         data: Optional[str] = None,
         fallback: Optional[str] = None,
+        body: Optional[SlotInput] = None,
         # TODO_V1: Use `fallback` kwarg instead of `default`
         default: Optional[str] = None,
     ) -> str:
@@ -1015,12 +1057,19 @@ class FillNode(BaseNode):
                 f" and slot data ({FILL_DATA_KWARG}=...)"
             )
 
+        if body is not None and self.contents:
+            raise TemplateSyntaxError(
+                f"Fill '{name}' received content both through '{FILL_BODY_KWARG}' kwarg and '{{% fill %}}' body. "
+                f"Use only one method."
+            )
+
         fill_data = FillWithData(
             fill=self,
             name=name,
             fallback_var=fallback,
             data_var=data,
             extra_context={},
+            body=body,
         )
 
         self._extract_fill(context, fill_data)
@@ -1036,10 +1085,13 @@ class FillNode(BaseNode):
         #       ...
         #     {% endfill %}
         #   {% endfor %}
-        collected_fills: List[FillWithData] = context.get(FILL_GEN_CONTEXT_KEY, None)
+        collected_fills: Optional[List[FillWithData]] = context.get(FILL_GEN_CONTEXT_KEY, None)
 
         if collected_fills is None:
-            return
+            raise RuntimeError(
+                "FillNode.render() (AKA {% fill ... %} block) cannot be rendered outside of a Component context. "
+                "Make sure that the {% fill %} tags are nested within {% component %} tags."
+            )
 
         # To allow using variables which were defined within the template and to which
         # the `{% fill %}` tag has access, we need to capture those variables too.
@@ -1107,6 +1159,17 @@ class FillWithData(NamedTuple):
     fill: FillNode
     name: str
     """Name of the slot to be filled, as set on the `{% fill %}` tag."""
+    body: Optional[SlotInput]
+    """
+    Slot fill as set by the `body` kwarg on the `{% fill %}` tag.
+
+    E.g.
+    ```django
+    {% component "mycomponent" %}
+        {% fill "footer" body=my_slot / %}
+    {% endcomponent %}
+    ```
+    """
     fallback_var: Optional[str]
     """Name of the FALLBACK variable, as set on the `{% fill %}` tag."""
     data_var: Optional[str]
@@ -1224,7 +1287,12 @@ def resolve_fills(
         # NOTE: If slot fills are explicitly defined, we use them even if they are empty (or only whitespace).
         #       This is different from the default slot, where we ignore empty content.
         for fill in maybe_fills:
-            slots[fill.name] = _nodelist_to_slot(
+            # Case: Slot fill was explicitly defined as `{% fill body=... / %}`
+            if fill.body is not None:
+                slot_fill = fill.body if isinstance(fill.body, Slot) else Slot(fill.body)
+            # Case: Slot fill was defined as the body of `{% fill / %}...{% endfill %}`
+            else:
+                slot_fill = _nodelist_to_slot(
                     component_name=component_name,
                     slot_name=fill.name,
                     nodelist=fill.fill.nodelist,
@@ -1235,6 +1303,7 @@ def resolve_fills(
                     # Escaped because this was defined in the template
                     escaped=True,
                 )
+            slots[fill.name] = slot_fill
 
     return slots
 

tests/test_slots.py

@@ -99,17 +99,13 @@ class TestSlot:
 
         with pytest.raises(
             TemplateSyntaxError,
-            match=re.escape(
+            match=re.escape("Slot 'first' is marked as 'required' (i.e. non-optional), yet no fill is provided."),
-                "Slot 'first' is marked as 'required' (i.e. non-optional), yet no fill is provided."
-            ),
         ):
             SimpleComponent.render()
 
         with pytest.raises(
             TemplateSyntaxError,
-            match=re.escape(
+            match=re.escape("Slot 'first' is marked as 'required' (i.e. non-optional), yet no fill is provided."),
-                "Slot 'first' is marked as 'required' (i.e. non-optional), yet no fill is provided."
-            ),
         ):
             SimpleComponent.render(
                 slots={"first": None},
@@ -408,3 +404,73 @@ class TestSlot:
         assert second_nodelist[0].s == "\n                FROM_INSIDE_NAMED_SLOT\n              "
 
         assert first_slot_func.contents == second_slot_func.contents
+
+    def test_pass_body_to_fill__slot(self):
+        @register("test")
+        class SimpleComponent(Component):
+            template: types.django_html = """
+                {% load component_tags %}
+                {% slot "first" default %}
+                {% endslot %}
+            """
+
+        template_str: types.django_html = """
+            {% load component_tags %}
+            {% component "test" %}
+              {% fill "first" body=my_slot / %}
+            {% endcomponent %}
+        """
+        template = Template(template_str)
+
+        my_slot: Slot = Slot(lambda ctx: "FROM_INSIDE_NAMED_SLOT")
+        rendered: str = template.render(Context({"my_slot": my_slot}))
+
+        assert rendered.strip() == "FROM_INSIDE_NAMED_SLOT"
+
+    def test_pass_body_to_fill__string(self):
+        @register("test")
+        class SimpleComponent(Component):
+            template: types.django_html = """
+                {% load component_tags %}
+                {% slot "first" default %}
+                {% endslot %}
+            """
+
+        template_str: types.django_html = """
+            {% load component_tags %}
+            {% component "test" %}
+              {% fill "first" body=my_slot / %}
+            {% endcomponent %}
+        """
+        template = Template(template_str)
+
+        rendered: str = template.render(Context({"my_slot": "FROM_INSIDE_NAMED_SLOT"}))
+
+        assert rendered.strip() == "FROM_INSIDE_NAMED_SLOT"
+
+    def test_pass_body_to_fill_raises_on_body(self):
+        @register("test")
+        class SimpleComponent(Component):
+            template: types.django_html = """
+                {% load component_tags %}
+                {% slot "first" default %}
+                {% endslot %}
+            """
+
+        template_str: types.django_html = """
+            {% load component_tags %}
+            {% component "test" %}
+              {% fill "first" body=my_slot %}
+                FROM_INSIDE_NAMED_SLOT
+              {% endfill %}
+            {% endcomponent %}
+        """
+        template = Template(template_str)
+
+        my_slot: Slot = Slot(lambda ctx: "FROM_INSIDE_NAMED_SLOT")
+
+        with pytest.raises(
+            TemplateSyntaxError,
+            match=re.escape("Fill 'first' received content both through 'body' kwarg and '{% fill %}' body."),
+        ):
+            template.render(Context({"my_slot": my_slot}))