return a Dependency instance from Blocks.load event listener (#4304)

* return a Dependency instance from Blocks.load event listener

* a test case for chaining then from load event

* update CHANGELOG

* add test for load.then with blocks re-used

* fixes

---------

Co-authored-by: Abubakar Abid <abubakar@huggingface.co>

CHANGELOG.md

@@ -2,7 +2,7 @@
 
 ## New Features:
 
-No changes to highlight.
+* Make `Blocks.load` behave like other event listeners (allows chaining `then` off of it) [@anentropic](https://github.com/anentropic/) in [PR 4304](https://github.com/gradio-app/gradio/pull/4304)
 
 ## Bug Fixes:
 

gradio/analytics.py

@@ -114,7 +114,10 @@ def launched_analytics(blocks: gradio.Blocks, data: dict[str, Any]) -> None:
 
     for x in blocks.dependencies:
         targets_telemetry = targets_telemetry + [
-            str(blocks.blocks[y]) for y in x["targets"]
+            # Sometimes the target can be the Blocks object itself, so we need to check if its in blocks.blocks
+            str(blocks.blocks[y])
+            for y in x["targets"]
+            if y in blocks.blocks
         ]
         inputs_telemetry = inputs_telemetry + [
             str(blocks.blocks[y]) for y in x["inputs"]

gradio/blocks.py

@@ -1482,7 +1482,9 @@ Received outputs:
                 name=name, src=src, hf_token=api_key, alias=alias, **kwargs
             )
         else:
-            return self_or_cls.set_event_trigger(
+            from gradio.events import Dependency
+
+            dep, dep_index = self_or_cls.set_event_trigger(
                 event_name="load",
                 fn=fn,
                 inputs=inputs,
@@ -1498,7 +1500,8 @@ Received outputs:
                 max_batch_size=max_batch_size,
                 every=every,
                 no_target=True,
-            )[0]
+            )
+            return Dependency(self_or_cls, dep, dep_index)
 
     def clear(self):
         """Resets the layout of the Blocks object."""

gradio/routes.py

@@ -411,7 +411,7 @@ class App(FastAPI):
             dependency = app.get_blocks().dependencies[fn_index_inferred]
             target = dependency["targets"][0] if len(dependency["targets"]) else None
             event_data = EventData(
-                app.get_blocks().blocks[target] if target else None,
+                app.get_blocks().blocks.get(target) if target else None,
                 body.event_data,
             )
             batch = dependency["batch"]

test/test_events.py

@@ -1,8 +1,12 @@
+import os
+
 import pytest
 from fastapi.testclient import TestClient
 
 import gradio as gr
 
+os.environ["GRADIO_ANALYTICS_ENABLED"] = "False"
+
 
 class TestEvent:
     def test_clear_event(self):
@@ -69,6 +73,47 @@ class TestEvent:
         assert not parent.config["dependencies"][2]["trigger_only_on_success"]
         assert parent.config["dependencies"][3]["trigger_only_on_success"]
 
+    def test_load_chaining(self):
+        calls = 0
+
+        def increment():
+            nonlocal calls
+            calls += 1
+            return str(calls)
+
+        with gr.Blocks() as demo:
+            out = gr.Textbox(label="Call counter")
+            demo.load(increment, inputs=None, outputs=out).then(
+                increment, inputs=None, outputs=out
+            )
+
+        assert demo.config["dependencies"][0]["trigger"] == "load"
+        assert demo.config["dependencies"][0]["trigger_after"] is None
+        assert demo.config["dependencies"][1]["trigger"] == "then"
+        assert demo.config["dependencies"][1]["trigger_after"] == 0
+
+    def test_load_chaining_reuse(self):
+        calls = 0
+
+        def increment():
+            nonlocal calls
+            calls += 1
+            return str(calls)
+
+        with gr.Blocks() as demo:
+            out = gr.Textbox(label="Call counter")
+            demo.load(increment, inputs=None, outputs=out).then(
+                increment, inputs=None, outputs=out
+            )
+
+        with gr.Blocks() as demo2:
+            demo.render()
+
+        assert demo2.config["dependencies"][0]["trigger"] == "load"
+        assert demo2.config["dependencies"][0]["trigger_after"] is None
+        assert demo2.config["dependencies"][1]["trigger"] == "then"
+        assert demo2.config["dependencies"][1]["trigger_after"] == 0
+
 
 class TestEventErrors:
     def test_event_defined_invalid_scope(self):