Migrate streamlit hello to MPAv2 (streamlit#8806)

## Describe your changes

This PR converts `streamlit hello` to use Multipage Apps v2. We overall
kept the change very simple to mimic the same look and feel. We can
adjust it accordingly.

A minor change to improve types is here when we cannot find the context.
This should be impossible in a Streamlit app, but for bare execution and
to make the return type to ALWAYS be a Page, we return the default page
to run.

## Testing Plan

- Same E2E and unit tests should pass

---

**Contribution License Agreement**

By submitting this pull request you agree that all contributions to this
project are made under the Apache 2.0 license.

e2e_playwright/hello_app.py

@@ -14,18 +14,10 @@
 
 import numpy as np
 
-from streamlit import source_util
-from streamlit.hello import Hello
+from streamlit import runtime
+from streamlit.hello import streamlit_app
 
 # Set random seed to always get the same results in the plotting demo
 np.random.seed(0)
-
-# This is a trick to setup the MPA hello app programmatically
-source_util._cached_pages = None
-source_util._cached_pages = source_util.get_pages(Hello.__file__)
-source_util._on_pages_changed.send()
-
-# TODO(lukasmasuch): Once we migrate the hello app to the new programmatic
-# MPA API, we can remove this workaround.
-
-Hello.run()
+if runtime.exists():
+    streamlit_app.run()

lib/streamlit/commands/navigation.py

@@ -55,7 +55,7 @@ def navigation(
     pages: list[StreamlitPage] | dict[SectionHeader, list[StreamlitPage]],
     *,
     position: Literal["sidebar", "hidden"] = "sidebar",
-) -> StreamlitPage | None:
+) -> StreamlitPage:
     """
     Configure the available pages in a multipage app.
 
@@ -91,10 +91,6 @@ def navigation(
     >>>	st.title("My Awesome App")
     >>> pg.run()
     """
-    ctx = get_script_run_ctx()
-    if not ctx:
-        return None
-
     nav_sections = {"": pages} if isinstance(pages, list) else pages
     page_list = pages_from_nav_sections(nav_sections)
 
@@ -162,6 +158,13 @@ def navigation(
             p.section_header = section_header
             p.url_pathname = page.url_path
 
+    ctx = get_script_run_ctx()
+    if not ctx:
+        # This should never run in Streamlit, but we want to make sure that
+        # the function always returns a page
+        default_page._can_be_called = True
+        return default_page
+
     # Inform our page manager about the set of pages we have
     ctx.pages_manager.set_pages(pagehash_to_pageinfo)
     found_page = ctx.pages_manager.get_page_script(

lib/streamlit/hello/Hello.py

@@ -14,35 +14,29 @@
 
 import streamlit as st
 
+st.set_page_config(
+    page_title="Hello",
+    page_icon="👋",
+)
 
-def run():
-    st.set_page_config(
-        page_title="Hello",
-        page_icon="👋",
-    )
+st.write("# Welcome to Streamlit! 👋")
 
-    st.write("# Welcome to Streamlit! 👋")
+st.sidebar.success("Select a demo above.")
 
-    st.sidebar.success("Select a demo above.")
-
-    st.markdown(
-        """
-        Streamlit is an open-source app framework built specifically for
-        Machine Learning and Data Science projects.
-        **👈 Select a demo from the sidebar** to see some examples
-        of what Streamlit can do!
-        ### Want to learn more?
-        - Check out [streamlit.io](https://streamlit.io)
-        - Jump into our [documentation](https://docs.streamlit.io)
-        - Ask a question in our [community
-          forums](https://discuss.streamlit.io)
-        ### See more complex demos
-        - Use a neural net to [analyze the Udacity Self-driving Car Image
-          Dataset](https://github.com/streamlit/demo-self-driving)
-        - Explore a [New York City rideshare dataset](https://github.com/streamlit/demo-uber-nyc-pickups)
+st.markdown(
     """
-    )
-
-
-if __name__ == "__main__":
-    run()
+    Streamlit is an open-source app framework built specifically for
+    Machine Learning and Data Science projects.
+    **👈 Select a demo from the sidebar** to see some examples
+    of what Streamlit can do!
+    ### Want to learn more?
+    - Check out [streamlit.io](https://streamlit.io)
+    - Jump into our [documentation](https://docs.streamlit.io)
+    - Ask a question in our [community
+      forums](https://discuss.streamlit.io)
+    ### See more complex demos
+    - Use a neural net to [analyze the Udacity Self-driving Car Image
+      Dataset](https://github.com/streamlit/demo-self-driving)
+    - Explore a [New York City rideshare dataset](https://github.com/streamlit/demo-uber-nyc-pickups)
+"""
+)

lib/streamlit/hello/streamlit_app.py

@@ -0,0 +1,37 @@
+# Copyright (c) Streamlit Inc. (2018-2022) Snowflake Inc. (2022-2024)
+#
+# Licensed under the Apache License, Version 2.0 (the "License");
+# you may not use this file except in compliance with the License.
+# You may obtain a copy of the License at
+#
+#     http://www.apache.org/licenses/LICENSE-2.0
+#
+# Unless required by applicable law or agreed to in writing, software
+# distributed under the License is distributed on an "AS IS" BASIS,
+# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+# See the License for the specific language governing permissions and
+# limitations under the License.
+
+from pathlib import Path
+
+import streamlit as st
+
+dir_path = Path(__file__).parent
+
+
+def run():
+    page = st.navigation(
+        [
+            st.Page(dir_path / "Hello.py"),
+            st.Page(dir_path / "Animation_Demo.py"),
+            st.Page(dir_path / "Plotting_Demo.py"),
+            st.Page(dir_path / "Mapping_Demo.py"),
+            st.Page(dir_path / "Dataframe_Demo.py"),
+        ]
+    )
+
+    page.run()
+
+
+if __name__ == "__main__":
+    run()

lib/streamlit/web/cli.py

@@ -180,10 +180,10 @@ def main_docs():
 @configurator_options
 def main_hello(**kwargs):
     """Runs the Hello World script."""
-    from streamlit.hello import Hello
+    from streamlit.hello import streamlit_app
 
     bootstrap.load_config_options(flag_options=kwargs)
-    filename = Hello.__file__
+    filename = streamlit_app.__file__
     _main_run(filename, flag_options=kwargs)
 
 

lib/streamlit/web/server/server.py

@@ -396,9 +396,9 @@ def browser_is_connected(self) -> bool:
 
     @property
     def is_running_hello(self) -> bool:
-        from streamlit.hello import Hello
+        from streamlit.hello import streamlit_app
 
-        return self._main_script_path == Hello.__file__
+        return self._main_script_path == streamlit_app.__file__
 
     def stop(self) -> None:
         cli_util.print_to_cli("  Stopping...", fg="blue")

lib/tests/streamlit/web/cli_test.py

@@ -319,14 +319,14 @@ def test_docs_command(self):
 
     def test_hello_command(self):
         """Tests the hello command runs the hello script in streamlit"""
-        from streamlit.hello import Hello
+        from streamlit.hello import streamlit_app
 
         with patch("streamlit.web.cli._main_run") as mock_main_run:
             self.runner.invoke(cli, ["hello"])
 
             mock_main_run.assert_called_once()
             positional_args = mock_main_run.call_args[0]
-            self.assertEqual(positional_args[0], Hello.__file__)
+            self.assertEqual(positional_args[0], streamlit_app.__file__)
 
     @patch("streamlit.logger.get_logger")
     def test_hello_command_with_logs(self, get_logger):