Page switching in AppTest (#8280)

## Describe your changes
Adds page switching to AppTest

## GitHub Issue Link (if applicable)
Enables a fix for #8154

## Testing Plan

App tests added

lib/streamlit/testing/v1/app_test.py

@@ -15,10 +15,10 @@
 
 import hashlib
 import inspect
-import pathlib
 import tempfile
 import textwrap
 import traceback
+from pathlib import Path
 from typing import Any, Callable, Sequence
 from unittest.mock import MagicMock
 from urllib import parse
@@ -84,7 +84,7 @@
 )
 from streamlit.testing.v1.local_script_runner import LocalScriptRunner
 from streamlit.testing.v1.util import patch_config_options
-from streamlit.util import HASHLIB_KWARGS
+from streamlit.util import HASHLIB_KWARGS, calc_md5
 
 TMP_DIR = tempfile.TemporaryDirectory()
 
@@ -162,6 +162,7 @@ def __init__(
         self.secrets: dict[str, Any] = {}
         self.args = args
         self.kwargs = kwargs
+        self._page_hash = ""
 
         tree = ElementTree()
         tree._runner = self
@@ -203,7 +204,7 @@ def _from_string(
         hasher = hashlib.md5(bytes(script, "utf-8"), **HASHLIB_KWARGS)
         script_name = hasher.hexdigest()
 
-        path = pathlib.Path(TMP_DIR.name, script_name)
+        path = Path(TMP_DIR.name, script_name)
         aligned_script = textwrap.dedent(script)
         path.write_text(aligned_script)
         return AppTest(
@@ -284,14 +285,14 @@ def from_file(cls, script_path: str, *, default_timeout: float = 3) -> AppTest:
             executed via ``.run()``.
 
         """
-        if pathlib.Path.is_file(pathlib.Path(script_path)):
+        if Path.is_file(Path(script_path)):
             path = script_path
         else:
             # TODO: Make this not super fragile
             # Attempt to find the test file calling this method, so the
             # path can be relative to there.
             stack = traceback.StackSummary.extract(traceback.walk_stack(None))
-            filepath = pathlib.Path(stack[1].filename)
+            filepath = Path(stack[1].filename)
             path = str(filepath.parent / script_path)
         return AppTest(path, default_timeout=default_timeout)
 
@@ -334,7 +335,9 @@ def _run(
             self._script_path, self.session_state, args=self.args, kwargs=self.kwargs
         )
         with patch_config_options({"global.appTest": True}):
-            self._tree = script_runner.run(widget_state, self.query_params, timeout)
+            self._tree = script_runner.run(
+                widget_state, self.query_params, timeout, self._page_hash
+            )
             self._tree._runner = self
         # Last event is SHUTDOWN, so the corresponding data includes query string
         query_string = script_runner.event_data[-1]["client_state"].query_string
@@ -373,6 +376,30 @@ def run(self, *, timeout: float | None = None) -> AppTest:
         """
         return self._tree.run(timeout=timeout)
 
+    def switch_page(self, page_path: str) -> AppTest:
+        """Switch to another page of the app.
+
+        Parameters
+        ----------
+        page_path: str
+            Path of the page to switch to. The path must be relative to the
+            location of the main script (e.g. ``"pages/my_page.py"``).
+
+        Returns
+        -------
+        AppTest
+            self
+        """
+        main_dir = Path(self._script_path).parent
+        full_page_path = main_dir / page_path
+        if not full_page_path.is_file():
+            raise ValueError(
+                f"Unable to find script at {page_path}, make sure the page given is relative to the main script."
+            )
+        page_path_str = str(full_page_path.resolve())
+        self._page_hash = calc_md5(page_path_str)
+        return self
+
     @property
     def main(self) -> Block:
         """Sequence of elements within the main body of the app.

lib/streamlit/testing/v1/local_script_runner.py

@@ -98,6 +98,7 @@ def run(
         widget_state: WidgetStates | None = None,
         query_params=None,
         timeout: float = 3,
+        page_hash: str = "",
     ) -> ElementTree:
         """Run the script, and parse the output messages for querying
         and interaction.
@@ -109,7 +110,11 @@ def run(
         if query_params:
             query_string = parse.urlencode(query_params, doseq=True)
 
-        rerun_data = RerunData(widget_states=widget_state, query_string=query_string)
+        rerun_data = RerunData(
+            widget_states=widget_state,
+            query_string=query_string,
+            page_script_hash=page_hash,
+        )
         self.request_rerun(rerun_data)
         if not self._script_thread:
             self.start()

lib/tests/streamlit/testing/app_test_test.py

@@ -234,3 +234,27 @@ def code():
     at = AppTest.from_function(code).run()
     # The script run should finish instead of recurring and timing out
     at.button[0].click().run()
+
+
+def test_switch_page():
+    at = AppTest.from_file("test_data/main.py").run()
+    assert at.text[0].value == "main page"
+
+    at.switch_page("pages/page1.py").run()
+    assert at.text[0].value == "page 1"
+
+    with pytest.raises(ValueError) as e:
+        # Pages must be relative to main script path
+        at.switch_page("test_data/pages/page1.py")
+        assert "relative to the main script path" in str(e.value)
+
+
+def test_switch_page_widgets():
+    at = AppTest.from_file("test_data/main.py").run()
+    at.slider[0].set_value(5).run()
+    assert at.slider[0].value == 5
+
+    at.switch_page("pages/page1.py").run()
+    assert not at.slider
+    at.switch_page("main.py").run()
+    assert at.slider[0].value == 0

lib/tests/streamlit/testing/test_data/main.py

@@ -0,0 +1,19 @@
+# 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.
+
+import streamlit as st
+
+st.text("main page")
+
+st.slider("slider", min_value=0, max_value=10)

lib/tests/streamlit/testing/test_data/pages/page1.py

@@ -0,0 +1,17 @@
+# 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.
+
+import streamlit as st
+
+st.text("page 1")