Fix widget id stability

lib/streamlit/elements/slider.py

@@ -376,19 +376,6 @@ def _slider(
 
         maybe_raise_label_warnings(label, label_visibility)
 
-        if value is None:
-            # Set value from session_state if exists.
-            session_state = get_session_state().filtered_state
-
-            # we look first to session_state value of the widget because
-            # depending on the value (single value or list/tuple) the slider should be
-            # initializing differently (either as range or single value slider)
-            if key is not None and key in session_state:
-                value = session_state[key]
-            else:
-                # Set value default.
-                value = min_value if min_value is not None else 0
-
         SUPPORTED_TYPES = {
             Integral: SliderProto.INT,
             Real: SliderProto.FLOAT,
@@ -398,6 +385,33 @@ def _slider(
         }
         TIMELIKE_TYPES = (SliderProto.DATETIME, SliderProto.TIME, SliderProto.DATE)
 
+        if value is None:
+            # We need to know if this is a single or range slider, but don't have
+            # a default value. We will try looking at the value in session state,
+            # but we must be careful, because we don't want the slider's identity
+            # to be derived from the value, or it will change every time the value
+            # changes, which would be a problem if we stop garbage collecting
+            # widgets that haven't been used recently.
+
+            single_value = True
+
+            session_state = get_session_state().filtered_state
+
+            if key is not None and key in session_state:
+                state_value = session_state[key]
+                single_value = isinstance(state_value, tuple(SUPPORTED_TYPES.keys()))
+
+            # Set value default. We assume single vs range doesn't change in the
+            # lifetime of a slider, so it is safe to depend on, but we use the
+            # min/max values and constants for the actual default, to prevent
+            # depending on the value.
+            if single_value:
+                value = min_value if min_value is not None else 0
+            else:
+                mn = min_value if min_value is not None else 0
+                mx = max_value if max_value is not None else 100
+                value = [mn, mx]
+
         # Ensure that the value is either a single value or a range of values.
         single_value = isinstance(value, tuple(SUPPORTED_TYPES.keys()))
         range_value = isinstance(value, (list, tuple)) and len(value) in (0, 1, 2)

lib/streamlit/runtime/state/widgets.py

@@ -148,6 +148,26 @@
         For both paths a widget return value is provided, allowing the widgets
         to be used in a non-streamlit setting.
     """
+    # Some fields are common to most widgets and not part of the widget identity,
+    # so they should not be set before registration and we can check them here.
+    # They don't exist on every widget, so we ignore fields that don't exist.
+    try:
+        assert not element_proto.disabled
+    except AttributeError:
+        pass
+    try:
+        assert not element_proto.label_visibility.value
+    except AttributeError:
+        pass
+    try:
+        assert not element_proto.value
+    except AttributeError:
+        pass
+    try:
+        assert not element_proto.set_value
+    except AttributeError:
+        pass
+
     widget_id = compute_widget_id(element_type, element_proto, user_key)
     element_proto.id = widget_id
 

lib/tests/streamlit/elements/slider_test.py

@@ -25,6 +25,7 @@
 from streamlit.errors import StreamlitAPIException
 from streamlit.js_number import JSNumber
 from streamlit.proto.LabelVisibilityMessage_pb2 import LabelVisibilityMessage
+from streamlit.testing.script_interactions import InteractiveScriptTests
 from tests.delta_generator_test_case import DeltaGeneratorTestCase
 
 
@@ -300,3 +301,20 @@ def test_label_visibility_wrong_value(self):
             "Unsupported label_visibility option 'wrong_value'. Valid values are "
             "'visible', 'hidden' or 'collapsed'.",
         )
+
+
+class SliderInteractiveTest(InteractiveScriptTests):
+    def test_id_stability(self):
+        script = self.script_from_string(
+            """
+        import streamlit as st
+
+        st.slider("slider", key="slider")
+        """
+        )
+        sr = script.run()
+        s1 = sr.slider[0]
+        sr2 = s1.set_value(5).run()
+        s2 = sr2.slider[0]
+
+        assert s1.id == s2.id