Add tests, update changelog

Author: cpsievert

CHANGELOG.md

@@ -5,6 +5,16 @@ All notable changes to Shiny for Python will be documented in this file.
 The format is based on [Keep a Changelog](https://keepachangelog.com/en/1.0.0/),
 and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0.html).
 
+## [UNRELEASED]
+
+### New features
+
+* A `default` parameter is now available to reactive reads (e.g., `input.val(default=None)`). When provided, and the read results in a `MISSING` value, the `default` value is returned instead of raising a `SilentException` (#2100)
+
+### Improvements
+
+* A warning now occurs when a reactive read (e.g., `input.val()`) results in a `SilentException`. This most commonly occurs when attempting to read an input that doesn't exist in the UI. Since this behavior is sometimes desirable (mainly for dynamic UI reasons), the warning can be suppressed via `input.val(warn_if_missing=False)`. (#2100)
+
 ## [1.5.0] - 2025-09-11
 
 ### New features

shiny/reactive/_reactives.py

@@ -72,8 +72,8 @@ class Value(Generic[T]):
     value
         An optional initial value.
     read_only
-        If ``True``, then the reactive value cannot be `set()`. For internal use,
-        this value may also be a string (of the input ID).
+        If ``True``, then the reactive value cannot be `set()`. For internal purposes,
+        a string containing an input ID can also be provided.
 
     Returns
     -------

tests/pytest/test_reactives.py

@@ -101,14 +101,14 @@ async def test_reactive_value_unset():
     with isolate():
         assert v.is_set() is False
         with pytest.raises(SilentException):
-            v()
+            v(warn_if_missing=False)
 
     val: int = 0
 
     @effect()
     def o():
         nonlocal val
-        val = v()
+        val = v(warn_if_missing=False)
 
     await flush()
     assert o._exec_count == 1
@@ -128,7 +128,7 @@ def o():
     with isolate():
         assert v.is_set() is False
         with pytest.raises(SilentException):
-            v()
+            v(warn_if_missing=False)
 
 
 # ======================================================================
@@ -1021,7 +1021,7 @@ async def test_event_silent_exception():
     x = Value[bool]()
 
     @effect()
-    @event(x)
+    @event(lambda: x(warn_if_missing=False))
     def _():
         nonlocal n_times
         n_times += 1
@@ -1052,7 +1052,7 @@ async def test_event_silent_exception_async():
 
     async def req_fn() -> int:
         await asyncio.sleep(0)
-        x()
+        x(warn_if_missing=False)
         return 1234
 
     @effect()
@@ -1365,3 +1365,170 @@ async def obs():
     a.set(4)
     await flush()
     assert obs._exec_count == 2
+
+
+# ======================================================================
+# reactive.Value.get() with default parameter
+# ======================================================================
+@pytest.mark.asyncio
+async def test_reactive_value_get_with_default():
+    # Test with unset value - should return default
+    v = Value[int]()
+
+    with isolate():
+        assert v.get(default=42) == 42
+        assert v.get(default=0) == 0
+        assert v.get(default=None) is None
+
+    # Test after setting value - should return value, not default
+    v.set(100)
+    with isolate():
+        assert v.get(default=42) == 100
+        assert v.get(default=0) == 100
+
+    # Test after unsetting - should return default again
+    v.unset()
+    with isolate():
+        assert v.get(default=99) == 99
+
+
+@pytest.mark.asyncio
+async def test_reactive_value_call_with_default():
+    # Test that __call__ also supports default parameter
+    v = Value[str]()
+
+    with isolate():
+        assert v(default="hello") == "hello"
+        assert v(default=None) is None
+
+    v.set("world")
+    with isolate():
+        assert v(default="hello") == "world"
+
+    v.unset()
+    with isolate():
+        assert v(default="fallback") == "fallback"
+
+
+@pytest.mark.asyncio
+async def test_reactive_value_default_in_reactive_context():
+    # Test using default parameter within reactive contexts
+    v = Value[int]()
+    result = None
+
+    @effect()
+    def obs():
+        nonlocal result
+        result = v.get(default=10)
+
+    await flush()
+    assert result == 10
+    assert obs._exec_count == 1
+
+    # Setting the value should invalidate and return new value
+    v.set(50)
+    await flush()
+    assert result == 50
+    assert obs._exec_count == 2
+
+    # Unsetting should use default again
+    v.unset()
+    await flush()
+    assert result == 10
+    assert obs._exec_count == 3
+
+
+@pytest.mark.asyncio
+async def test_reactive_value_no_default_raises_silent_exception():
+    # Test that without default, unset value still raises SilentException
+    v = Value[int]()
+
+    with isolate():
+        with pytest.raises(SilentException):
+            v.get()
+        with pytest.raises(SilentException):
+            v()
+
+
+# ======================================================================
+# reactive.Value warning behavior
+# ======================================================================
+@pytest.mark.asyncio
+async def test_reactive_value_missing_warning():
+    # Test that reading unset value with warn_if_missing=True logs warning
+    v = Value[int]()
+
+    with isolate():
+        with pytest.warns(
+            ReactiveWarning, match="Attempted to read a `reactive.value`"
+        ):
+            try:
+                v.get()
+            except SilentException:
+                pass
+
+
+@pytest.mark.asyncio
+async def test_reactive_value_suppress_warning():
+    # Test that warn_if_missing=False suppresses the warning
+    v = Value[int]()
+
+    with isolate():
+        # Should not produce a warning
+        with pytest.raises(SilentException):
+            v.get(warn_if_missing=False)
+
+
+@pytest.mark.asyncio
+async def test_reactive_value_default_no_warning():
+    # Test that providing a default doesn't generate warning
+    v = Value[int]()
+
+    with isolate():
+        # Should not raise or warn when default is provided
+        result = v.get(default=42)
+        assert result == 42
+
+
+@pytest.mark.asyncio
+async def test_input_value_missing_warning():
+    # Test that reading unset input value shows input-specific warning
+    conn = MockConnection()
+    session = App(ui.TagList(), None)._create_session(conn)
+    input = session.input
+
+    # Access an input that doesn't exist yet
+    with isolate():
+        with pytest.warns(
+            ReactiveWarning, match=r"Attempted to read `input\.test_input\(\)`"
+        ):
+            try:
+                input.test_input()
+            except SilentException:
+                pass
+
+
+@pytest.mark.asyncio
+async def test_input_value_with_default():
+    # Test that input values can use default parameter
+    conn = MockConnection()
+    session = App(ui.TagList(), None)._create_session(conn)
+    input = session.input
+
+    with isolate():
+        # Should return default without warning or exception
+        result = input.test_input(default="fallback")
+        assert result == "fallback"
+
+
+@pytest.mark.asyncio
+async def test_input_value_suppress_warning():
+    # Test that warn_if_missing=False suppresses input warning
+    conn = MockConnection()
+    session = App(ui.TagList(), None)._create_session(conn)
+    input = session.input
+
+    with isolate():
+        # Should not produce a warning
+        with pytest.raises(SilentException):
+            input.test_input(warn_if_missing=False)