Add a default value parameter to get_value()

[mikeprosserni]

What does this Pull Request accomplish?

Users want to be able to specify a default value for get_value() to return if the value has not been set yet.

We can implement this by simply catching the NOT_FOUND exception and returning the default, if there is one.

Why should this Pull Request be merged?

Implements AB#3161227

What testing has been done?

Manual testing:

with col3:
    st.markdown("**Default Values**")
    st.write(panel.get_value("missing_string", "0"))
    st.write(panel.get_value("missing_int", 0))
    st.write(panel.get_value("missing_float", 0.0))
    st.write(panel.get_value("missing_bool", False))
    st.write(panel.get_value("missing_list", [0.0]))

Also, new autotests:
test___set_value___get_value_ignores_default
test___get_value_returns_default_when_value_not_set
test___panel___get_unset_value_with_no_default___raises_exception

[github-actions]

Test Results

 10 files  ± 0   10 suites  ±0   15s ⏱️ -7s
 80 tests + 2   80 ✅ + 2  0 💤 ±0  0 ❌ ±0 
770 runs  +20  770 ✅ +20  0 💤 ±0  0 ❌ ±0 

Results for commit 8b42206. ± Comparison against base commit 96498f7.

This pull request removes 1 and adds 3 tests. Note that renamed tests count towards both.

tests.unit.test_streamlit_panel ‑ test___panel___get_unset_value___raises_exception

tests.unit.test_streamlit_panel ‑ test___get_value_returns_default_when_value_not_set
tests.unit.test_streamlit_panel ‑ test___panel___get_unset_value_with_no_default___raises_exception
tests.unit.test_streamlit_panel ‑ test___set_value___get_value_ignores_default

♻️ This comment has been updated with latest results.

[bkeryan]

Do we want the default value to affect type inference?

_T = TypeVar("_T")

@overload
def get_value(self, value_id: str) -> object: ...

@overload
def get_value(self, value_id: str, default_value: _T | None = None) -> _T: ...

[bkeryan]

Also see how the stdlib does it: https://github.com/python/typeshed/blob/main/stdlib/typing.pyi#L745