bug: StrEnum doesn't support optional

[nh13]

The new test_str_enum_optional unit test fails when the default value is set to StrChoice.A but passes when the default value is set to "A". I was expecting the former to work, not the latter. CC: @geoffjentry

[anntzer]

diff --git i/src/defopt.py w/src/defopt.py
index cb902b7..37dc654 100644
--- i/src/defopt.py
+++ w/src/defopt.py
@@ -1178,6 +1178,10 @@ def _is_constructible_from_str(type_):
 def _make_enum_parser(enum, value=None):
     if value is None:
         return functools.partial(_make_enum_parser, enum)
+    if isinstance(value, enum):
+        # If the default is a StrEnum it will be passed to the converter, which
+        # must pass it through.
+        return value
     try:
         return enum[value]
     except KeyError:

looks like the fix? Can you make a PR?

[nh13]

I can probably simplify the test case to just use an enum and not a strenum, so 3.7 works.

[nh13]

@anntzer ready to go!

[anntzer]

I think the previously failing case occurred specifically when the enum class did also inherit from str, no? You can just directly create the double-inherited enum class yourself,

class StrChoice(str, Enum):
    A = "A"
    B = "B"

(or something along these lines).

[nh13]

Good idea, done

[anntzer]

Thanks for the PR.