re-work backend parameters again to work on 3.9 and 3.11

src/chap/core.py

@@ -11,7 +11,17 @@ import pathlib
 import pkgutil
 import subprocess
 from dataclasses import MISSING, Field, dataclass, fields
-from typing import Any, AsyncGenerator, Callable, Optional, Union, cast
+from typing import (
+    Any,
+    AsyncGenerator,
+    Callable,
+    Optional,
+    Union,
+    cast,
+    get_origin,
+    get_args,
+)
+import sys
 
 import click
 import platformdirs
@@ -21,8 +31,11 @@ from typing_extensions import Protocol
 from . import backends, commands
 from .session import Message, Session, System, session_from_file
 
-# 3.9 compatible version of `from types import UnionType`
-UnionType = type(Union[int, list])
+UnionType: type
+if sys.version_info >= (3, 10):
+    from types import UnionType
+else:
+    UnionType = type(Union[int, float])
 
 conversations_path = platformdirs.user_state_path("chap") / "conversations"
 conversations_path.mkdir(parents=True, exist_ok=True)
@@ -69,10 +82,22 @@ def new_session_path(opt_path: Optional[pathlib.Path] = None) -> pathlib.Path:
     )
 
 
-def convert_str_to_field(ctx: click.Context, field: Field[Any], value: str) -> Any:
+def get_field_type(field: Field[Any]) -> Any:
     field_type = field.type
-    if isinstance(field_type, UnionType):
-        field_type = field_type.__args__[0]
+    if isinstance(field_type, str):
+        raise RuntimeError(
+            "parameters dataclass may not use 'from __future__ import annotations"
+        )
+    origin = get_origin(field_type)
+    if origin in (Union, UnionType):
+        for arg in get_args(field_type):
+            if arg is not None:
+                return arg
+    return field_type
+
+
+def convert_str_to_field(ctx: click.Context, field: Field[Any], value: str) -> Any:
+    field_type = get_field_type(field)
     try:
         if field_type is bool:
             tv = click.types.BoolParamType().convert(value, None, ctx)
@@ -181,9 +206,7 @@ def format_backend_help(api: Backend, formatter: click.HelpFormatter) -> None:
             if doc:
                 doc += " "
             doc += f"(Default: {default!r})"
-            f_type = f.type
-            if isinstance(f_type, UnionType):
-                f_type = f_type.__args__[0]
+            f_type = get_field_type(f)
             typename = f_type.__name__
             rows.append((f"-B {name}:{typename.upper()}", doc))
         formatter.write_dl(rows)