Merge pull request #709 from rouge8/field-number-kind

`Field.number(kind=...)` supports any function that returns a number

.coveragerc

@@ -15,3 +15,4 @@ source=
 exclude_lines =
     pragma: no cover
     if TYPE_CHECKING:
+    \s*\.\.\.$

src/klein/_form.py

@@ -12,8 +12,9 @@
     NoReturn,
     Optional,
     Sequence,
-    Type,
+    TypeVar,
     cast,
+    overload,
 )
 
 import attr
@@ -29,6 +30,7 @@
 
 from ._app import KleinRenderable, _call
 from ._decorators import bindable
+from ._typing_compat import Protocol
 from .interfaces import (
     EarlyExit,
     IDependencyInjector,
@@ -41,6 +43,23 @@
 )
 
 
+_Self = TypeVar("_Self")
+
+
+class _Numeric(Protocol):
+    def __float__(self) -> float:
+        ...
+
+    def __lt__(self: _Self, other: _Self) -> bool:
+        ...
+
+    def __gt__(self: _Self, other: _Self) -> bool:
+        ...
+
+
+_N = TypeVar("_N", bound=_Numeric)
+
+
 class CrossSiteRequestForgery(Resource):
     """
     Cross site request forgery detected.  Request aborted.
@@ -255,12 +274,46 @@ def hidden(cls, name: str, value: str, **kw: Any) -> "Field":
             **kw,
         ).maybeNamed(name)
 
+    @overload
+    @classmethod
+    def number(
+        cls,
+        minimum: Optional[float] = None,
+        maximum: Optional[float] = None,
+        kind: Callable[[str], float] = float,
+        **kw: Any,
+    ) -> "Field":
+        ...
+
+    @overload
+    @classmethod
+    def number(
+        cls,
+        minimum: Optional[_N] = None,
+        maximum: Optional[_N] = None,
+        *,
+        kind: Callable[[str], _N],
+        **kw: Any,
+    ) -> "Field":
+        ...
+
+    @overload
+    @classmethod
+    def number(
+        cls,
+        minimum: _N,
+        maximum: _N,
+        kind: Callable[[str], _N],
+        **kw: Any,
+    ) -> "Field":
+        ...
+
     @classmethod
     def number(
         cls,
-        minimum: Optional[int] = None,
-        maximum: Optional[int] = None,
-        kind: Type = float,
+        minimum: Optional[_N] = None,
+        maximum: Optional[_N] = None,
+        kind: Callable[[str], _N] = float,  # type:ignore[assignment]
         **kw: Any,
     ) -> "Field":
         """