fix typing

CHANGELOG.md

@@ -3,3 +3,4 @@
 * `memoize` will release the per-function lock while calling target function
 * added `cache_memoize`
 * added `words_to_int`
+* added covariance to hashable for `satella.coding.typing.K`

docs/coding/typing.rst

@@ -8,6 +8,7 @@ They are as follows:
 * `ExceptionClassType` - base type of exception class
 * `Number` - an amalgam of int and float
 * `T`, `U`, `K`, `V` - type vars to use
+    * Note that `K` must be hashable
 * `Iteratable` - a generic iterator or an iterable of `T`
 * `NoArgCallable` - a callable with no arguments returning `T`
 * `Appendable` - an object which has the `append` method

satella/coding/typing.py

@@ -6,7 +6,7 @@ T = tp.TypeVar('T')
 Iteratable = tp.Union[tp.Iterator[T], tp.Iterable[T]]
 U = tp.TypeVar('U')
 V = tp.TypeVar('V')
-K = tp.TypeVar('K')
+K = tp.TypeVar('K', covariant=tp.Hashable)
 KVTuple = tp.Tuple[K, V]
 Number = tp.Union[int, float]
 NoArgCallable = tp.Callable[[], T]