Rewrite cache decorator to reference args by name

`@cache.delete('user', 'user_id')` is easier to read and understand than
`@cache.delete('user', key_from_args=[1])`. This will become even more
apparent if we have to start doing stuff like `key_from_args=[1, 5]`,
which is a lot more opaque than just saying
`'service_id', 'template_id'`.

It does make the implementation a bit more complex, but I’m not too
worried about that because:
- the tests are solid
- it’s nicely encapsulated

app/notify_client/cache.py

@@ -1,27 +1,51 @@
 import json
+from contextlib import suppress
 from datetime import timedelta
 from functools import wraps
+from inspect import signature
 
 TTL = int(timedelta(hours=24).total_seconds())
 
 
-def _make_key(prefix, args, key_from_args):
+def _get_argument(argument_name, args, kwargs, client_method):
 
-    if key_from_args is None:
-        key_from_args = [0]
+    with suppress(KeyError):
+        return kwargs[argument_name]
 
+    with suppress(ValueError):
+        return args[list(signature(client_method).parameters).index(argument_name) - 1]
+
+    raise TypeError("{}() takes no argument called '{}'".format(
+        client_method.__name__, argument_name
+    ))
+
+
+def list_of_strings(list_of_stuff):
+    return list(map(str, filter(None, list_of_stuff)))
+
+
+def _make_key(prefix, key_from_args, local_variables):
     return '-'.join(
-        [prefix] + [args[index] for index in key_from_args]
+        [
+            local_variables['prefix']
+        ] + list_of_strings(
+            _get_argument(
+                argument_name,
+                local_variables['args'],
+                local_variables['kwargs'],
+                local_variables['client_method']
+            ) for argument_name in key_from_args
+        )
     )
 
 
-def set(prefix, key_from_args=None):
+def set(prefix, *key_from_args):
 
     def _set(client_method):
 
         @wraps(client_method)
         def new_client_method(client_instance, *args, **kwargs):
-            redis_key = _make_key(prefix, args, key_from_args)
+            redis_key = _make_key(prefix, key_from_args, locals())
             cached = client_instance.redis_client.get(redis_key)
             if cached:
                 return json.loads(cached.decode('utf-8'))
@@ -37,13 +61,13 @@ def set(prefix, key_from_args=None):
     return _set
 
 
-def delete(prefix, key_from_args=None):
+def delete(prefix, *key_from_args):
 
     def _delete(client_method):
 
         @wraps(client_method)
         def new_client_method(client_instance, *args, **kwargs):
-            redis_key = _make_key(prefix, args, key_from_args)
+            redis_key = _make_key(prefix, key_from_args, locals())
             client_instance.redis_client.delete(redis_key)
             return client_method(client_instance, *args, **kwargs)