Add ":signatures: short" to autosummary directive

This formats functions and classes with (…) if they have
arguments and () if they don't have arguments. This makes
it easier to distinguish callables from attributes and
properties.

doc/usage/extensions/autosummary.rst

@@ -95,9 +95,11 @@ The :mod:`sphinx.ext.autosummary` extension does this in two parts:
 
       - ``long`` (*default*): use a long signature. This is still cut off so that name
         plus signature do not exceeed a certain length.
+      - ``short``: Function and class signatures are displayed as ``(…)`` if they have
+        arguments and as ``()`` if they don't have arguments.
       - ``none``: do not show signatures.
 
-      .. versionadded:: 9.0
+      .. versionadded:: 8.2
 
    .. rst:directive:option:: nosignatures
 

sphinx/ext/autosummary/__init__.py

@@ -320,11 +320,11 @@ def get_items(self, names: list[str]) -> list[tuple[str, str | None, str, str]]:
 
         signatures_option = self.options.get('signatures')
         if signatures_option is None:
-            signatures_option = 'none' if self.options.get('nosignatures') else 'long'
-        if signatures_option not in ['none', 'long']:
-            raise ValueError(f"Invalid value for autosummary :signatures: option: "
-                             f"{signatures_option!r}. "
-                             f"Valid values are 'none', 'long'")
+            signatures_option = 'none' if 'nosignatures' in self.options else 'long'
+        if signatures_option not in {'none', 'short', 'long'}:
+            msg = ("Invalid value for autosummary :signatures: option: "
+                   f"{signatures_option!r}. Valid values are 'none', 'short', 'long'")
+            raise ValueError(msg)
 
         max_item_chars = 50
 
@@ -384,10 +384,12 @@ def get_items(self, names: list[str]) -> list[tuple[str, str | None, str, str]]:
                 except TypeError:
                     # the documenter does not support ``show_annotation`` option
                     sig = documenter.format_signature()
-
                 if not sig:
                     sig = ''
-                else:
+                elif signatures_option == 'short':
+                    if sig != '()':
+                        sig = '(…)'
+                else:  # signatures_option == 'long'
                     max_chars = max(10, max_item_chars - len(display_name))
                     sig = mangle_signature(sig, max_chars=max_chars)