Speedup _setattr usage and fix slight performance regressions (#991)

* Speedup `_setattr` usage and fix performance regressions

* Add changelog file

Co-authored-by: Hynek Schlawack <hs@ox.cx>

changelog.d/991.change.rst

@@ -0,0 +1 @@
+Fix slight performance regression in classes with custom ``__setattr__`` and speedup even more.

docs/how-does-it-work.rst

@@ -94,9 +94,9 @@ This is (still) slower than a plain assignment:
         -s "import attr; C = attr.make_class('C', ['x', 'y', 'z'], slots=True, frozen=True)" \
         "C(1, 2, 3)"
   .........................................
-  Mean +- std dev: 450 ns +- 26 ns
+  Mean +- std dev: 425 ns +- 16 ns
 
-So on a laptop computer the difference is about 230 nanoseconds (1 second is 1,000,000,000 nanoseconds).
+So on a laptop computer the difference is about 200 nanoseconds (1 second is 1,000,000,000 nanoseconds).
 It's certainly something you'll feel in a hot loop but shouldn't matter in normal code.
 Pick what's more important to you.
 

src/attr/_make.py

@@ -915,7 +915,7 @@ def slots_setstate(self, state):
             """
             Automatically created by attrs.
             """
-            __bound_setattr = _obj_setattr.__get__(self, Attribute)
+            __bound_setattr = _obj_setattr.__get__(self)
             for name, value in zip(state_attr_names, state):
                 __bound_setattr(name, value)
 
@@ -2007,6 +2007,7 @@ def _make_init(
         cache_hash,
         base_attr_map,
         is_exc,
+        needs_cached_setattr,
         has_cls_on_setattr,
         attrs_init,
     )
@@ -2019,7 +2020,7 @@ def _make_init(
     if needs_cached_setattr:
         # Save the lookup overhead in __init__ if we need to circumvent
         # setattr hooks.
-        globs["_setattr"] = _obj_setattr
+        globs["_cached_setattr_get"] = _obj_setattr.__get__
 
     init = _make_method(
         "__attrs_init__" if attrs_init else "__init__",
@@ -2036,15 +2037,15 @@ def _setattr(attr_name, value_var, has_on_setattr):
     """
     Use the cached object.setattr to set *attr_name* to *value_var*.
     """
-    return "_setattr(self, '%s', %s)" % (attr_name, value_var)
+    return "_setattr('%s', %s)" % (attr_name, value_var)
 
 
 def _setattr_with_converter(attr_name, value_var, has_on_setattr):
     """
     Use the cached object.setattr to set *attr_name* to *value_var*, but run
     its converter first.
     """
-    return "_setattr(self, '%s', %s(%s))" % (
+    return "_setattr('%s', %s(%s))" % (
         attr_name,
         _init_converter_pat % (attr_name,),
         value_var,
@@ -2086,6 +2087,7 @@ def _attrs_to_init_script(
     cache_hash,
     base_attr_map,
     is_exc,
+    needs_cached_setattr,
     has_cls_on_setattr,
     attrs_init,
 ):
@@ -2101,6 +2103,14 @@ def _attrs_to_init_script(
     if pre_init:
         lines.append("self.__attrs_pre_init__()")
 
+    if needs_cached_setattr:
+        lines.append(
+            # Circumvent the __setattr__ descriptor to save one lookup per
+            # assignment.
+            # Note _setattr will be used again below if cache_hash is True
+            "_setattr = _cached_setattr_get(self)"
+        )
+
     if frozen is True:
         if slots is True:
             fmt_setter = _setattr
@@ -2315,7 +2325,7 @@ def fmt_setter_with_converter(
         if frozen:
             if slots:
                 # if frozen and slots, then _setattr defined above
-                init_hash_cache = "_setattr(self, '%s', %s)"
+                init_hash_cache = "_setattr('%s', %s)"
             else:
                 # if frozen and not slots, then _inst_dict defined above
                 init_hash_cache = "_inst_dict['%s'] = %s"
@@ -2428,7 +2438,7 @@ def __init__(
         )
 
         # Cache this descriptor here to speed things up later.
-        bound_setattr = _obj_setattr.__get__(self, Attribute)
+        bound_setattr = _obj_setattr.__get__(self)
 
         # Despite the big red warning, people *do* instantiate `Attribute`
         # themselves.
@@ -2525,7 +2535,7 @@ def __setstate__(self, state):
         self._setattrs(zip(self.__slots__, state))
 
     def _setattrs(self, name_values_pairs):
-        bound_setattr = _obj_setattr.__get__(self, Attribute)
+        bound_setattr = _obj_setattr.__get__(self)
         for name, value in name_values_pairs:
             if name != "metadata":
                 bound_setattr(name, value)

tests/test_functional.py

@@ -745,6 +745,7 @@ class D(C):
 
         src = inspect.getsource(D.__init__)
 
-        assert "_setattr(self, 'x', x)" in src
-        assert "_setattr(self, 'y', y)" in src
+        assert "_setattr = _cached_setattr_get(self)" in src
+        assert "_setattr('x', x)" in src
+        assert "_setattr('y', y)" in src
         assert object.__setattr__ != D.__setattr__