bpo-31581: Reduce the number of imports for functools (GH-3757)

Lib/functools.py

@@ -19,8 +19,7 @@ except ImportError:
     pass
 from abc import get_cache_token
 from collections import namedtuple
-from types import MappingProxyType
-from weakref import WeakKeyDictionary
+# import types, weakref  # Deferred to single_dispatch()
 from reprlib import recursive_repr
 from _thread import RLock
 
@@ -753,10 +752,14 @@ def singledispatch(func):
     function acts as the default implementation, and additional
     implementations can be registered using the register() attribute of the
     generic function.
-
     """
+    # There are many programs that use functools without singledispatch, so we
+    # trade-off making singledispatch marginally slower for the benefit of
+    # making start-up of such applications slightly faster.
+    import types, weakref
+
     registry = {}
-    dispatch_cache = WeakKeyDictionary()
+    dispatch_cache = weakref.WeakKeyDictionary()
     cache_token = None
 
     def dispatch(cls):
@@ -803,7 +806,7 @@ def singledispatch(func):
     registry[object] = func
     wrapper.register = register
     wrapper.dispatch = dispatch
-    wrapper.registry = MappingProxyType(registry)
+    wrapper.registry = types.MappingProxyType(registry)
     wrapper._clear_cache = dispatch_cache.clear
     update_wrapper(wrapper, func)
     return wrapper

Lib/test/test_functools.py

@@ -2019,6 +2019,8 @@ class TestSingleDispatch(unittest.TestCase):
 
     def test_cache_invalidation(self):
         from collections import UserDict
+        import weakref
+
         class TracingDict(UserDict):
             def __init__(self, *args, **kwargs):
                 super(TracingDict, self).__init__(*args, **kwargs)
@@ -2033,9 +2035,9 @@ class TestSingleDispatch(unittest.TestCase):
                 self.data[key] = value
             def clear(self):
                 self.data.clear()
-        _orig_wkd = functools.WeakKeyDictionary
+
         td = TracingDict()
-        functools.WeakKeyDictionary = lambda: td
+        with support.swap_attr(weakref, "WeakKeyDictionary", lambda: td):
             c = collections.abc
             @functools.singledispatch
             def g(arg):
@@ -2116,7 +2118,6 @@ class TestSingleDispatch(unittest.TestCase):
             self.assertEqual(g(l), "list")
             g._clear_cache()
             self.assertEqual(len(td), 0)
-        functools.WeakKeyDictionary = _orig_wkd
 
 
 if __name__ == '__main__':