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