Extract function for initializing the master working set. Fixes #373.

pkg_resources/__init__.py

@@ -3053,20 +3053,34 @@ def _initialize(g):
             g[name] = getattr(_manager, name)
 _initialize(globals())
 
-# Prepare the master working set and make the ``require()`` API available
-working_set = WorkingSet._build_master()
-_declare_state('object', working_set=working_set)
-
-require = working_set.require
-iter_entry_points = working_set.iter_entry_points
-add_activation_listener = working_set.subscribe
-run_script = working_set.run_script
-# backward compatibility
-run_main = run_script
-# Activate all distributions already on sys.path, and ensure that
-# all distributions added to the working set in the future (e.g. by
-# calling ``require()``) will get activated as well.
-add_activation_listener(lambda dist: dist.activate())
-working_set.entries=[]
-# match order
-list(map(working_set.add_entry, sys.path))
+
+def _initialize_master_working_set():
+    """
+    Prepare the master working set and make the ``require()``
+    API available.
+
+    This function has explicit effects on the global state
+    of pkg_resources. It is intended to be invoked once at
+    the initialization of this module.
+
+    Invocation by other packages is unsupported and done
+    at their own risk.
+    """
+    working_set = WorkingSet._build_master()
+    _declare_state('object', working_set=working_set)
+
+    require = working_set.require
+    iter_entry_points = working_set.iter_entry_points
+    add_activation_listener = working_set.subscribe
+    run_script = working_set.run_script
+    # backward compatibility
+    run_main = run_script
+    # Activate all distributions already on sys.path, and ensure that
+    # all distributions added to the working set in the future (e.g. by
+    # calling ``require()``) will get activated as well.
+    add_activation_listener(lambda dist: dist.activate())
+    working_set.entries=[]
+    # match order
+    list(map(working_set.add_entry, sys.path))
+    globals().update(locals())
+_initialize_master_working_set()