Restore dummy_threading and _dummy_thread, but deprecate them (bpo-31370) (#3648)

1 files changed, 78 insertions, 0 deletions

diff --git a/Lib/dummy_threading.py b/Lib/dummy_threading.py
new file mode 100644
index 0000000..1bb7eee
--- /dev/null
+++ b/Lib/dummy_threading.py
@@ -0,0 +1,78 @@
+"""Faux ``threading`` version using ``dummy_thread`` instead of ``thread``.
+
+The module ``_dummy_threading`` is added to ``sys.modules`` in order
+to not have ``threading`` considered imported.  Had ``threading`` been
+directly imported it would have made all subsequent imports succeed
+regardless of whether ``_thread`` was available which is not desired.
+
+"""
+from sys import modules as sys_modules
+
+import _dummy_thread
+
+# Declaring now so as to not have to nest ``try``s to get proper clean-up.
+holding_thread = False
+holding_threading = False
+holding__threading_local = False
+
+try:
+    # Could have checked if ``_thread`` was not in sys.modules and gone
+    # a different route, but decided to mirror technique used with
+    # ``threading`` below.
+    if '_thread' in sys_modules:
+        held_thread = sys_modules['_thread']
+        holding_thread = True
+    # Must have some module named ``_thread`` that implements its API
+    # in order to initially import ``threading``.
+    sys_modules['_thread'] = sys_modules['_dummy_thread']
+
+    if 'threading' in sys_modules:
+        # If ``threading`` is already imported, might as well prevent
+        # trying to import it more than needed by saving it if it is
+        # already imported before deleting it.
+        held_threading = sys_modules['threading']
+        holding_threading = True
+        del sys_modules['threading']
+
+    if '_threading_local' in sys_modules:
+        # If ``_threading_local`` is already imported, might as well prevent
+        # trying to import it more than needed by saving it if it is
+        # already imported before deleting it.
+        held__threading_local = sys_modules['_threading_local']
+        holding__threading_local = True
+        del sys_modules['_threading_local']
+
+    import threading
+    # Need a copy of the code kept somewhere...
+    sys_modules['_dummy_threading'] = sys_modules['threading']
+    del sys_modules['threading']
+    sys_modules['_dummy__threading_local'] = sys_modules['_threading_local']
+    del sys_modules['_threading_local']
+    from _dummy_threading import *
+    from _dummy_threading import __all__
+
+finally:
+    # Put back ``threading`` if we overwrote earlier
+
+    if holding_threading:
+        sys_modules['threading'] = held_threading
+        del held_threading
+    del holding_threading
+
+    # Put back ``_threading_local`` if we overwrote earlier
+
+    if holding__threading_local:
+        sys_modules['_threading_local'] = held__threading_local
+        del held__threading_local
+    del holding__threading_local
+
+    # Put back ``thread`` if we overwrote, else del the entry we made
+    if holding_thread:
+        sys_modules['_thread'] = held_thread
+        del held_thread
+    else:
+        del sys_modules['_thread']
+    del holding_thread
+
+    del _dummy_thread
+    del sys_modules