bpo-31975 (PEP 565): Show DeprecationWarning in __main__ (GH-4458)

- primary change is to add a new default filter entry for
  'default::DeprecationWarning:__main__'
- secondary change is an internal one to cope with plain
  strings in the warning module's internal filter list
  (this avoids the need to create a compiled regex object
  early on during interpreter startup)
- assorted documentation updates, including many more
  examples of configuring the warnings settings
- additional tests to ensure that both the pure Python and
  the C accelerated warnings modules have the expected
  default configuration

1 files changed, 3 insertions, 1 deletions

diff --git a/Lib/warnings.py b/Lib/warnings.py
index f4331c8..76ad4da 100644
--- a/Lib/warnings.py
+++ b/Lib/warnings.py
@@ -519,8 +519,10 @@ except ImportError:
 # Module initialization
 _processoptions(sys.warnoptions)
 if not _warnings_defaults:
-    # Several warning categories are ignored by default in Py_DEBUG builds
+    # Several warning categories are ignored by default in regular builds
     if not hasattr(sys, 'gettotalrefcount'):
+        filterwarnings("default", category=DeprecationWarning,
+                       module="__main__", append=1)
         simplefilter("ignore", category=DeprecationWarning, append=1)
         simplefilter("ignore", category=PendingDeprecationWarning, append=1)
         simplefilter("ignore", category=ImportWarning, append=1)