asyncio, Tulip issue #136: Add get/set_debug() methods to BaseEventLoopTests.

Add also a PYTHONASYNCIODEBUG environment variable to debug coroutines since
Python startup, to be able to debug coroutines defined directly in the asyncio
module.

Lib/asyncio/base_events.py

@@ -123,6 +123,7 @@ class BaseEventLoop(events.AbstractEventLoop):
         self._running = False
         self._clock_resolution = time.get_clock_info('monotonic').resolution
         self._exception_handler = None
+        self._debug = False
 
     def _make_socket_transport(self, sock, protocol, waiter=None, *,
                                extra=None, server=None):
@@ -795,3 +796,9 @@ class BaseEventLoop(events.AbstractEventLoop):
             if not handle._cancelled:
                 handle._run()
         handle = None  # Needed to break cycles when an exception occurs.
+
+    def get_debug(self):
+        return self._debug
+
+    def set_debug(self, enabled):
+        self._debug = enabled

Lib/asyncio/events.py

@@ -345,6 +345,14 @@ class AbstractEventLoop:
     def call_exception_handler(self, context):
         raise NotImplementedError
 
+    # Debug flag management.
+
+    def get_debug(self):
+        raise NotImplementedError
+
+    def set_debug(self, enabled):
+        raise NotImplementedError
+
 
 class AbstractEventLoopPolicy:
     """Abstract policy for accessing the event loop."""

Lib/asyncio/tasks.py

@@ -12,6 +12,8 @@ import concurrent.futures
 import functools
 import inspect
 import linecache
+import os
+import sys
 import traceback
 import weakref
 
@@ -28,7 +30,8 @@ from .log import logger
 # before you define your coroutines.  A downside of using this feature
 # is that tracebacks show entries for the CoroWrapper.__next__ method
 # when _DEBUG is true.
-_DEBUG = False
+_DEBUG = (not sys.flags.ignore_environment
+          and bool(os.environ.get('PYTHONASYNCIODEBUG')))
 
 
 class CoroWrapper: