bpo-32311: Implement asyncio.create_task() shortcut (#4848)

* Implement functionality
* Add documentation

7 files changed, 112 insertions, 89 deletions

diff --git a/Lib/asyncio/base_futures.py b/Lib/asyncio/base_futures.py
index 2ee82c3..008812e 100644
--- a/Lib/asyncio/base_futures.py
+++ b/Lib/asyncio/base_futures.py
@@ -3,7 +3,7 @@ __all__ = ()
 import concurrent.futures._base
 import reprlib
 
-from . import events
+from . import format_helpers
 
 Error = concurrent.futures._base.Error
 CancelledError = concurrent.futures.CancelledError
@@ -38,7 +38,7 @@ def _format_callbacks(cb):
         cb = ''
 
     def format_cb(callback):
-        return events._format_callback_source(callback, ())
+        return format_helpers._format_callback_source(callback, ())
 
     if size == 1:
         cb = format_cb(cb[0])
diff --git a/Lib/asyncio/constants.py b/Lib/asyncio/constants.py
index dfe97f4..52169c3 100644
--- a/Lib/asyncio/constants.py
+++ b/Lib/asyncio/constants.py
@@ -6,5 +6,5 @@ ACCEPT_RETRY_DELAY = 1
 
 # Number of stack entries to capture in debug mode.
 # The larger the number, the slower the operation in debug mode
-# (see extract_stack() in events.py).
+# (see extract_stack() in format_helpers.py).
 DEBUG_STACK_DEPTH = 10
diff --git a/Lib/asyncio/coroutines.py b/Lib/asyncio/coroutines.py
index bca7fe3..e3c0162 100644
--- a/Lib/asyncio/coroutines.py
+++ b/Lib/asyncio/coroutines.py
@@ -9,9 +9,9 @@ import types
 
 from collections.abc import Awaitable, Coroutine
 
-from . import constants
-from . import events
 from . import base_futures
+from . import constants
+from . import format_helpers
 from .log import logger
 
 
@@ -48,7 +48,7 @@ class CoroWrapper:
         assert inspect.isgenerator(gen) or inspect.iscoroutine(gen), gen
         self.gen = gen
         self.func = func  # Used to unwrap @coroutine decorator
-        self._source_traceback = events.extract_stack(sys._getframe(1))
+        self._source_traceback = format_helpers.extract_stack(sys._getframe(1))
         self.__name__ = getattr(gen, '__name__', None)
         self.__qualname__ = getattr(gen, '__qualname__', None)
 
@@ -243,7 +243,7 @@ def _format_coroutine(coro):
         func = coro
 
     if coro_name is None:
-        coro_name = events._format_callback(func, (), {})
+        coro_name = format_helpers._format_callback(func, (), {})
 
     try:
         coro_code = coro.gi_code
@@ -260,7 +260,7 @@ def _format_coroutine(coro):
     if (isinstance(coro, CoroWrapper) and
             not inspect.isgeneratorfunction(coro.func) and
             coro.func is not None):
-        source = events._get_function_source(coro.func)
+        source = format_helpers._get_function_source(coro.func)
         if source is not None:
             filename, lineno = source
         if coro_frame is None:
diff --git a/Lib/asyncio/events.py b/Lib/asyncio/events.py
index a00f861..974a4a2 100644
--- a/Lib/asyncio/events.py
+++ b/Lib/asyncio/events.py
@@ -11,86 +11,14 @@ __all__ = (
     '_get_running_loop',
 )
 
-import functools
-import inspect
 import os
-import reprlib
 import socket
 import subprocess
 import sys
 import threading
-import traceback
 
 from . import constants
-
-
-def _get_function_source(func):
-    func = inspect.unwrap(func)
-    if inspect.isfunction(func):
-        code = func.__code__
-        return (code.co_filename, code.co_firstlineno)
-    if isinstance(func, functools.partial):
-        return _get_function_source(func.func)
-    if isinstance(func, functools.partialmethod):
-        return _get_function_source(func.func)
-    return None
-
-
-def _format_args_and_kwargs(args, kwargs):
-    """Format function arguments and keyword arguments.
-
-    Special case for a single parameter: ('hello',) is formatted as ('hello').
-    """
-    # use reprlib to limit the length of the output
-    items = []
-    if args:
-        items.extend(reprlib.repr(arg) for arg in args)
-    if kwargs:
-        items.extend(f'{k}={reprlib.repr(v)}' for k, v in kwargs.items())
-    return '({})'.format(', '.join(items))
-
-
-def _format_callback(func, args, kwargs, suffix=''):
-    if isinstance(func, functools.partial):
-        suffix = _format_args_and_kwargs(args, kwargs) + suffix
-        return _format_callback(func.func, func.args, func.keywords, suffix)
-
-    if hasattr(func, '__qualname__'):
-        func_repr = getattr(func, '__qualname__')
-    elif hasattr(func, '__name__'):
-        func_repr = getattr(func, '__name__')
-    else:
-        func_repr = repr(func)
-
-    func_repr += _format_args_and_kwargs(args, kwargs)
-    if suffix:
-        func_repr += suffix
-    return func_repr
-
-
-def _format_callback_source(func, args):
-    func_repr = _format_callback(func, args, None)
-    source = _get_function_source(func)
-    if source:
-        func_repr += f' at {source[0]}:{source[1]}'
-    return func_repr
-
-
-def extract_stack(f=None, limit=None):
-    """Replacement for traceback.extract_stack() that only does the
-    necessary work for asyncio debug mode.
-    """
-    if f is None:
-        f = sys._getframe().f_back
-    if limit is None:
-        # Limit the amount of work to a reasonable amount, as extract_stack()
-        # can be called for each coroutine and future in debug mode.
-        limit = constants.DEBUG_STACK_DEPTH
-    stack = traceback.StackSummary.extract(traceback.walk_stack(f),
-                                           limit=limit,
-                                           lookup_lines=False)
-    stack.reverse()
-    return stack
+from . import format_helpers
 
 
 class Handle:
@@ -106,7 +34,8 @@ class Handle:
         self._cancelled = False
         self._repr = None
         if self._loop.get_debug():
-            self._source_traceback = extract_stack(sys._getframe(1))
+            self._source_traceback = format_helpers.extract_stack(
+                sys._getframe(1))
         else:
             self._source_traceback = None
 
@@ -115,7 +44,8 @@ class Handle:
         if self._cancelled:
             info.append('cancelled')
         if self._callback is not None:
-            info.append(_format_callback_source(self._callback, self._args))
+            info.append(format_helpers._format_callback_source(
+                self._callback, self._args))
         if self._source_traceback:
             frame = self._source_traceback[-1]
             info.append(f'created at {frame[0]}:{frame[1]}')
@@ -145,7 +75,8 @@ class Handle:
         try:
             self._callback(*self._args)
         except Exception as exc:
-            cb = _format_callback_source(self._callback, self._args)
+            cb = format_helpers._format_callback_source(
+                self._callback, self._args)
             msg = f'Exception in callback {cb}'
             context = {
                 'message': msg,
diff --git a/Lib/asyncio/format_helpers.py b/Lib/asyncio/format_helpers.py
new file mode 100644
index 0000000..39cfcee
--- /dev/null
+++ b/Lib/asyncio/format_helpers.py
@@ -0,0 +1,75 @@
+import functools
+import inspect
+import reprlib
+import traceback
+
+from . import constants
+
+
+def _get_function_source(func):
+    func = inspect.unwrap(func)
+    if inspect.isfunction(func):
+        code = func.__code__
+        return (code.co_filename, code.co_firstlineno)
+    if isinstance(func, functools.partial):
+        return _get_function_source(func.func)
+    if isinstance(func, functools.partialmethod):
+        return _get_function_source(func.func)
+    return None
+
+
+def _format_callback_source(func, args):
+    func_repr = _format_callback(func, args, None)
+    source = _get_function_source(func)
+    if source:
+        func_repr += f' at {source[0]}:{source[1]}'
+    return func_repr
+
+
+def _format_args_and_kwargs(args, kwargs):
+    """Format function arguments and keyword arguments.
+
+    Special case for a single parameter: ('hello',) is formatted as ('hello').
+    """
+    # use reprlib to limit the length of the output
+    items = []
+    if args:
+        items.extend(reprlib.repr(arg) for arg in args)
+    if kwargs:
+        items.extend(f'{k}={reprlib.repr(v)}' for k, v in kwargs.items())
+    return '({})'.format(', '.join(items))
+
+
+def _format_callback(func, args, kwargs, suffix=''):
+    if isinstance(func, functools.partial):
+        suffix = _format_args_and_kwargs(args, kwargs) + suffix
+        return _format_callback(func.func, func.args, func.keywords, suffix)
+
+    if hasattr(func, '__qualname__'):
+        func_repr = getattr(func, '__qualname__')
+    elif hasattr(func, '__name__'):
+        func_repr = getattr(func, '__name__')
+    else:
+        func_repr = repr(func)
+
+    func_repr += _format_args_and_kwargs(args, kwargs)
+    if suffix:
+        func_repr += suffix
+    return func_repr
+
+
+def extract_stack(f=None, limit=None):
+    """Replacement for traceback.extract_stack() that only does the
+    necessary work for asyncio debug mode.
+    """
+    if f is None:
+        f = sys._getframe().f_back
+    if limit is None:
+        # Limit the amount of work to a reasonable amount, as extract_stack()
+        # can be called for each coroutine and future in debug mode.
+        limit = constants.DEBUG_STACK_DEPTH
+    stack = traceback.StackSummary.extract(traceback.walk_stack(f),
+                                           limit=limit,
+                                           lookup_lines=False)
+    stack.reverse()
+    return stack
diff --git a/Lib/asyncio/futures.py b/Lib/asyncio/futures.py
index d46a295..b310962 100644
--- a/Lib/asyncio/futures.py
+++ b/Lib/asyncio/futures.py
@@ -11,6 +11,7 @@ import sys
 
 from . import base_futures
 from . import events
+from . import format_helpers
 
 
 CancelledError = base_futures.CancelledError
@@ -79,7 +80,8 @@ class Future:
             self._loop = loop
         self._callbacks = []
         if self._loop.get_debug():
-            self._source_traceback = events.extract_stack(sys._getframe(1))
+            self._source_traceback = format_helpers.extract_stack(
+                sys._getframe(1))
 
     _repr_info = base_futures._future_repr_info
 
diff --git a/Lib/asyncio/tasks.py b/Lib/asyncio/tasks.py
index c5122f7..172057e 100644
--- a/Lib/asyncio/tasks.py
+++ b/Lib/asyncio/tasks.py
@@ -1,7 +1,7 @@
 """Support for tasks, coroutines and the scheduler."""
 
 __all__ = (
-    'Task',
+    'Task', 'create_task',
     'FIRST_COMPLETED', 'FIRST_EXCEPTION', 'ALL_COMPLETED',
     'wait', 'wait_for', 'as_completed', 'sleep',
     'gather', 'shield', 'ensure_future', 'run_coroutine_threadsafe',
@@ -67,13 +67,19 @@ class Task(futures.Future):
         return {t for t in cls._all_tasks if t._loop is loop}
 
     def __init__(self, coro, *, loop=None):
-        assert coroutines.iscoroutine(coro), repr(coro)
         super().__init__(loop=loop)
         if self._source_traceback:
             del self._source_traceback[-1]
-        self._coro = coro
-        self._fut_waiter = None
+        if not coroutines.iscoroutine(coro):
+            # raise after Future.__init__(), attrs are required for __del__
+            # prevent logging for pending task in __del__
+            self._log_destroy_pending = False
+            raise TypeError(f"a coroutine was expected, got {coro!r}")
+
         self._must_cancel = False
+        self._fut_waiter = None
+        self._coro = coro
+
         self._loop.call_soon(self._step)
         self.__class__._all_tasks.add(self)
 
@@ -263,6 +269,15 @@ else:
     Task = _CTask = _asyncio.Task
 
 
+def create_task(coro):
+    """Schedule the execution of a coroutine object in a spawn task.
+
+    Return a Task object.
+    """
+    loop = events.get_running_loop()
+    return loop.create_task(coro)
+
+
 # wait() and as_completed() similar to those in PEP 3148.
 
 FIRST_COMPLETED = concurrent.futures.FIRST_COMPLETED