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

* Implement functionality
* Add documentation

12 files changed, 201 insertions, 95 deletions

diff --git a/Doc/library/asyncio-task.rst b/Doc/library/asyncio-task.rst
index 0d0569f0..72fae5e 100644
--- a/Doc/library/asyncio-task.rst
+++ b/Doc/library/asyncio-task.rst
@@ -371,10 +371,21 @@ with the result.
 Task
 ----
 
+.. function:: create_task(coro)
+
+   Wrap a :ref:`coroutine <coroutine>` *coro* into a task and schedule
+   its execution.  Return the task object.
+
+   The task is executed in :func:`get_running_loop` context,
+   :exc:`RuntimeError` is raised if there is no running loop in
+   current thread.
+
+   .. versionadded:: 3.7
+
 .. class:: Task(coro, \*, loop=None)
 
-   Schedule the execution of a :ref:`coroutine <coroutine>`: wrap it in a
-   future. A task is a subclass of :class:`Future`.
+   A unit for concurrent running of :ref:`coroutines <coroutine>`,
+   subclass of :class:`Future`.
 
    A task is responsible for executing a coroutine object in an event loop.  If
    the wrapped coroutine yields from a future, the task suspends the execution
@@ -399,7 +410,7 @@ Task
    <coroutine>` did not complete. It is probably a bug and a warning is
    logged: see :ref:`Pending task destroyed <asyncio-pending-task-destroyed>`.
 
-   Don't directly create :class:`Task` instances: use the :func:`ensure_future`
+   Don't directly create :class:`Task` instances: use the :func:`create_task`
    function or the :meth:`AbstractEventLoop.create_task` method.
 
    This class is :ref:`not thread safe <asyncio-multithreading>`.
@@ -547,9 +558,15 @@ Task functions
    .. versionchanged:: 3.5.1
       The function accepts any :term:`awaitable` object.
 
+   .. note::
+
+      :func:`create_task` (added in Python 3.7) is the preferable way
+      for spawning new tasks.
+
    .. seealso::
 
-      The :meth:`AbstractEventLoop.create_task` method.
+      The :func:`create_task` function and
+      :meth:`AbstractEventLoop.create_task` method.
 
 .. function:: wrap_future(future, \*, loop=None)
 
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
diff --git a/Lib/test/test_asyncio/test_tasks.py b/Lib/test/test_asyncio/test_tasks.py
index a5563ba..a32dca1 100644
--- a/Lib/test/test_asyncio/test_tasks.py
+++ b/Lib/test/test_asyncio/test_tasks.py
@@ -2054,6 +2054,43 @@ class BaseTaskTests:
 
         self.assertEqual(self.Task.all_tasks(self.loop), set())
 
+    def test_create_task_with_noncoroutine(self):
+        with self.assertRaisesRegex(TypeError,
+                                    "a coroutine was expected, got 123"):
+            self.new_task(self.loop, 123)
+
+    def test_create_task_with_oldstyle_coroutine(self):
+
+        @asyncio.coroutine
+        def coro():
+            pass
+
+        task = self.new_task(self.loop, coro())
+        self.assertIsInstance(task, self.Task)
+        self.loop.run_until_complete(task)
+
+    def test_create_task_with_async_function(self):
+
+        async def coro():
+            pass
+
+        task = self.new_task(self.loop, coro())
+        self.assertIsInstance(task, self.Task)
+        self.loop.run_until_complete(task)
+
+    def test_bare_create_task(self):
+
+        async def inner():
+            return 1
+
+        async def coro():
+            task = asyncio.create_task(inner())
+            self.assertIsInstance(task, self.Task)
+            ret = await task
+            self.assertEqual(1, ret)
+
+        self.loop.run_until_complete(coro())
+
 
 def add_subclass_tests(cls):
     BaseTask = cls.Task
diff --git a/Lib/test/test_asyncio/utils.py b/Lib/test/test_asyncio/utils.py
index 560db9f..a1a9bb3 100644
--- a/Lib/test/test_asyncio/utils.py
+++ b/Lib/test/test_asyncio/utils.py
@@ -28,6 +28,7 @@ except ImportError:  # pragma: no cover
 
 from asyncio import base_events
 from asyncio import events
+from asyncio import format_helpers
 from asyncio import futures
 from asyncio import tasks
 from asyncio.log import logger
@@ -429,7 +430,7 @@ class MockPattern(str):
 
 
 def get_function_source(func):
-    source = events._get_function_source(func)
+    source = format_helpers._get_function_source(func)
     if source is None:
         raise ValueError("unable to get the source of %r" % (func,))
     return source
diff --git a/Misc/NEWS.d/next/Library/2017-12-14-17-28-54.bpo-32311.DL5Ytn.rst b/Misc/NEWS.d/next/Library/2017-12-14-17-28-54.bpo-32311.DL5Ytn.rst
new file mode 100644
index 0000000..e2d1095
--- /dev/null
+++ b/Misc/NEWS.d/next/Library/2017-12-14-17-28-54.bpo-32311.DL5Ytn.rst
@@ -0,0 +1 @@
+Implement asyncio.create_task(coro) shortcut
diff --git a/Modules/_asynciomodule.c b/Modules/_asynciomodule.c
index 01c38b8..9ac1c44 100644
--- a/Modules/_asynciomodule.c
+++ b/Modules/_asynciomodule.c
@@ -26,6 +26,7 @@ static PyObject *all_tasks;
 static PyObject *current_tasks;
 static PyObject *traceback_extract_stack;
 static PyObject *asyncio_get_event_loop_policy;
+static PyObject *asyncio_iscoroutine_func;
 static PyObject *asyncio_future_repr_info_func;
 static PyObject *asyncio_task_repr_info_func;
 static PyObject *asyncio_task_get_stack_func;
@@ -1461,16 +1462,38 @@ _asyncio_Task___init___impl(TaskObj *self, PyObject *coro, PyObject *loop)
 /*[clinic end generated code: output=9f24774c2287fc2f input=8d132974b049593e]*/
 {
     PyObject *res;
+    int tmp;
     _Py_IDENTIFIER(add);
 
     if (future_init((FutureObj*)self, loop)) {
         return -1;
     }
 
+    if (!PyCoro_CheckExact(coro)) {
+        // fastpath failed, perfom slow check
+        // raise after Future.__init__(), attrs are required for __del__
+        res = PyObject_CallFunctionObjArgs(asyncio_iscoroutine_func,
+                                           coro, NULL);
+        if (res == NULL) {
+            return -1;
+        }
+        tmp = PyObject_Not(res);
+        Py_DECREF(res);
+        if (tmp < 0) {
+            return -1;
+        }
+        if (tmp) {
+            self->task_log_destroy_pending = 0;
+            PyErr_Format(PyExc_TypeError,
+                         "a coroutine was expected, got %R",
+                         coro, NULL);
+            return -1;
+        }
+    }
+
     self->task_fut_waiter = NULL;
     self->task_must_cancel = 0;
     self->task_log_destroy_pending = 1;
-
     Py_INCREF(coro);
     self->task_coro = coro;
 
@@ -2604,6 +2627,7 @@ module_free(void *m)
     Py_CLEAR(traceback_extract_stack);
     Py_CLEAR(asyncio_get_event_loop_policy);
     Py_CLEAR(asyncio_future_repr_info_func);
+    Py_CLEAR(asyncio_iscoroutine_func);
     Py_CLEAR(asyncio_task_repr_info_func);
     Py_CLEAR(asyncio_task_get_stack_func);
     Py_CLEAR(asyncio_task_print_stack_func);
@@ -2645,6 +2669,9 @@ module_init(void)
     GET_MOD_ATTR(asyncio_task_get_stack_func, "_task_get_stack")
     GET_MOD_ATTR(asyncio_task_print_stack_func, "_task_print_stack")
 
+    WITH_MOD("asyncio.coroutines")
+    GET_MOD_ATTR(asyncio_iscoroutine_func, "iscoroutine")
+
     WITH_MOD("inspect")
     GET_MOD_ATTR(inspect_isgenerator, "isgenerator")