Please use 4 spaces to ident your code (as the rest of _asynciomodule.c file).

Move ); to the previous line, so that

int class_has_attr = _PyObject_HasAttrId(
    class, &PyId__asyncio_future_blocking);

While this code is OK, you shouldn't use refs to Python objects after you DECREF them, it's a bad style. Please compare first, and decref after,

Why not use Future_CheckExact() / Future_Check() as very fast happy path here?

Good idea.

@asvetlov @1st1 isfuture check if _asyncio_future_blocking exists and is not None because mock object has _asyncio_future_blocking as None and we want to return isfuture() as False in this case.
See test case

Do we really want to use Future_CheckExact to return fast?

It is better to use Py_TYPE() for convenience and speed . There is a subtle difference between the type and the __class__ attribute, but it is usually ignored when write C accelerations.

I suggested to @jimmylai to get the class attribute to make sure that the C implementation behaves as the Python implementation: see PEP 399. If someone wants to use type(), I would prefer to see the same change in the Python implementation as well.

@1st1, @asvetlov: Do you know the rationale for checking class here?

I suggested to use Py_TYPE as well.

I think it was I who used __class__ there, and there's no rationale behind that :)

So the Python version can be updated to use type() too.

@1st1 @serhiy-storchaka
use type() won't work.
Original isfuture: return (hasattr(obj.__class__, '_asyncio_future_blocking') and obj._asyncio_future_blocking is not None)

Use type: return (hasattr(type(obj), '_asyncio_future_blocking') and obj._asyncio_future_blocking is not None)

Unit tests fail when use type()
Did I use type() wrong?

======================================================================
FAIL: test_isfuture (test.test_asyncio.test_futures.CFutureTests)
----------------------------------------------------------------------
Traceback (most recent call last):
  File "/Users/jimmylai/workspace/cpython/Lib/test/test_asyncio/test_futures.py", line 131, in test_isfuture
    self.assertTrue(asyncio.isfuture(mock.Mock(type(f))))
AssertionError: False is not true

======================================================================
FAIL: test_isfuture (test.test_asyncio.test_futures.CSubFutureTests)
----------------------------------------------------------------------
Traceback (most recent call last):
  File "/Users/jimmylai/workspace/cpython/Lib/test/test_asyncio/test_futures.py", line 131, in test_isfuture
    self.assertTrue(asyncio.isfuture(mock.Mock(type(f))))
AssertionError: False is not true

======================================================================
FAIL: test_isfuture (test.test_asyncio.test_futures.PyFutureTests)
----------------------------------------------------------------------
Traceback (most recent call last):
  File "/Users/jimmylai/workspace/cpython/Lib/test/test_asyncio/test_futures.py", line 131, in test_isfuture
    self.assertTrue(asyncio.isfuture(mock.Mock(type(f))))
AssertionError: False is not true

Use 4-space indentation.

fixed.

Make the argument positional-only.

In pratice, it means adding "/" on a new line, aligned with "obj", and run "make clinic" again.

Is it needed?

class may not have attr _asyncio_future_blocking
It's needed.

obj_attr is leaked here.

added Py_DECREF(obj_attr);