In any case, this can be done now with a helper class used as a decorator.

  >>> class At:
    __slots__= 'fun',
    def __init__(self, fun):
      self.fun = fun
    def __matmul__(self, arg):
      return self.fun(arg)
    def __call__(self, *args, **kwargs):
      return self.fun(*args, **kwargs)

  >>> @At
  def square(x):
    return x*x

  >>> square(2)
  4
  >>> square@2
  4