import ast


def offset(source):
  function_def = ast.parse(source).body[0]
  decorator = function_def.decorator_list[0]
  return decorator.col_offset


assert offset("""
@a()
def b(): 0
""") == 0

assert offset("""
@a(x)
def b(): 0
""") == 1

assert offset("""
@a
def b(): 0
""") == 1