如何使用设置为“无”的可选参数

Question

使用命名参数，如何告诉接收器方法使用参数的“未提供”版本？无发送功能不起作用。以下是我的特定代码，请特别注意以下部分：

args=launch[1:] if launch[4] is not None else None

我想尽可能保持列表理解

procs = [Process(name=key, target=launch[0],

               args=launch[1:] if launch[4] is not None else None)

       for key, launch in zip(procinfos.keys(), launches)]

结果是选择了进程的单参数版本，然后抱怨参数为无：

File "<stdin>", line 15, in parallel

          for key, launch in zip(procinfos.keys(), launches)]

File "/usr/lib/python2.7/multiprocessing/process.py", line 104, in __init__

 self._args = tuple(args)

TypeError：“ NoneType”对象不可迭代

当然，有一种蛮力方法：即复制理解的一部分，而只是避免指定args =参数。我可能最终会走那条路线..除非在这里神奇地出现了替代方法;）

Answer 1

您可以使用参数解包将指定的参数指定为字典，args如果则不存在launch[4] is None，例如：procs = []for key, launch in zip(procinfos.keys(), launches):&nbsp; &nbsp; &nbsp;params = {"name": key, "target": launch[0]}&nbsp; &nbsp; &nbsp;if launch[4] is not None:&nbsp; &nbsp; &nbsp; &nbsp; &nbsp;params["args"] = launch[1:]&nbsp; &nbsp; &nbsp;procs.append(Process(**params))

Answer 2

的默认值args是一个空的元组，不是None：launch[1:] if launch[4] is not None else ()我真的会避免编写三行单行代码。常规for循环没有错：processes = []for key, launch in zip(procinfos, launches):&nbsp; &nbsp; args = launch[1:] if launch[4] is not None else ()&nbsp; &nbsp; process = Process(name=key, target=launch[0], args=args)&nbsp; &nbsp; processes.append(process)

Answer 3

替换None为空的元组：()procs = [Process(name=key, target=launch[0],&nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp;args=launch[1:] if launch[4] is not None else ())&nbsp; &nbsp;for key, launch in zip(procinfos.keys(), launches)]