在Python的__init__中使用* args和** kwargs的SyntaxError

Question

I'm trying to define a class RecurringInterval which uses the rrule class from dateutil.rrule through composition, and in addition has the attribute period which by default is None . I've tried to initialize it in this way:

class RecurringInterval(object):
    def __init__(self, *args, period=None, **kwargs):
        self.period = period
        self.rrule = dateutil.rrule.rrule(*args, **kwargs)

recurring_interval = RecurringInterval(dateutil.rrule.DAILY, count=1)

However, I get a SyntaxError :

  File "/home/kurt/dev/scratch/Furion_scheduler/recurring_interval.py", line 7
    def __init__(self, *args, period=None, **kwargs):
                                   ^
SyntaxError: invalid syntax

As I understand it, positional arguments should come before keyword arguments, so this is how I would expect the syntax to be; how would I correct it? (From https://docs.python.org/2/tutorial/controlflow.html#unpacking-argument-lists this is not yet clear to me).

I tried bringing period=None forward, like so:

class RecurringInterval(object):
    def __init__(self, period=None, *args, **kwargs):
        self.period = period
        self.rrule = dateutil.rrule.rrule(*args, **kwargs)

but this gives rise to a TypeError :

Traceback (most recent call last):
  File "/home/kurt/dev/scratch/Furion_scheduler/recurring_interval.py", line 9, in <module>
    recurring_interval = RecurringInterval(dateutil.rrule.DAILY, count=1)
  File "/home/kurt/dev/scratch/Furion_scheduler/recurring_interval.py", line 7, in __init__
    self.rrule = dateutil.rrule.rrule(*args, **kwargs)
TypeError: __init__() takes at least 2 arguments (2 given)

How can I initialize the RecurringInterval in the intended fashion?

Answer 1

它应该是：

def __init__(self, period=None, *args, **kwargs):

Answer 2

Updated answer

Following Python, default keyword arguments after variable length positional arguments , the following works in Python 3:

class RecurringInterval(object):
    def __init__(self, *args, duration=datetime.timedelta(seconds=0), **kwargs):    # Initializing in this way only works in Python 3
        self.duration = duration
        self.rrule = dateutil.rrule.rrule(*args, **kwargs)

Old answer

Following this Github article , I found a solution using kwargs.pop :

class RecurringInterval(object):
    def __init__(self, *args, **kwargs):
        self.period = kwargs.pop('period', None)
        self.rrule = dateutil.rrule.rrule(*args, **kwargs)

recurring_interval = RecurringInterval(dateutil.rrule.DAILY, count=1, period=datetime.timedelta(days=2))

This way, period is given as a keyword argument, it is assigned to self.period , the default value of which is None , while the remaining args and kwargs are used to initialize the RecurringInterval 's self.rrule .