尝试创建从Python列表继承的类时语法无效

Question

I am creating a small class that inherits from a Python list.

class palette(list):

    def __init__(
        self,
        *args,
        name   = None, # string name
        colors = None, # list of colors
        ):
        super().__init__(self, *args)   
        self._name   = name
        self.extend(colors)

When I run this, I get a syntax error at the name argument of the initialisation function. Why is this? How can I fix it? I am working with both Python 2 and 3.

Answer 1

*args must come after the other arguments, not before or in the middle of them. (At least in Python 2.)

So this is valid:

def myfunc(foo, bar, *args):
    pass

but this is not:

def myfunc(foo, *args, bar):
    pass

--

As @Padraic Cunningham pointed out in the comments, also note that you have a second syntax error: the trailing comma after colors = None . And your call to super() will fail at run time. You probably want super(palette, self).__init__(*args) .

Answer 2

An argument signature like func(self, *args, name=None, colors=None) specifies name and colors as keyword-only arguments . This is only possible in Python 3. If you want your code to work with Python 2, you need to move the *args to the end of the argument list (and accept that arguments passed positionally will fill in name and colors ).