了解python类变量

Question

Why do we have template_name = None as class variable here?(from django source code)

It's because if self.template_name is None, would raise an error?
(self.template_name would look for instance variable and if it's not there, would return the class variable)
If so, wouldn't it be better to have def __init__(self): self.template_name = None ?

class TemplateResponseMixin(object):
    """                                                                                                                                                                                                                                                                       
    A mixin that can be used to render a template.                                                                                                                                                                                                                            
    """
    template_name = None
    response_class = TemplateResponse

    def render_to_response(self, context, **response_kwargs):
        """                                                                                                                                                                                                                                                                   
        Returns a response with a template rendered with the given context.                                                                                                                                                                                                   
        """
        return self.response_class(
            request = self.request,
            template = self.get_template_names(),
            context = context,
            **response_kwargs
        )

    def get_template_names(self):
        """                                                                                                                                                                                                                                                                   
        Returns a list of template names to be used for the request. Must return                                                                                                                                                                                              
        a list. May not be called if render_to_response is overridden.                                                                                                                                                                                                        
        """
        if self.template_name is None:
            raise ImproperlyConfigured(
                "TemplateResponseMixin requires either a definition of "
                "'template_name' or an implementation of 'get_template_names()'")
        else:
            return [self.template_name]

Answer 1

TemplateResponseMixin is a mixin which is not using an init, to make it easier to use in a mutiple inheritence. It does not need its own state, so it does not need a constructor. This also makes the inheritence easier because you don' t need to call a constructor on it in your subclass.

The template_name is set as a class instance obviously because there is no constructor. It implies that it should be set from the subclass. Also, changing the value of it will affect all future instances of that mixin.