IF语句导致webpy内部服务器错误

我有这个类:

class View(object):
    def main_page(self, extra_placeholders = None):
        file = '/media/Shared/sites/www/subdomains/pypular/static/layout.tmpl'

        placeholders = { 'site_name' : 'pypular' } 

        # If we passed placeholders vars, append them
        if extra_placeholders  != None:
            for k, v in extra_placeholders.iteritems():
                placeholders[k] = v

如您所见,该函数接受一个参数(extra_placeholders),这是一个字典。

如果我不向main_page()传递参数,

if extra_placeholders  == None:
    return 'i executed'

运行良好。然而,

if extra_placeholders  != None:
    return 'i cause error'