导致Webpy内部服务器错误的if语句

我有这门课:

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

上面代码中的问题是if语句

如您所见,函数接受一个参数(额外的占位符),它是一个dict。

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'

不起作用。它会导致500个内部服务器错误。为什么?