带有空列表的 Python 错误答案

【问题标题】：Python bug with empty list带有空列表的 Python 错误
【发布时间】：2017-01-28 20:55:00
【问题描述】：

我正在使用 python 来尝试模拟 Inform 7 (inform7.com) 的基本功能。当我尝试设置房间时打印出其中的所有对象，并且房间中没有任何对象，它仍然会打印出In this room there is a。代码的相关部分在这里。

def __repr__(self):
    if self.room_firstrun:
        for i in things:
            if things[i] == self.name:
                self.objects_in_room.append(i)
        self.room_firstrun = False

    fullstring = 'In this room, there is a '
    for i in self.objects_in_room:
        if not self.objects_in_room:
            fullstring = ''
            break
        elif i == self.objects_in_room[len(self.objects_in_room) - 1] and len(self.objects_in_room) > 2:
            stringtoappend = ', and a ' + i + '.'
        elif i == self.objects_in_room[len(self.objects_in_room) - 1] and len(self.objects_in_room) == 2:
            stringtoappend = ' and a ' + i + '.'
        elif i == self.objects_in_room[len(self.objects_in_room) - 1] and len(self.objects_in_room) == 1:
            stringtoappend = i + '.'
        elif i == self.objects_in_room[0]:
            stringtoappend = i
        else:
            stringtoappend = ', a ' + i
    fullstring += stringtoappend
    stringtoappend = ''
    return '\n----------' + self.name + '----------\n\n' + self.desc + '\n\n' + fullstring

。 if not self.objects_in_room: fullstring = '' break 行永远不会到达，即使我通过删除所有项目来验证列表是空的。我一直在尝试修复它一段时间，但它一直没有工作。也很烦人，当我在一个房间里有两件物品时，它不会显示第一个。也许我应该把它放在另一个问题中。提前感谢您的回答！

【问题讨论】：

将该语句移出循环。如果列表为空，则循环根本不会运行。其次，为什么要使用 stringtoappend 而不是将它们添加到完整字符串中
嗯....我试过了，但是没用。
哦，没关系，我不小心删除了它facepalm

标签： python if-statement python-3.6

【解决方案1】：

if not self.objects_in_room 从未到达，因为它位于 for 循环内，并且 for 循环仅在 self.objects_in_room 不为空时运行。要解决此问题，请将 if not self.objects_in_room 放在循环之外

def __repr__(self):
    if self.room_firstrun:
        for i in things:
            if things[i] == self.name:
                self.objects_in_room.append(i)
        self.room_firstrun = False

    fullstring = 'In this room, there is a '
    if not self.objects_in_room:
            fullstring = ''
    for i in self.objects_in_room:

        if i == self.objects_in_room[len(self.objects_in_room) - 1] and len(self.objects_in_room) > 2:
            fullstring += ', and a {0}.'.format(i)
        elif i == self.objects_in_room[len(self.objects_in_room) - 1] and len(self.objects_in_room) == 2:
            fullstring += ' and a {0}.'.format(i)
        elif i == self.objects_in_room[len(self.objects_in_room) - 1] and len(self.objects_in_room) == 1:
            fullstring += '{0}.'.format(i)
        elif i == self.objects_in_room[0]:
            fullstring += i
        else:
            fullstring += ', a {0}'.format(i)
    return '\n----------' + self.name + '----------\n\n' + self.desc + '\n\n' + fullstring

或者您可以这样做：如果您使用的是 python 3，则使用 for..else 语句。

def __repr__(self):
    if self.room_firstrun:
        for i in things:
            if things[i] == self.name:
                self.objects_in_room.append(i)
        self.room_firstrun = False

    fullstring = 'In this room, there is a '

    for i in self.objects_in_room:

        if i == self.objects_in_room[len(self.objects_in_room) - 1] and len(self.objects_in_room) > 2:
            fullstring += ', and a {0}.'.format(i)
        elif i == self.objects_in_room[len(self.objects_in_room) - 1] and len(self.objects_in_room) == 2:
            fullstring += ' and a {0}.'.format(i)
        elif i == self.objects_in_room[len(self.objects_in_room) - 1] and len(self.objects_in_room) == 1:
            fullstring += '{0}.'.format(i)
        elif i == self.objects_in_room[0]:
            fullstring += i
        else:
            fullstring += ', a {0}'.format(i)
    else:
        fullstring = ''
    return '\n----------' + self.name + '----------\n\n' + self.desc + '\n\n' + fullstring

你为什么使用stringtoappend？为什么不直接将其添加到完整字符串中？

【讨论】：