描述符'getter'需要一个'property'对象但接收到一个'函数

Question

我有下面这段代码，是关于一个表格对象的，它里面有一个字段名称的属性。

class Table(object):
    '''A CSV backed SQL table.'''
    @property
    def fieldnames(self):
        with open(self.filename) as f:
            return csv.DictReader(f).fieldnames

    @property.setter
    def fieldnames(self, fieldnames):
        with open(self.filename, 'w') as f:
            dr = csv.reader(f)
            dw = csv.DictWriter(f, fieldnames=fieldnames)
            dw.writerow(dict((field, field) for field in fieldnames))
            for row in self:
                dw.writerow(row)

当我尝试导入这个文件时，出现了这个错误：

seas486:PennAppSuite ceasarbautista$ python
Python 2.7.1 (r271:86832, Jun 25 2011, 05:09:01) 
[GCC 4.2.1 (Based on Apple Inc. build 5658) (LLVM build 2335.15.00)] on darwin
Type "help", "copyright", "credits" or "license" for more information.
>>> import table
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "table.py", line 7, in <module>
    class Table(object):
  File "table.py", line 9, in Table
    @property.getter
TypeError: descriptor 'getter' requires a 'property' object but received a 'function'

有没有人能解释一下这个错误是什么意思？

Answer 1

我想这和 TypeError: unbound method ... must be called with ... instance as first argument (got ... instance instead) 的意思差不多。要通过装饰器给一个属性添加一个设置器，你需要把 .setter 作为属性对象的一个成员/方法来使用，而不是作为 property 的静态方法或类方法来用。代码应该像这样：

class Table(object):
    '''A CSV backed SQL table.'''
    @property
    def fieldnames(self):
        with open(self.filename) as f:
            return csv.DictReader(f).fieldnames

    @fieldnames.setter # <<<
    def fieldnames(self, fieldnames):
        with open(self.filename, 'w') as f:
            dr = csv.reader(f)
            dw = csv.DictWriter(f, fieldnames=fieldnames)
            dw.writerow(dict((field, field) for field in fieldnames))
            for row in self:
                dw.writerow(row)

你还可以查看 文档中的例子。