Question

So I have this code below for a Table object, and it has a property for fieldnames.

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)

When I try to import the file, I get the error:

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'

Can anybody explain what this error means?

Answer 1

I guess it’s the equivalent to TypeError: unbound method ... must be called with ... instance as first argument (got ... instance instead). To add a setter to a property via a decorator, you have to use .setter as a member/method of the property object, not as a static method/classmethod of property. The code is supposed to look like this:

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)

Also see the example in the documentation.