Question

Currently __setattr__ only works for instance. Is there any similar method for class? I am asking this question because I want to collect the list of defined attribute in order when user define it in class as below:

class CfgObj(object):
    _fields = []
    def __setattr__(self, name, value):
        self._fields.append([name, value])
        object.__setattr__(self, name, value)

class ACfg(CfgObj):
    setting1 = Field(str, default='set1', desc='setting1 ...')
    setting2 = Field(int, default=5, desc='setting2...')

I know the above code will not work as expected because the __setattr__ only called by instance as below:

acfg = ACfg()
acfg.c = 1
acfg._fields == [['c', 1]]

So, is there any equivalent __setattr__ for python class? The main purpose is to collect the define attribute in order when user define it in class.

Answer 1

What I would do to solve your problem – but not your question – is to set the timestamp of the field creation create a counter of Field objects and set the current value of the counter to the created one:

class Field(object):
    count = 0
    def __init__(self, value, default=None, desc=None):
        self.value = value
        self.default = default
        self.desc = desc
        # Here comes the magic
        self.nth = Field.count
        Field.count += 1
        # self.created_at = time.time()

Then I would create a method for returning all fields ordered by its counter value:

class CfgObj(object):
    def params(self):
        ns = dir(self)
        fs = [getattr(self, field) 
                    for field in ns 
                    if isinstance(getattr(self, field), Field)]
        # fs = sorted(fs, key=lambda f: f.created_at)
        fs = sorted(fs, key=lambda f: f.nth)
        return fs

Its usage is intuitive:

class ACfg(CfgObj):
    setting1 = Field(str, default='set1', desc='setting1 ...')
    setting2 = Field(int, default=5, desc='setting2...')

print ACfg().params()

Clearly the fields are ordered by time of object creation, not field creation, but it can be enough for you. Is it?