Create a 'standard property' for class attributes in Python

Question

I have a class, that is used to translate binary stream to human readable. I want to translate it both ways, because I send and receive binary messages. Attributes of this class are made mostly the same way - take the bytes from startbyte to stopbyte and decode them - so I made a decision to use a property to do that. But can I make a general "property" that will be used when defining my class attributes?

class Packet(object):
    def __init__(self, data):
        self.data = data

    def standard_getter(startbyte, stopbyte):
        def getter(self):
            return decode(self.data[startbyte:stopbyte])
        return getter

    def standard_setter(startbyte, stopbyte):
        def setter(self, value):
            self.data[startbyte:stopbyte] = encode(value)
    return setter

    # the way I define properties by now:
    protocol_type = property(standard_getter(16, 18), standard_setter(16, 18))
    protocol_sub_type = property(standard_getter(18, 20), standard_setter(18, 20))

    # the way I would like to do it:
    protocol_type = property(standard_property(16, 18))
    # or
    protocol_type = standard_property(16, 18)

I tried to define a function, that takes two arguments and returns property(getter, setter), but always I'm stuck in giving "self" instance to the function. Is there a nice way I can make it?

Martijn Pieters · Accepted Answer · 2016-08-01 12:21:03Z

Have your function produce both the getter and setter, and return the property object for those two functions:

def standard_property(startbyte, stopbyte):
    def getter(self):
        return decode(self.data[startbyte:stopbyte])
    def setter(self, value):
        self.data[startbyte:stopbyte] = encode(value)
    return property(getter, setter)

Then use the return value directly:

protocol_type = standard_property(16, 18)
protocol_sub_type = standard_property(18, 20)

Note that the standard_property() function doesn't even need to live in your class; it could be a top-level function too:

>>> def standard_property(startbyte, stopbyte):
...     def getter(self):
...         return decode(self.data[startbyte:stopbyte])
...     def setter(self, value):
...         self.data[startbyte:stopbyte] = encode(value)
...     return property(getter, setter)
...
>>> encode = lambda v: list(v)
>>> decode = lambda v: ''.join(v)
>>> class Packet(object):
...     def __init__(self, data):
...         self.data = data
...     protocol_type = standard_property(16, 18)
...     protocol_sub_type = standard_property(18, 20)
...
>>> p = Packet(list('foo bar baz spam ham eggs'))
>>> p.protocol_type
' h'
>>> p.protocol_sub_type
'am'
>>> p.protocol_type = '_c'
>>> p.protocol_sub_type = 'an'
>>> ''.join(p.data)
'foo bar baz spam_can eggs'

Great one, that's what I needed. I know I can put it outside the class, but by now it makes no sense for me.

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