So I'm trying to make a class that extends list, with the extra ability that certain special attributes are mapped to refer to certain parts of the list. Using this Py3k doc page, I created the following code. The idea is that (say I have a sequence
instance of this class) sequence.seq
should act exactly like sequence[0]
, and sequence.index
should act exactly like sequence[2]
, etc.
It seems to work great, except that I can't seem to access the class variable mapping attributes to the list.
I found this SO question, but either the answer there is wrong, or something is different within methods. I could also use self.__class__.__map__
, but since I need the class variable inside __getattribute__
, that sends me into an infinite recursion loop.
>>> class Sequence(list):
... __map__ = {'seq': 0,
... 'size': 1,
... 'index': 2,
... 'fdbid': 3,
... 'guide': 4,
... 'factors': 5,
... 'clas': 6,
... 'sorttime': 7,
... 'time': 8,
... 'res': 9,
... 'driver': 10 }
...
... def __setattr__(self, name, value): # "Black magic" meta programming to make certain attributes access the list
... print('Setting atr', name, 'with val', value)
... try:
... self[__map__[name]] = value
... except KeyError:
... object.__setattr__(self, name, value)
...
... def __getattribute__(self, name):
... print('Getting atr', name)
... try:
... return self[__map__[name]]
... except KeyError:
... return object.__getattribute__(self, name)
...
... def __init__(self, seq=0, size=0, index=0, fdbid=0, guide=None, factors=None,
... sorttime=None, time=None):
... super().__init__([None for i in range(11)]) # Be sure the list has the necessary length
... self.seq = seq
... self.index = index
... self.size = size
... self.fdbid = fdbid
... self.guide = ''
... self.time = time
... self.sorttime = sorttime
... self.factors = factors
... self.res = ''
... self.driver = ''
...
>>> a = Sequence()
Setting atr seq with val 0
Traceback (most recent call last):
File "<stdin>", line 1, in <module>
File "<stdin>", line 31, in __init__
File "<stdin>", line 17, in __setattr__
NameError: global name '__map__' is not defined
Since none of the methods are called until after Sequence
is fully defined, you can refer to Sequence.__map__
without any trouble. For example:
def __setattr(self, name, value):
print('Setting atr', name, 'with val', value)
try:
self[Sequence.__map__[name]] = value
except KeyError:
object.__setattr__(self, name, value)
As an aside, here's a demonstration that class attributes may be accessed via objects as long as an instance attribute with the same name does not also exist:
class Foo:
i = 3
def __init__(self, overwrite):
if overwrite:
self.i = 4
f = Foo(False)
id(f.i) == id(Foo.i) # Should be True
f = Foo(True)
id(f.i) == id(Foo.i) # Should be False