I've a question about Python2.2 properties, setattr, getattr and friends.
I've implemented a file object, which depending on how you open it picks a
different implementation of a stream and uses that.
Currently I have code that looks like this:
def __setattr__(self, attr, value):
"Make some attributes readonly."
if attr in ['mode', 'name', 'closed', 'encoding']:
if hasattr(self, attr):
raise AttributeError, "'%s' is a read-only Attribute" % attr
# actually, write-once
object.__setattr__(self, attr, value)
(in file, which inherits from object). This works fine. When it comes to
__getattr__ on the file object, what I want, if file does not provide its own
attribute is getattr(self.fd, attr) of whatever stream I have ended up making
fd.
But Armin said that 2.2 provided properties (which I hadn't paid any attention
to) which would do this in a neater way. Hmmm, I thought. But I cannot
seem to get them to work. According to Guido's doc which I read here,
http://www.python.org/2.2.3/descrintro.html#property
it looks to me as if they don't do what I want. First I experimented with this:
Python 2.3.4 (#2, Sep 24 2004, 08:39:09)
[GCC 3.3.4 (Debian 1:3.3.4-12)] on linux2
Type "help", "copyright", "credits" or "license" for more information.
>>> class C(object):
... def __init__(self):
... self.mode = 'rU'
... def getval(self, val):
... return self.__dict__[val]
... mode = property(getval, None)
...
>>> a = C()
Traceback (most recent call last):
File "<stdin>", line 1, in ?
File "<stdin>", line 3, in __init__
AttributeError: can't set attribute
---
Ok, this property takes effect even in __init__, rats.
Try again:
>>> class C(object):
... def __init__(self):
... self.mode = 'rU'
... def getval(self, val):
... return self.__dict__[val]
... def setval(self, attr, val):
... if hasattr(self, attr):
... raise AttributeError, "'%s' is a read-only Attribute" % attr
... # actually, write-once
... mode = property(getval, setval)
...
>>> a = C()
Traceback (most recent call last):
File "<stdin>", line 1, in ?
File "<stdin>", line 3, in __init__
TypeError: setval() takes exactly 3 arguments (2 given)
---
Well, this looks as if I would have to write a separate setval for each
attribute
I want to pass once, and there is no way to give them all the same setval
method.
Am I missing something obvious? Probably. But properties look singularily
useless
to me right now. I suppose you could get around this limitation using
metaclasses,
but that looks like getting out a monkeywrench to crack a very small nut ...
Laura
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