Setting a read-only attribute

[James Stroud]

tleeuwenburg at gmail.com wrote:
> I have an object and wish to set an attribute on it which,
> unfortunately for me, is read-only.
> 
> How can I go about this?

Can you simply subclass the object's class, intercept __setattr__ and 
__getattribute__, and spoof the read-only attribute? E.g.:

class A(object):
   def get_ro(self):
     return 'readonly'
   ro = property(get_ro)

class B(A):
   def __setattr__(self, anatt, aval):
      if anatt == 'ro':
        anatt = '_ro'
      super(A, self).__setattr__(anatt, aval)
   def __getattribute__(self, anatt):
     if anatt == 'ro' and hasattr(self, '_ro'):
       anatt = '_ro'
     return super(A, self).__getattribute__(anatt)

The output:

py> class A(object):
...   def get_ro(self):
...     return 'readonly'
...   ro = property(get_ro)
...
py> class B(A):
...   def __setattr__(self, anatt, aval):
...      if anatt == 'ro':
...        anatt = '_ro'
...      super(A, self).__setattr__(anatt, aval)
...   def __getattribute__(self, anatt):
...     if anatt == 'ro' and hasattr(self, '_ro'):
...       anatt = '_ro'
...     return super(A, self).__getattribute__(anatt)
...
py> a = A()
py> print a.ro
readonly
py> a.ro = 4
------------------------------------------------------------
Traceback (most recent call last):
   File "<ipython console>", line 1, in <module>
<type 'exceptions.AttributeError'>: can't set attribute

py>
py> b = B()
py> b.x = 10
py> print b.x
10
py> print b.ro
readonly
py> b.ro = 4
py> print b.ro
4


I'm not so sure this is the least clunky way to do this.

James

-- 
James Stroud
UCLA-DOE Institute for Genomics and Proteomics
Box 951570
Los Angeles, CA 90095

http://www.jamesstroud.com/