Read-only attributes in Python.

gistfile1.py

In [1]: class Foo:
   ...:     def __init__(self,x):
   ...:         self._x = x
   ...:     def getx(self):
   ...:         return self._x
   ...:     x = property(getx)
   ...:     
   ...:     

In [2]: f = Foo(5)

In [3]: f.x
Out[3]: 5

In [4]: f.x = 6

In [5]: f.x
Out[5]: 6

In [6]: f._x
Out[6]: 5

In [7]: class Foo(object):
   ...:     def __init__(self,x):
   ...:         self._x = x
   ...:     def getx(self):
   ...:         return self._x
   ...:     x = property(getx)
   ...:     
   ...:     

In [8]: f = Foo(5)

In [9]: f.x
Out[9]: 5

In [10]: f.x = 6
---------------------------------------------------------------------------
AttributeError                            Traceback (most recent call last)

/home/seanh/<ipython console> in <module>()

AttributeError: can't set attribute

In [11]: 

To create a read-only attribute in Python you simply use the property built-in and pass an fget parameter but not an fset. Now if user code tries to set the value of the read-only attribute AttributeError will be raised.

What confused me for ages is that for property() to work properly the class has to be a new-style class, i.e. it has to inherit from object. (I think in Python 3 maybe all classes will inherit from object implicitly, like in Java.)

In the first attempt above Foo is an old-style class, and the read-only attribute doesn't work as expected, writing the attribute produces unexpected behaviour. In the second attempt Foo inherits from object, and attempting to write the read-only attribute raises an exception.

Also see: Properties: attributes managed by get/set methods from Guido's article Unifying types and classes in Python 2.2.