Python classes, difference between setting an attribute and using __setattr__()

Question

I'm trying to set attributes to a class of which I don't know the name a-priori. I also want to avoid users to write to that attribute, so I use a property factory with getters and setters which returns a property object. However, when calling the property object, I get the reference to that object, instead of whatever the getter should be returning.

So I try to do this:

def property_factory(name):
 def getter(self):
 return self.__getattribute__(name)
 def setter(self, value):
 raise Exception('Cannot set a value')
 return property(getter, setter)
# This is just a read_file placeholder
class read_file(object):
 def __init__(self):
 self.name = 'myName'
 self.value = 'myValue'
 def __iter__(self):
 return self
class a(object):
 list1 = read_file()
 def __init__(self):
 list1 = read_file()
 self.__setattr__('_' + list1.name, list1.value)
 # this doesn't work:
 self.__setattr__(list1.name, property_factory('_' + list1.name))
 
 # this actually does work, but with the wrong attribute name:
 notMyName = property_factory('_' + list1.name)

Then I get this:

In [38]: b = a()
In [39]: b.myName
Out[39]: <property at 0x2883d454450>
In [40]: b.notMyName
Out[40]: 'myValue'
In [41]: b.notMyName = 'true'
---------------------------------------------------------------------------
Exception Traceback (most recent call last)
Cell In[41], line 1
----> 1 b.notMyName = 'true'
Cell In[37], line 6, in property_factory.<locals>.setter(self, value)
 5 def setter(self, value):
----> 6 raise Exception('Cannot set a value')
Exception: Cannot set a value

What I want is this:

In [39]: b.myName
Out[40]: 'myValue'
In [41]: b.MyName = 'true'
---------------------------------------------------------------------------
Exception Traceback (most recent call last)
Cell In[41], line 1
----> 1 b.MyName = 'true'
Cell In[37], line 6, in property_factory.<locals>.setter(self, value)
 5 def setter(self, value):
----> 6 raise Exception('Cannot set a value')
Exception: Cannot set a value

How do I do this?

• ken: property must be attached to a class object, not to an instance. Try: setattr(self.__class__, list1.name, property_factory('_' + list1.name)). However, in this way all instances of a will have the same named properties.
• Mezzerine: This actually also works indeed. Apply property to a class, need to remember that :). Thanks!

Answer 1

Why do you want to do this? I've always gone back to this answer whenever I have an idea that uses the notion of dynamically-named attributes -- which is essentially what you're trying to do here if I'm not mistaken (with added read-only "protection" applied only to the keys in list1). Do you need to use a property factory? You could do something like this:

class A(object):
 list1 = read_file()
 def __init__(self):
 self.__dict__[A.list1.name] = A.list1.value
 def __setattr__(self, name, value):
 if name == A.list1.name:
 raise Exception('Cannot set a value for this key!')

Now at least this works:

>>> b = A()
>>> b.myName
'myValue'
>>> b.myName = 'true'
Traceback (most recent call last):
 File "<stdin>", line 1, in <module>
 File "<stdin>", line 7, in __setattr__
Exception: Cannot set a value for this key!

However both methods will be susecptable to the following:

>>> b.__dict__['myName'] = 'true'
>>> b.myName
'true'

Obviously there's a lot of optimization to be done here, adding sentinels, name mangling, etc, plus I'd need a lot more information regarding ultimately what you're trying to achieve and why -- but is this getting a little closer to what you want? I'll delete this answer (or tidy) if necessary, too long for a comment. Also, typo:

In [41]: b.MyName = 'true'

Should be myName.

• Mezzerine: Thanks! The property factory was not a goal on its own, just my way of implementing write protection for stuff in list1. Your method is much cleaner, I like it.