run-time object creation in Python

Question

I wish to make a relatively fleeting object in Python. In Javascript, which has a similar internal semantic for object management (a lookup table) you can do the following:

/* Hopefully I'm not so out of practice with JS that this would cause an error: */
var not_a_defined_class;
not_a_defined_class.this_property_exists_as_of_this_line = 1

In Python, you cannot. The equivalent would be something like the following:

not_a_defined_class = object()
not_a_defined_class.__dict__['this_property_exists_as_of_this_line'] = 1

Evidently, dot-notation to access a member of a class is syntactic sugar:

class DefinedClass(object):
    __init(self):
        self.predefined_property = 2

defined_object = DefinedClass()
defined_object.predefined_property = 5
# Is syntactic sugar for:
defined_object.__dict__['predefined_property'] = 5
# But is read-only
defined_object.undefined_property = 6 # AttributeError

My questions then are as follows:

Is there a difference between .__dict__['predefined_property'] = 5 and .predefined_property = 5?
Is dot-notation read-only outside class definitions (i.e. other than self.new_property =)? (As far as I can tell this is the case)
If so, why? Type safety?
Is there a way I can work around this? Is there a method called by dot-notation that I can recklessly override in my own class, say MessyObject?

Of course, I could use a dictionary object to similar effect. I'm really asking this question to learn more.

Aaron Digulla · Accepted Answer · 2014-07-03 10:24:08Z

3

The reason you get an error is that object is a Python class defined in C. Those can't be extended. The same is true for other internal / C-based classes like str and list:

 > a = 'x'
 > a.foo = 1
 Traceback (most recent call last):
   File "<stdin>", line 1, in <module>
 AttributeError: 'str' object has no attribute 'foo'

But you can extend those classes:

>>> class MyString(str): pass
... 
>>> a = MyString()
>>> a.foo = 1
>>> len(a)
0

Re #1: For classes defined in Python code: Usually not. There are some corner cases which is why you should use setattr() instead.

Re #2 and #3: No. As I said above, this is only true for internal types.

Re #4: See above.

answered Jul 3, 2014 at 10:24

Aaron Digulla

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