Using __dict__ in Python and Private Attributes

In the last phase of the World Crises project, I was designed with creating the search functionality. Given the reasonable amount of data types in our Google app engine datastore, I soon tried to search out a way to iterate over the datastore without creating multiple loops with the same body. The problem is that these types are represented as fields of different app engine model objects. How would someone iterate over the instance fields of a class?

Well, the solution I came up with was to use the __dict__ variable in python. __dict__ is a "private" (Is anything really private in Python?) variable which contains the attributes in a class instance: all functions, variables, etc. Using this special field I am able to make a list of the model names I want to search, and then access them using the __dict__ dictionary.

Why does the variable start with two underscores? This is how private attributes are handled in Python. They aren't actually private, of course, because you can still access them, but they do lend themselves to be fairly awkward and ugly if you do try to use them. If you try to create attributes that start with two underscores, they will have their names mangled like so:


>>> class A:
...     def __init__(self) :
...             self.__x = 5
... 
>>> A().__x
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
AttributeError: A instance has no attribute '__x'
>>> A()._A__x
5

As you can see, if you try and make a variable starting with two underscores yourself, Python will mangle it by prepending an underscore with the class name. Another interesting thing is that names starting with _ will not be imported with "from X import *".

The more you know...