Why does python use two underscores for certain things?

Question

I'm fairly new to actual programming languages, and Python is my first one. I know my way around Linux a bit, enough to get a summer job with it (I'm still in high school), and on the job, I have a lot of free time which I'm using to learn Python.

One thing's been getting me though. What exactly is different in Python when you have expressions such as

x.__add__(y) <==> x+y x.__getattribute__('foo') <==> x.foo 

I know what methods do and stuff, and I get what they do, but my question is: How are those double underscore methods above different from their simpler looking equivalents?

P.S., I don't mind being lectured on programming history, in fact, I find it very useful to know :) If these are mainly historical aspects of Python, feel free to start rambling.

Answer 1

Here is the creator of Python explaining it:

... rather than devising a new syntax for special kinds of class methods (such as initializers and destructors), I decided that these features could be handled by simply requiring the user to implement methods with special names such as __init__, __del__, and so forth. This naming convention was taken from C where identifiers starting with underscores are reserved by the compiler and often have special meaning (e.g., macros such as __FILE__ in the C preprocessor).

...

I also used this technique to allow user classes to redefine the behavior of Python's operators. As previously noted, Python is implemented in C and uses tables of function pointers to implement various capabilities of built-in objects (e.g., “get attribute”, “add” and “call”). To allow these capabilities to be defined in user-defined classes, I mapped the various function pointers to special method names such as __getattr__, __add__, and __call__. There is a direct correspondence between these names and the tables of function pointers one has to define when implementing new Python objects in C.