Can bin() be overloaded like oct() and hex() in Python 2.6?

As you've already discovered, you can't override bin(), but it doesn't sound like you need to do that. You just want a 0-padded binary value. Unfortunately in python 2.5 and previous, you couldn't use "%b" to indicate binary, so you can't use the "%" string formatting operator to achieve the result you want.

Luckily python 2.6 does offer what you want, in the form of the new str.format() method. I believe that this particular bit of line-noise is what you're looking for:

>>> '{0:010b}'.format(19)'0000010011'

The syntax for this mini-language is under "format specification mini-language" in the docs. To save you some time, I'll explain the string that I'm using:

1. parameter zero (i.e. 19) should be formatted, using
2. a magic "0" to indicate that I want 0-padded, right-aligned number, with
3. 10 digits of precision, in
4. binary format.

You can use this syntax to achieve a variety of creative versions of alignment and padding.

I think the short answer is 'No, bin() can't be overloaded like oct() and hex().'

As to why, the answer must lie with Python 3.0, which uses __index__ to overload hex(), oct() and bin(), and has removed the __oct__ and __hex__ special functions altogether.

So the Python 2.6 bin() looks very much like it's really a Python 3.0 feature that has been back-ported without much consideration that it's doing things the new Python 3 way rather than the old Python 2 way. I'd also guess that it's unlikely to get fixed, even if it is considered to be a bug.

The bin function receives it's value from the object's __index__ function. So for an object, you can define the value converted to binary, but you can't define the format of the string.