__index__

[Ned Batchelder]

On 11/1/14 12:56 PM, duncan smith wrote:
> Hello,
>        I have a Bloom filter class and want to (partially) serialize
> instances using hex() or oct(). Instances are mutable, so I can't
> inherit from long. I thought I'd found the answer when I came across
> __index__,
> https://docs.python.org/2/reference/datamodel.html#object.__index__. But
> it doesn't seem to work as I expected it to.
>
>
>>>> class MyClass(object):
> 	def __init__(self):
> 		self.val = 7
> 	def __index__(self):
> 		return self.val
>
> 	
>>>> x = MyClass()
>>>> oct(x)
>
> Traceback (most recent call last):
>    File "<pyshell#76>", line 1, in <module>
>      oct(x)
> TypeError: oct() argument can't be converted to oct
>>>> oct(x.__index__())
> '07'
>>>>
>
>
> Can someone please explain why my thinking is wrong on this? TIA.

Just above your link in the docs is __oct__ and __hex__, which are used 
to implement oct() and hex(): 
https://docs.python.org/2/reference/datamodel.html#object.__oct__

That said, I would simply add a .hex() method on the class.  I would 
never expect a complex thing like a Bloom filter to be turned into 
useful hex with the hex() builtin.

For example, when making an MD5 hash, you use the md5.hexdigest() 
method, not hex(md5).

-- 
Ned Batchelder, http://nedbatchelder.com