[Tutor] totalViruses[i] /= float(numTrials),

[Alan Gauld]

On 24/04/13 20:32, Oscar Benjamin wrote:
> On 24 April 2013 20:07, Alan Gauld <alan.gauld at btinternet.com> wrote:
>> On 24/04/13 16:52, Dave Angel wrote:
>>
>>>> Does it mean? ;  totalViruses[i] = totalViruses[i]/float(numTrials)
>>>>
>>> As the others have said, that's exactly right, at least if
>>> totalViruses[i] is immutable, like an int or a float.
>>
>>
>> What difference does immutability make here?

> The subtle distinction that Dave is referring to is about modifying an
> object in place vs rebinding a name to a newly created object.

Sure, the bejhaviour with regard to the objects after the opreation is 
slightly different. But it makes noi dsifference to the duality of

x += n
v
x = x + n

The mutability issues are identical.

> Which behaviour occurs is entirely up to the author of the __iadd__
> method of the class in question. This method can modify and return
> self or it can return a different object.

This does make a difference because iadd (etc) can make a difference and 
I had forgotten that there are a separate set of operator methods for 
the ixxx operations, I was assuming that ixxx called the standard xxx. 
So in the special case of a user type implementing xxx differently to 
ixxx the behaviour might differ. But for all other cases I can think of 
the duality will still work? Apart from this exception

x += y

will yield the same result as

x = x + y

I think....

-- 
Alan G
Author of the Learn to Program web site
http://www.alan-g.me.uk/