[Tutor] Question about equality of sets

[Jim Byrnes]

On 04/05/2014 01:15 PM, Steven D'Aprano wrote:
> On Sat, Apr 05, 2014 at 12:46:19PM -0500, Jim Byrnes wrote:
>> Ubuntu 12.04 python 3.3
>>
>> I was working through an exercise about sets. I needed to find the
>> duplicates in a list and put them in a set.  I figured the solution had
>> to do with sets not supporting duplicates.  I finally figured it out but
>> along the way I was experimenting in idle and got some results I don't
>> understand.
>>
>>>>> s = {1,2,3}
>>>>> s
>> {1, 2, 3}
>>>>> s.add(1) == s    # <1>
>> False
>>>>> s.add(1) == s.add(2)    # <2>
>> True
>>>>>
>>
>> Neither <1> or <2> changes s, so why is <1> False and <2> True ?
>
> You're making an assumption about what s.add returns. You're assuming it
> returns a new set. It doesn't. Try this:
>
>
> print(s.add(100))
>
>
> and see what it prints.

Actually my assumption was worse than that.  I was thinking that because 
it would not add a dup it would end up being {1,2,3} == {1,2,3} 
completely forgetting that the left side would return None.

Thanks,  Jim


> set.add modifies the set in place. So calling s.add(1) tries to change
> s, it doesn't create a new set. It is standard in Python that methods
> that change the object in place normally return None:
>
> list.append
> set.add
> list.sort
> list.reverse
> dict.update
>
> etc. So your examples try:
>
> None == s  # this is false
> None == None  # but this is true
>
>
>