[Tutor] Question about equality of sets

[Steven D'Aprano]

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.

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



-- 
Steven