boolean operations on sets

[Stargaming]

On Mon, 06 Aug 2007 14:13:51 +0000, Flavio wrote:

> Hi, I have been playing with set operations lately and came across a
> kind of surprising result given that it is not mentioned in the standard
> Python tutorial:
> 
> with python sets,   intersections  and unions are supposed to be done
> like this:
> In [7]:set('casa') & set('porca')
> Out[7]:set(['a', 'c'])
> 
> In [8]:set('casa') | set('porca')
> Out[8]:set(['a', 'c', 'o', 'p', 's', 'r'])
> 
> and they work correctly. Now what is confusing is that if you do:
> 
> In [5]:set('casa') and set('porca')
> Out[5]:set(['a', 'p', 'c', 'r', 'o'])
> 
> In [6]:set('casa') or set('porca')
> Out[6]:set(['a', 'c', 's'])
> 
> The results are not what you would expect from an AND  or OR operation,
> from the mathematical point of view! aparently the "and" operation is
> returning the the second set, and the "or" operation is returning the
> first.

That might be, because `and` and `or` are not mathematical in Python (at 
least not as you think). All the operator bits, e.g. `|` and `&`, are 
overloadable. You can just give them any meaning you want.

The `and` and `or` operator, though, are implemented in Python and there 
is no way you can make them behave different from how they do it by 
default. It has been discussed to remove this behaviour or make them 
overloadable as well but this hasn't made it far, as far as I remember.

> If python developers wanted these operations to reflect the traditional
> (Python) truth value for data structures: False for empty data
> structures and True otherwise, why not return simply True or False?

Because in the most cases, returning True of False simply has no 
advantage. But returning the actual operands has been of fairly large 
use, e.g. for replacing the if expression ("ternary operator") ``THEN if 
COND else DEFAULT`` with ``COND and THEN or DEFAULT`` (which has some bad 
corner cases, though).

> So My question is: Why has this been implemented in this way? I can see
> this confusing many newbies...

Hmm, you could be right there. But they shouldn't be biased by default 
boolean behaviour, then, anyways.