boolean operations on sets

Flavio fccoelho at
Mon Aug 6 16:13:51 CEST 2007

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.

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

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

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