Introducing Python to others

Mensanator mensanator at aol.com
Thu Mar 26 19:16:00 CET 2009


On Mar 26, 11:42 am, "andrew cooke" <and... at acooke.org> wrote:
> David C. Ullrich wrote:
> > In article <mailman.2701.1238060157.11746.python-l... at python.org>,
> >  "Paddy O'Loughlin" <patrick.olough... at gmail.com> wrote:
>
> > Here's my favorite thing about Python (you'd of course
> > remark that it's just a toy example, doing everything
> > in as dumb but easily understood way as possible):
>
> > x=[1,2]
>
> > print x+x
>
> > class Vector():
> >   def __init__(self, data):
> >     self.data = data
> >   def __repr__(self):
> >     return repr(self.data)
> >   def __add__(self, other):
> >     return Vector([self.data[0]+other.data[0],
> >                   self.data[1]+other.data[1]])
>
> > x = Vector([1,2])
>
> > print x+x
>
> that's cute, but if you show them 2.6 or 3 it's even cuter:
>
> >>> from operator import add
> >>> class Vector(list):
>
> ...   def __add__(self, other):
> ...     return map(add, self, other)
> ...>>> x = Vector([1,2])
> >>> x+x
>
> [2, 4]
>
> andrew

Mind if I ask a question? In DU's code, both operands have to
be instances of the Vector class?

>>> x = Vector([1,2])
>>> x+x
[2, 4]
>>> x+[3,3]

Traceback (most recent call last):
  File "<pyshell#60>", line 1, in <module>
    x+[3,3]
  File "<pyshell#55>", line 7, in __add__
    return SV([self.data[0]+other.data[0],self.data[1]+other.data[1]])
AttributeError: 'list' object has no attribute 'data'


Whereas with your version, "other" just has to be an iterable.

>>> x = Vector([1,2])
>>> x+x
[2, 4]
>>> x+[3,3]
[4, 5]
>>> x+(9,9)
[10, 11]
>>> x+{3:4,4:9}
[4, 6]

Although it does require the same number of elements (because that's
required by map and could be changed if necessary).

>>> x+[3,3,3]

Traceback (most recent call last):
  File "<pyshell#71>", line 1, in <module>
    x+[3,3,3]
  File "<pyshell#62>", line 3, in __add__
    return map(add,self,other)
TypeError: unsupported operand type(s) for +: 'NoneType' and 'int'


What would you have to do to make this work?

>>> x+x+x      # expecting [3,6]
[2, 4, 1, 2]




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