mapping a method to a list?

[Steven D. Majewski]

On Wed, 8 Aug 2001, Tim Hochberg wrote:

> 
> <gyromagnetic at excite.com> wrote in message
> news:3b716759.126576344 at 127.0.0.1...
>
> > Is there a way to map the .upper method to these items? Something
> > like, for example,
> > >>>>map(?.upper, val).
> 
> Did you try the obvious thing? It should work. For example:
> 
>    class Monty:
>        def spammify(self, text):
>            return "spam & %s" % text
> 
>    l = ["eggs", "baked beans", "sausage"]
>    m = Monty()
> 
>    print map(m.spammify, l)
  [...]
> 
> Or did you mean something completely different?

I think he meant (from the example) to map a method onto a list of
instances. 

If they are all instances of the same class, then you can map the 
unbound class method to the list of instances:

>>> class X:
...     def __init__( self, val ):
...             self.val = val
...     def value(self):
...             return self.val
... 
>>> things = map( X, range(5) )
>>> map( X.value, things )
[0, 1, 2, 3, 4]

However, if there are any subclasses in the list, you may not get
the answer you're expecting:

>>> class Y(X):
...     def value(self):
...             return self.__class__.__name__ + ':' + str(self.val)
... 
>>> Y(999).value()
'Y:999'
>>> things.append( Y( 999 ) )
>>> map( X.value, things )
[0, 1, 2, 3, 4, 999]


And it won't work at all if any of the classes are not subclasses of 
the class method's class:

>>> map( Y.value, things )
Traceback (most recent call last):
  File "<stdin>", line 1, in ?
TypeError: unbound method value() must be called with instance as first
argument



So the best method is either to wrap the method in a lambda function:

>>> map( lambda x: x.value(), things )
[0, 1, 2, 3, 4, 'Y:999']


Or, in 2.*, us a list comprehension:

>>> [ x.value() for x in things ]
[0, 1, 2, 3, 4, 'Y:999']


-- Steve M.