Newbie question: how to join a list of elements of user-defined types?

[Fernando Pérez]

Fortepianissimo wrote:

> Basically I want to have a subclass of list, like
> 
> class MyList (list):
>     ...
> 
> then I want to
> 
> l=MyList()
> ... do stuff to fill l
> s=" ".join(l)
> 
> with the hope that s will be a nicely joined string with " " as the
> delimiter. Given my C++ background, I tried to define __str__/__repr__
> for both MyList and the user-defined types of the list members,
> assuming the type conversion would silently be invoked. But I was
> wrong!
> 
> Even trying this failed:
> 
> l=[1,2,3]
> s=" ".join(l)
> 
> and I thought built-in types all have default __str__ and __repr__?
> 
> What am I missing here? Many thanks to someone who would take time to
> rescue a poor soul.

Well, this quick solution seems to work. It's probably an ugly/inefficient 
one,  so wait for the gurus for a deep/elegant one ;) But in the meantime 
this may get you going:

In [25]: class mystr(str):
   ....:   def join(self,alist):
   ....:     template = ('%s'+self.__str__()) * (len(alist)-1) + '%s'
   ....:     return template % tuple(alist)
   ....:

In [26]: mystr(' ').join(l)
Out[26]: '1 2 3'

In [27]: l
Out[27]: [1, 2, 3]

cheers,

f