Changes to UserList.py

[Jakob Schiotz]

--- Yesterday, I <schiotz1 at yahoo.com> wrote:
> Dear Python Gurus,
> 
> Apparently, classes derived from UserList can be initialized with
> real
> lists and with instances themselves derived from UserList but not
> with
> other list-like objects (i.e. objects defining the methods you would
> expect of a list).

The solution is probably to fix UserList.py.  I have attached a fix I
have submitted to GvR.  Here is a short description:

The problem
was that UserLists could not be initialized from any sequence, only
from true lists and other UserLists.  Furthermore, __init__
incorrectly assumed that anything not a true list is a UserList,
causing a confusing AttributeError.  __setslice__, __add__ and
__radd__ are more tolerant, they test if their argument is a
UserList or a true list, and if not they attempt to convert it to a
list with list(s).  The extend method, however, had the same problem
as __init__.

__init__ was changed from 

    def __init__(self, list=None):
        self.data = []
        if list is not None:
            if type(list) == type(self.data):
                self.data[:] = list
            else:
                self.data[:] = list.data[:]

to

    def __init__(self, lst=None):
        self.data = []
        if lst is not None:
            if isinstance(lst, UserList):
                self.data[:] = lst.data[:]
            elif type(lst) == type(self.data):
                self.data[:] = lst
            else:
                self.data = list(lst)

extend was changes similarly.

> [ ... ] and then
> the build-in function list() should be extended to work on any object
> having a __getitem__ method.  The latter change should probably be
> done
> anyway in a later version of Python.

Ooops, I managed to fool myself into thinking that list(x) didn't work,
when the problem was that I used it in __init__ above, but had kept the
argument name as "list" thus hiding the function.  



 Best regards,
 
 Jakob Schiotz
 



=====
Jakob Schiotz, CAMP and Department of Physics, Tech. Univ. of Denmark,
DK-2800 Lyngby, Denmark.  http://www.fysik.dtu.dk/~schiotz/
This email address is used for newsgroups and mailing lists
(spam protection).  Official email: schiotz @ fysik . dtu . dk
When spammed too much, I'll move on to schiotz2 at yahoo.com
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