"Collapsing" a list into a list of changes

[John J. Lee]

Steven Bethard <steven.bethard at gmail.com> writes:

> Mike C. Fletcher wrote:
[...]
> >  >>> def changes( dataset ):
> > ...     last = None
> > ...     for value in dataset:
> > ...         if value != last:
> > ...             yield value
> > ...         last = value
> > ...        >>> print list(changes(data ))
> > which is quite readable/elegant IMO.
> 
> But fails if the list starts with None:
> 
> py> lst = [None,0,0,1,1,1,2,2,3,3,3,2,2,2,4,4,4,5]
> py> def changes(dataset):
> ...     last = None
> ...     for value in dataset:
> ...         if value != last:
> ...             yield value
> ...         last = value
> ...
> py> list(changes(lst))
> [0, 1, 2, 3, 2, 4, 5]
> 
> A minor modification that does appear to work:
> 
> py> def changes(dataset):
> ...     last = object()
> ...     for value in dataset:
> ...         if value != last:
> ...             yield value
> ...         last = value
> ...
> py> list(changes(lst))
> [None, 0, 1, 2, 3, 2, 4, 5]

Unless the first object in the list has a weird __cmp__ (does
happen...).  OK, weird __cmp__s are nasty anyway, but still, why
compound it through cleverness when you can write a really plodding
function that *always* does what it says on the tin?

clever-is-evil-ly y'rs,


John