[Python-Dev] Iterable String Redux (aka String ABC)

[Terry Reedy]

"Armin Ronacher" <armin.ronacher at active-4.com> wrote in message 
news:loom.20080527T192243-415 at post.gmane.org...
| Basically *the* problematic situation with iterable strings is something 
like
| a `flatten` function that flattens out every iterable object except of 
strings.

In most real cases I can imagine, this is way too broad.  For instance, 
trying to 'flatten' an infinite iterable makes the flatten output one also. 
Flattening a set imposes an arbitrary order (but that is ok if one feeds 
the output to set(), which de-orders it).  Flattening a dict decouples keys 
and values.  Flattening iterable set-theoretic numbers (0={}, n = {n-1, 
{n-1}}, or something like that) would literaly yield nothing.

| Imagine it's implemented in a way similar to that::
|
|    def flatten(iterable):
|        for item in iterable:
|            try:
|                if isinstance(item, basestring):
|                    raise TypeError()
|                iterator = iter(item)
|            except TypeError:
|                yield item
|            else:
|                for i in flatten(iterator):
|                    yield i

I can more easily imagine wanting to flatten only certain classes, such and 
tuples and lists, or frozensets and sets.

def flatten(iterable, classes):
    for item in iterable:
        if type(item) in classes:
             for i in flatten(item, classes):
                 yield i
        else:
            yield item

| A problem comes up as soon as user defined strings (such as UserString) 
is
| passed to the function.  In my opinion a good solution would be a 
"String"
| ABC one could test against.

This might be a good idea regardless of my comments.

tjr