Guido van Rossum wrote:
> This PEP is incomplete without specifying exactly which built-in and
> stdlib types should be registered as String instances.
>
> I'm also confused -- the motivation seems mostly "so that you can skip
> iterating over it when flattening a nested sequence" but earlier you
> rejected my "Atomic" proposal, saying "Earlier in the thread it was
> made clear that that atomicity is not an intrinsic property of a type;
> instead it varies across applications [...]". Isn't this String
> proposal just that by another name?
>
> Finally, I fully expect lots of code writing isinstance(x, String) and
> making many more assumptions than promised by the String ABC. For
> example that s[0] has the same type as s (not true for bytes). Or that
> it is hashable (the Sequence class doesn't define __hash__). Or that
> s1+s2 will work (not in the Sequence class either). And many more.
I think the PEP also needs to explain why having multiple small one-off
string ABCs is a bad thing. The whole point of providing a standard ABC
mechanism is to enable exactly that: allowing a library to say "Here is
my concept of what a string class needs to provide - register with this
ABC to tell me that I can use your class without blowing up
unexpectedly". The library can then preregister a bunch of other classes
it knows about that do the right thing (such as the builtin str type)
That is, to write a flatten operation with ABC's you might do something
along the lines of:
from abc import ABCMeta
class Atomic(metaclass=ABCMeta):
"""ABC for iterables that the flatten function will not expand"""
Atomic.register(str) # Consider builtin strings to be atomic
def flatten(obj, atomic=Atomic):
itr = None
if not isinstance(obj, atomic):
try:
itr = iter(obj)
except (TypeError, AttributeError):
pass
if itr is not None:
for item in itr:
for x in flatten(item):
yield x
else:
yield obj
Cheers,
Nick.
--
Nick Coghlan | ncoghlan at gmail.com | Brisbane, Australia
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