Terry J. Reedy added the comment:
The problem with the Iterable ABC is that 'iterable' and 'iterator' are
*dynamically* defined, with a possibly infinite time required to possibly
destructively check either definition. In general, an algorithmic *static*
check can only guess whether an object is iterable, though humans analyzing
enough code can potentially get it right. Therefore, using isinstance(ob,
Iterable) is not 100% reliable, and in my opinion *should not be used* as the
definition of lower-case 'iterable'.
Definition: Object ob is iterable if 'iter(ob)' returns an iterator. For the
reasons given above, iter may return a non-iterator, but it will if ob
implements either the old or new iterator protocol. If ob has .__iter__, iter
returns ob.__iter__(). If ob has .__getitem__, iter returns iterator(ob),
where iterator is a hidden internal class that embodies the old iterator
protocol by defining a .__next__ method that calls .__getitem__. In both
cases, iter does the best it can by assuming that the methods are correctly
written as per one of the two protocols.
Loose definition: Object 'it' is iterable if it can be looped over.
Python definition: Object 'it' is iterable if repeated 'next(it)' calls either
return an object or raise StopIteration. This means that
try:
while True:
next(it)
except StopIteration:
pass
runs, possibly forever, without raising.
As Raymond noted, an iterator can be created multiple ways: IteratorClass(),
iter(ob), iter(func, sentinal), generator_func().
---
Iterable versus iter with respect to classes with __getitem__:
Iter was added in 2.2. Built-in iterables were only gradually converted from
old to new protocol, by adding a new .__iter__. So even ignoring user classes,
iter *had* to respect .__getitem__. Even today, though only a small fraction
of classes with .__getitem__ are iterable, people do not generally call iter()
on random objects.
Iterable (added 2.6) is documented as the "ABC for classes that provide the
__iter__() method." In other words, isinstance(ob, Iterable) replaces
hasattr(ob, '__iter__'). Except that the former is more than that. The magic
word 'register' does not appear in the collections.ABC doc, and I think that
this is the omission to be remedied.
"ABC for classes that provide the __iter__() method, or that provide a
__getitem__ method that implements the old iterator protocol and register
themselves as Iterable."
An example could be given using a patched version of IsIterable.
If one adds two lines of code
from collections.abc import Iterable
...
Iterable.register(IsIterable)
then isinstance(IsIterable(3), Iterable) is True, except that this is a lie in
the other direction.
Traceback (most recent call last):
File "F:\Python\mypy\tem.py", line 17, in <module>
for i in it2:
File "F:\Python\mypy\tem.py", line 7, in __getitem__
return self.data[key]
TypeError: 'int' object is not subscriptable
Either IsIterable.__init__ must check that data itself has .__getitem__ or
IsIterable.__next__ must capture exceptions and raise IndexError instead.
def __getitem__(self, key):
try:
return self.data[key]
except Exception:
raise IndexError
----------
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Python tracker <rep...@bugs.python.org>
<http://bugs.python.org/issue18558>
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