On 3/31/2013 1:57 PM, Byron Ruth wrote:
I submitted this as bug last night: http://bugs.python.org/issue17584 and was
*honored* to be rejected by Raymond Hettinger. However, I would like feedback
on whether my concern (this bug) is justified and clarity if not.
Consider:
```python
class A(object):
def __init__(self):
self.r = iter(range(5))
def __iter__(self):
return self
@property
def next(self):
return next(self.r)
```
The `next` method is a property, however:
A competent Python programmer should not do that. In Py3, the method is
properly renamed '__next__', which should make doing that accidentally
even less likely.
```python
from collections import Iterator
a = A()
isinstance(a, Iterator) # True
next(a) # TypeError: 'int' object is not callable
```
I am using `collections.Iterator` as the means to check if the object is an
iterator,
Being an Iterator only means that it *might* be an iterator.
> however I am not sure if that is _root_ problem here. My
understanding of the iterator protocol is that is assumes the __iter__
and next *methods* are implemented. In the example, `A.next` is defined
as a property, but is still identified as an iterator. To me, this is
incorrect behavior since it's not conforming to the iterator protocol
requirements (i.e. a `next` method, not a property).
There is more to any protocol than can be statically checked.
Raymond stated: "The design of ABCs are to check for the existence to required
named; none of them verify the signature."
Having the required attributes is currently the definition of being an
instance of an ABC. Adding 'not a property' would be possible. but
hardly worthwhile. Checking signatures would be worthwhile, but
signatures are not yet available to Python for C-coded methods, let
alone other implementations.
I think I understand _why_ this is the case.. but I downstream
libraries use `collections.Iterator` to determine if an object _is one_:
see
https://github.com/django/django/blob/master/django/utils/itercompat.py#L22-L31
Who's job is it to check if `next` (and technically `__iter__`) are methods?
The programmer, and a user who does not trust the competence of the
programmer. But this is the least of the possible errors.
--
Terry Jan Reedy
--
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