On 04/25/2017 05:50 PM, Mark Jones wrote:
I'm keying off hasattr(obj, 'keys') which is working. Still seems
strange that it is returning a non-exported class though.
Python's own namedtuple() produces a custom class that itself is only a
subclass of tuple:
>>> from collections import namedtuple
>>> n = namedtuple('hi', ['x', 'y', 'z'])
>>> n1 = n(1, 2, 3)
>>> type(n1)
<class '__main__.hi'>
>>> isinstance(n1, n)
True
>>> isinstance(n1, namedtuple)
Traceback (most recent call last):
File "<stdin>", line 1, in <module>
TypeError: isinstance() arg 2 must be a class, type, or tuple of classes
and types
>>> isinstance(n1, tuple)
True
Above, there's no "exported" class we can compare this custom tuple
against except for the ad-hoc class that is linked to the specific keys,
there's no way to see that it was derived from namedtuple, which itself
is not a class. SQLAlchemy's own named tuple object is going for the
same type of thing - it's a tuple with keys on it, and that's all anyone
needs to know. If SQLAlchemy began using the regular
collections.namedtuple() approach (which we currently do not as it does
not have the best performance characteristics for SQLA's use case), this
would again break code that is tying to a specific kind of tuple
implementation.
Here's a Python3 standard library example. the list_iterator type.
Where is it exported?
>>> iter_ = iter([1, 2, 3])
>>> type(iter_)
<class 'list_iterator'>
Seems to claim it's in "builtins":
>>> type(iter_).__module__
'builtins'
but it's not!
>>> import builtins
>>> builtins.list_iterator
Traceback (most recent call last):
File "<stdin>", line 1, in <module>
AttributeError: module 'builtins' has no attribute 'list_iterator'
other things in "builtins" seem to be...
>>> getattr.__module__
'builtins'
>>> builtins.getattr
<built-in function getattr>
How do we test if something is a list_iterator then? Check it against
the ABC in this case collections.Iterable:
>>> import collections
>>> isinstance(iter_, collections.Iterable)
True
list_iterator itself is not mentioned in the Python 3 documentation at all:
https://docs.python.org/3/search.html?q=list_iterator&check_keywords=yes&area=default
There's lots of precedent for libraries returning sequences, iterables,
and mappings that themselves are not an "exported" type (I have
recollections that this might be commonplace in libraries like lxml,
numpy, but I'd have to test), but that one is from the standard library
itself.
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