Is calling __init__ from __new__ in OWSLib/owslib/feature/wfs110.py necessary?

[vjf]

I have been trying to 'pickle' instances of "WebFeatureService_1_1_0", but this is difficult because the code calls __init__ from within __new__.

When unpickling the object, 'pickle' calls __new__ to create the instance

(From https://docs.python.org/3/library/pickle.html#pickling-class-instances)

def load(cls, attributes):
obj = cls.__new__(cls)
obj.__dict__.update(attributes)
return obj

then because this calls __init__, it tries to fully re-create the object, which makes the use of pickle pointless!

In the end, I had to override WebFeatureService_1_1_0.__new__ with one that does not call __init__ to get pickle to work.

Also I understand that python calls __new__ then calls __init__ when creating objects, (https://docs.python.org/3/reference/datamodel.html#object.__new__)

This means that __init__ is called twice!

Is this a bug, or is there a good reason for calling __init__ from within __new__?

If it is a bug, I'd be happy to submit a pull request for this and the other WFS versions.

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[geographika]

Resolved with #548

[geographika]

@vjf - do you have an example of pickling the WFS class? I get errors as lxml classes can't be pickled:

import pickle
from owslib.wfs import WebFeatureService

wfs_url = "https://demo.mapserver.org/cgi-bin/wfs?"
wfs = WebFeatureService(url=wfs_url, version="2.0.0")

with open("wfs_object.pkl", "wb") as f:
    pickle.dump(wfs, f)
    # TypeError: cannot pickle 'lxml.etree._Element' object

Just wondering what approach you used (the dill library also fails).

[vjf]

@geographika

It was a long time ago, I think I was using 'diskcache'

This will fail with a TypeError when calling 'pickle.load()' if OWSLib < v0.32.1:

from owslib.wfs import WebFeatureService
from diskcache import Cache
import copyreg
from lxml import etree

wfs_url = "https://demo.mapserver.org/cgi-bin/wfs?"
wfs_dk = WebFeatureService(url=wfs_url, version="2.0.0")


def element_unpickler(data):
    return etree.fromstring(data)

def element_pickler(element):
    data = etree.tostring(element)
    return element_unpickler, (data,)

copyreg.pickle(etree._Element, element_pickler, element_unpickler)

def elementtree_unpickler(data):
    data = StringIO(data)
    return etree.parse(data)

def elementtree_pickler(tree):
    data = StringIO()
    tree.write(data)
    return elementtree_unpickler, (data.getvalue(),)

copyreg.pickle(etree._ElementTree, elementtree_pickler, elementtree_unpickler)


cache = Cache()
key = cache.push(wfs_dk)
print(cache.pop(key))