Why do the return value of the first and second calls of image field accessor differ?

Question

The first call of the accessor of the image field of a BlobWrapper returns None and the second call returns a BlobWrapper. Some hint?

In [1]: id = app.Plone.invokeFactory('Image', 'myimage', title='MyImage')

In [2]: image=app.Plone[id]

In [3]: blobWrapper=image.getBlobWrapper()

In [4]: accessor=blobWrapper.getField('image').getAccessor(blobWrapper)

In [5]: accessor()

In [6]: accessor()
Out[6]: <plone.app.blob.field.BlobWrapper at 0xa35a194>

Answer 1

The getAccessor method of the field takes the object in this case your ATImage instance as parameter not a blobWrapper instance

>>> accessor = image.getField('image').getAccessor(image)
>>> accessor()
<plone.app.blob.field.BlobWrapper object at 0x1050446e0>

Check this github link

security.declarePublic('getAccessor')
def getAccessor(self, instance):
    """Return the accessor method for getting data out of this
    field"""
    if self.accessor:
        return getattr(instance, self.accessor, None)
    return None

UPDATE: Explenation why the first call of the accessor is None and second not.

I has something to do with the magic's of Products.Archetypes. If you pass the BlobWrapper instance instead of a ATImage instance, the following happens:

First call: At some point the get method of Archetypes ObjectField gets called, See https://github.com/plone/Products.Archetypes/blob/master/Products/Archetypes/Field.py#L785

It catches the AttributeError (AttributeError: Attribut...'image'), which btw. usually happens if you add a new Field to a ContentType, because the BlobWrapper instance has no Attribute image (The ATImage would have this attribute).

Then it initializes your BlobWrapper like a new field on the ATImage. This persists some crap on your Blobwrapper instance, which you don't want :-) The return value is None, see https://github.com/plone/Products.Archetypes/blob/master/Products/Archetypes/Field.py#L803

Second call: Since the first call did some modifications to your BlobWrapper instance, the second call works and returns the correct value. https://github.com/plone/Products.Archetypes/blob/master/Products/Archetypes/Field.py#L785

Technically the first call initializes an AnnotationStorage on your BlobWrapper instance.

> /Users/maethu/.buildout/eggs-cache/Products.Archetypes-1.9.4-py2.7.egg/Products/Archetypes/Storage/annotation.py(62)get()
-> def get(self, name, instance, **kwargs):
(Pdb) l
 57             """Clean up data in set method
 58             """
 59             raise NotImplementedError
 60
 61         security.declarePrivate('get')
 62  ->     def get(self, name, instance, **kwargs):
 63             ann = getAnnotation(instance)
 64             value = ann.getSubkey(self._key, subkey=name, default=_marker)
 65             if value is _marker:
 66                 if self._migrate:
 67                     value = self._migration(name, instance, **kwargs)
(Pdb) name
'image'
(Pdb) instance
<plone.app.blob.field.BlobWrapper object at 0x10d4042a8>
(Pdb) getAnnotation(instance)
{'Archetypes.storage.AnnotationStorage-image': <plone.app.blob.field.BlobWrapper object at 0x10d430a28>}

I really think you should not do this :-)

If I got something wrong pls correct... it's kinda hard to understand this Archetypes behaviour.