`ImageFieldFile.url` raises an AttributeError exception after retrieving it from cache

[Pierre Dulac]

Hi everyone,

To reproduce the exception, just cache an instance of ImageFieldFile then retrieve it and call the url property on this retrieved instance.

Here is a minimal code to reproduce :

from django.core.cache import cache
from django.db import models

class A(models.Model):
    pictogram = models.ImageField(upload_to='images')
    class Meta:
        app_label = 'test'

a = A()
a.pictogram = 'fake/test/url.png'
original_picto = a.pictogram

cache_key = 'picto'
cache.set(cache_key, original_picto, 60)
cached_picto = cache.get(cache_key)

print original_picto.url
print cached_picto.url

and you get

AttributeError: 'ImageFieldFile' object has no attribute 'storage'

Didn't tried to fix it yet...

Have a nice day,

Pierre

[Tim Graham]

ImageFieldFile inherits from ImageFile and FieldFile. It looks like the problem is due to the fact that FieldFile.__init__ isn't called when the object is retrieved from the cache. I think the inheritance chain is broken because ImageFile.__init__ (inheriting from django.core.files.base.File) doesn't call super.

[Vajrasky Kok]

Here is the pull request (based on master/1.7) that can fix this problem: ​https://github.com/django/django/pull/1781

[Tim Graham]

I thought fixing the inheritance chain by having django.core.files.base.File.__init__ call super() would be the way to fix this. Thoughts?

[Vajrasky Kok]

I don't understand. django.core.files.base.File has only one parent, which is django.core.files.utils.FileProxyMixin which does not have __init__ method.

I try to do this in django/core/files/base.py:

    def __init__(self, file, name=None):
+       super(File, self).__init__()
        self.file = file
        if name is None:
            name = getattr(file, 'name', None)
        self.name = name
        if hasattr(file, 'mode'):
            self.mode = file.mode
+       # or even this, but it does not work
+       # super(File, self).__init__()

Before resorting to __reduce__ way, I did try to solve the problem by playing with super and __init__ in some classes related with ImageFieldFile, either I missed something or it did not work that way.

[Tim Graham]

Ok, thanks for the response -- I guess my initial hunch wasn't correct. To be honest, I'm not super familiar with the internals of how pickle and __reduce__ work so I have some reading to do. If you could include a quick explanation of your understanding of the problem and why this solution works, that would be quite helpful for me.

[Vajrasky Kok]

Sorry, the original PR does not work with Python 2.7. Also, __reduce__ is prone to error. It's better to use __getstate__. I'll already updated the PR. ​https://github.com/django/django/pull/1781

But first, let me give you the lesson of pickling. :) Naturally, without human intervention, the pickling (pickle.dumps) will copy the instance's dict. So why in this case the all ImageFieldFile instance's dict members (notably the storage attribute) do not get copied. The ImageFieldFile class does not define any method that can intervene the pickling process (__getinitargs__, __getnewargs__, __getstate__, __setstate__, __reduce__, __reduce_ex__). But the ancestor of ImageFieldFile, which is FieldFile, does have method that intervenes pickling process.

    def __getstate__(self):
        # FieldFile needs access to its associated model field and an instance
        # it's attached to in order to work properly, but the only necessary
        # data to be pickled is the file's name itself. Everything else will
        # be restored later, by FileDescriptor below.
        return {'name': self.name, 'closed': False, '_committed': True, '_file'

That's why the storage attribute of ImageFieldFile instance is missing because only name, closed, _committed attributes are copied in the pickling process.

There is an alternative solution for this problem beside my PR, which is removing the __getstate__ method from FieldFile. The downside is the dump result of FieldFile will be unnecessarily bigger (based on my understanding of the comment). I am not sure whether there is another ill effects or not. Attached the patch to do that.

[Tim Graham]

Won't ImageFieldFile be bigger if we override its __getstate__? I'm attaching a patch with a test that shows FieldFile has the same problem (AttributeError: 'FieldFile' object has no attribute 'storage' when unpickling) so I think we need a solution that covers both classes.

[Mariusz Felisiak <felisiak.mariusz@…>]

In aaea9de:

Refs #21238 -- Added more tests for pickling FileField and ImageField.

[Mariusz Felisiak <felisiak.mariusz@…>]

In f600e3f:

Fixed #21238 -- Fixed restoring attributes when pickling FileField and ImageField.