Empty BinaryField != b'' on Python 2

[wkschwartz@…]

Problem

In Python 2, at least under SQLite, the initial value for an empty binary field behaves inconsistently. The ORM thinks it's an empty bytes: b''. The database connection manager thinks it's a buffer. Now, the buffer evaluates to False and has zero length. So it'll mostly work. But not always -- and most importantly to me, not in my unit tests!

See ​http://stackoverflow.com/q/12625557/1286628

Note this was not a problem under Python 3.4.

Steps to Reproduce

Using Python 2.7.8, SQLite, and either Django 1.7rc2 or Django @ edcc75e5ac5b9dc2f174580e7adacd3be586f8bd (HEAD at the time of this writing; the error exists in both places)

1. Make a new project and a new app and add the app to settings.py
2. Fill in app/models.py as follows

from django.db import models                                                                                                         
class BinModel(models.Model):                                                                                                        
    data = models.BinaryField()                                                                                                      

3. Run from the command line:

(venv) $ ./manage.py makemigrations app && ./manage.py migrate && ./manage.py shell

4. Run from the resulting Python shell

>>> from app import models; m = models.BinModel(); m.save(); n = models.BinModel.objects.get()
>>> m.data
''
>>> m.data == b''
True
>>> n.data
<read-write buffer ptr 0x10eaa62b0, size 0 at 0x10eaa6270>
>>> n.data == b''
False
>>> bool(n.data)
False
>>> len(n.data)
0
>>> bytes(n.data)
''

Note that the same problem persisted when I had a default value for the field. There was no problem under Python 3.4.

[Collin Anderson]

I'm seeing the same behavior on django 1.6.0 and stable/1.6.x, so I don't think this is a release blocker.

[Areski Belaid]

I dont think it's SQLite related, I have the same output using PostgreSQL

[Baptiste Mispelon]

Marking this as accepted per previous comments.

This seem like it might be caused by six using buffers on python 2 [1].

Thanks.

[1] ​https://github.com/django/django/blob/master/django/utils/six.py#L681

[ppYang]

I'm not sure, if it's only a problem with empty BinaryField.

I try to add a test function in model_fields.tests.BinaryFieldTests, as below:

    def test_retrieve_compare_non_empty_field(self):
        dm = DataModel(data=b'test')
        self.assertTrue(dm.data == b'test')
        dm.save()
        dm = DataModel.objects.get(pk=dm.pk)
        self.assertTrue(dm.data == b'test')

On python2, the first assertion will be passed, and the second one will be failed.

As the data field is a buffer type after retrieving, we can't do a simple compare.

We can see the definition:

class BinaryField(Field):
    ...
    def get_db_prep_value(self, value, connection, prepared=False):
        value = super(BinaryField, self).get_db_prep_value(value, connection, prepared)
        if value is not None:
            return connection.Database.Binary(value)
        return value

I can't go further as I know little about six, and hope this can help:)