Field.get_default will stringify everything that isn't a callable

[Enrico Zini]

I'm using a JSON-encoded field that can take any python data structure as default, this way:

info = fields.ContactInfoField("Contact info", blank=True, default={"a":u"I ♥ Django"})

and found out that I was getting "{'a': u'I \\u2665 Django'}" as a default value in my model objects.

I tracked it down to Field.get_default, which stringifies all non-callables:

    def get_default(self):
        """
        Returns the default value for this field.
        """
        if self.has_default():
            if callable(self.default):
                return self.default()
            return force_unicode(self.default, strings_only=True)
        if (not self.empty_strings_allowed or (self.null and
                   not connection.features.interprets_empty_strings_as_nulls)):
            return None
        return ""

Indeed this works as expected:

info = fields.ContactInfoField("Contact info", blank=True, default=lambda:{{"a":u"I ♥ Django"})

I cannot understand the reason for the different treatment of values and callables, and comments don't help. Field documentation does not mention this behaviour either. From what little I can understand, this looks like a bug,

[Aymeric Augustin]

Duplicate of #8633, and the reason is explained there.

[Enrico Zini]

Thanks for linking this to #8633, I'm sorry I didn't find it myself.

I reckon the information in #8633 is rather relevant for people creating custom field types. Would it make sense to have it mentioned in the Field documentation, rather than hidden in an old bug log?

[Aymeric Augustin]

Well, it took me a bit of time to find it, and I wasn't aware of this reason :)

Accepting as a documentation issue.

[Tim Graham <timograham@…>]

In [6e2bb344e40dafdf462f6fb660837fa061faf549]:

Fixed #18478 - Documented how to use a mutable default in a model field.

[Tim Graham <timograham@…>]

In [cd5181f84c7d4f2002efb777b6cf042771135f7a]:

[1.4.X] Fixed #18478 - Documented how to use a mutable default in a model field.

Backport of 6e2bb344e4 from master