Warn about mutable defaults with ArrayField

[Bryan Rehbein]

I found an issue when upgrading to Django 1.8 and using the new ArrayField and a model's instance methods.

This code will cause Model#array_field's data to bleed between different instances.

def add_to_array(self, item):
    if item not in self.array_field:
        self.array_field.append(item)

However, this code does the correct thing:

def add_to_array(self, item):
    existing = self.array_field

    if item not in self.array_field:
        self.array_field = existing + [item]

I put together a django project/app with the code and tests that show the problems with self.array_field.append(). It also has a Vagrant file setup that should also exhibit the problems.

​https://github.com/redbeard0x0a/django18bug

[Marc Tamlyn]

The issue in your example code is that you have used default=[]. This default is shared between all instances and in this case it is mutable, so you are modifying the default array.

The correct code is to use default=list.

I don't consider this a bug in Django - this is the same behaviour as you would get if you have code like:

def myfunc(foo, bar=[]):
    bar.append(foo)
    return bar

The first call to myfunc(1) would return [1], the second [1, 1], etc.

[Tim Graham]

Could we add a warning about mutable defaults to the ArrayField docs? I guess this also applies to JSONField.

[MZ]

Pull Request Submitted

​https://github.com/django/django/pull/4806

[Tim Graham]

​PR

[Tim Graham <timograham@…>]

In f93e7f5:

Fixed #24690 -- Added a warning about mutable defaults for ArrayField/JSONField.

[Tim Graham <timograham@…>]

In ef5cf564:

[1.8.x] Fixed #24690 -- Added a warning about mutable defaults for ArrayField.

Backport of f93e7f5674436b230e6b4023570939a20c0c6055 from master