form.fields is still tied to class variables

[Michael Nelson]

From django.forms.forms.BaseForm.init, it seems the intention is that a form's self.fields should be safe to update

# The base_fields class attribute is the *class-wide* definition of
# fields. Because a particular *instance* of the class might want to
# alter self.fields, we create self.fields here by copying base_fields.
# Instances should always modify self.fields; they should not modify
# self.base_fields.
self.fields = deepcopy(self.base_fields)

Yet

In [1]: from django import forms


In [5]: class MyForm(forms.Form):
    title = forms.ChoiceField(choices=(('mr', 'Mr.'), ('mrs', 'Mrs.')))
   ...:     def __init__(self, *args, **kwargs):
   ...:         super(MyForm, self).__init__(*args, **kwargs)
   ...:         self.fields['title'].choices += [('you', 'who')]
   ...: 

In [7]: f1 = MyForm()

In [8]: f1.fields['title'].choices
Out[8]: [('mr', 'Mr.'), ('mrs', 'Mrs.'), ('you', 'who')]

In [9]: f2 = MyForm()

In [10]: f2.fields['title'].choices
Out[10]: [('mr', 'Mr.'), ('mrs', 'Mrs.'), ('you', 'who'), ('you', 'who')]

In [11]: f3 = MyForm()

In [12]: f3.fields['title'].choices
Out[12]: 
[('mr', 'Mr.'),
 ('mrs', 'Mrs.'),
 ('you', 'who'),
 ('you', 'who'),
 ('you', 'who')]

In [17]: f3.base_fields['title']
Out[17]: <django.forms.fields.ChoiceField object at 0x26cc910>

In [18]: f3.fields['title']
Out[18]: <django.forms.fields.ChoiceField object at 0x26ccb10>

In [19]: f3.base_fields['title'].choices
Out[19]: 
[('mr', 'Mr.'),
 ('mrs', 'Mrs.'),
 ('you', 'who'),
 ('you', 'who'),
 ('you', 'who')]

So it seems that the instance field is definitely a deep copy, but the attributes of are not. A friend then pointed out Field.deepcopy:

​http://code.djangoproject.com/browser/django/trunk/django/forms/fields.py#L193

which explains why the attributes are shared, but not whether this affect is intentional.