Multiple form fields can share the same widget instance

[Michal Petrucha]

Say we have a custom form field that defines a certain HTML class on its widget:

class CustomChoiceField(ChoiceField):
    widget = Select(attrs={'class': 'my-custom-class'})

If this field type is used for multiple fields in the same form, they will all share the same widget instance. This leads to conflicts when setting the fields' choices; since they all use the same widget instance, all fields will render the same choices from the widget even though the fields themselves might have different choices. The following test case fails, for example:

class TestForm(Form):
    field1 = CustomChoiceField(choices=[])
    field2 = CustomChoiceField(choices=[])

f = TestForm()
f.fields['field1'].choices = [("1", "1")]
f.fields['field2'].choices = [("2", "2")]
assert f.fields['field1'].widget.choices == [("1", "1")]
assert f.fields['field2'].widget.choices == [("2", "2")]

I took a deep dive, and found out that since the field instances themselves share the widget instance (as it's a class attribute that doesn't need to get instantiated in each field instance), when fields are getting deepcopied in BaseForm.__init__, their copies all share one new copy of the widget instance.

This is quite easy to fix by deepcopying the widget in the case where it was specified as an instance, rather than a widget class. Patch coming up in a couple of minutes.

[Tim Graham <timograham@…>]

In 09da1e79:

Fixed #27369 -- Prevented widgets from being shared between form field instances.