Description
When sending a multipart/form-data request to a serializer with a ListField, the default value returned for the field is always the empty list []. Any default= passed to field will be ignored due to the way html input is handled in the get_value() function.
- This functionality works properly when posting
jsondata - May be related to ListField default values not honored in multipart/form-data mode #5807 but I don't think so.
Steps to reproduce
class SimpleSerializer(Serializer):
a = IntegerField(required=True)
c = ListField(default=lambda: [1, 2, 3])
# simulate a multipart/form-data post (querydict is a confusing name here)
s1 = SimpleSerializer(data=QueryDict("a=3"))
s1.is_valid(raise_exception=True)
print("html:", s1.validated_data)
# simulate a JSON post
s2 = SimpleSerializer(data={'a': 3})
s2.is_valid(raise_exception=True)
print("json:", s2.validated_data)
>>> html: OrderedDict([(u'a', 3), (u'c', [])])
>>> json: OrderedDict([(u'a', 3), (u'c', [1, 2, 3])])Expected behavior
- Default value should be honored in HTML mode
Actual behavior
- Default value is always returned as
[]
Explanation and Workaround
The ListField.get_value() function always returns the value of html.parse_html_list from get_value when that field_name was not passed. There is a check at the top for a missing key, but that only affects partial posts. Missing keys on regular POSTs will still be processed normally and return [].
I'm not sure if it is OK to just remove that check for partial and always return empty, but that is what I have done for this version of my workaround. The list fields I use this way are in pure serializers and are only used for validating input. They are not used in ModelSerializers so for my case this is ok.
Initial Workaround
class ListFieldWithSaneDefault(ListField):
"""
This is used ONLY as a base class for other fields. When using it, please ensure that you
always provide a default value (at least `default=lambda: []`) if the field is not required.
Your derived class should take no parameters to __init__, it should be self contained
"""
def get_value(self, dictionary):
"""
When handling html multipart forms input (as opposed to json, which works properly)
the base list field returns `[]` for _missing_ keys. This override checks for that specific
case and returns `empty` so that standard default-value processing takes over
"""
if self.field_name not in dictionary:
return empty
return super(ListFieldWithSaneDefault, self).get_value(dictionary)