How do i flatten error messages to display a single consistent error message in Django? - django

Question

I am making a mobile app that uses Django Rest Framework. When one of my models fails validation, it uses one of the model.attributes as a key inside the error message, for example:
{'status_code': 400, 'name': [u'Ensure this field has no more than 32 characters.']}
{'status_code': 400, 'password': [u'Ensure this field has no more than 32 characters.']}
{'status_code': 400, 'arbitrary_field': [u'Ensure this field has no more than 32 characters.']}
This is very difficult to scale in a mobile, so I want to take the error messages and deliver a consistent 'error' key to the mobile device. For example,
{'status_code': 400, 'error': [u' Name: Ensure this field has no more than 32 characters.']}
{'status_code': 400, 'error': [u'Password: Ensure this field has no more than 32 characters.']}
{'status_code': 400, 'error': [u'Arbitrary Field: Ensure this field has no more than 32 characters.']}
In Rails, I could do this by saying:
model.errors.full_messages
But I'm not sure what the equivalent in Django is?
Thanks

Answer 1

You can define a property custom_full_errors in your serializer which will return the errors formatted according to your requirement. Doing serializer.custom_full_errors will give you the desired response.
We first call the serializer.errors to get the default errors dictionary. Then we iterate on this dictionary and create our desired response.
class MySerializer(serializers.ModelSerializer):
#property
def custom_full_errors(self):
"""
Returns full errors formatted as per requirements
"""
default_errors = self.errors # default errors dict
errors_messages = []
for field_name, field_errors in default_errors.items():
for field_error in field_errors:
error_message = '%s: %s'%(field_name, field_error)
errors_messages.append(error_message) # append error message to 'errors_messages'
return {'error': errors_messages}
...
For example:
# serializer.errors
{'name': [u'Ensure this field has no more than 32 characters.']}
will translate to
# serializer.custom_full_errors
{'error': [u'Name: Ensure this field has no more than 32 characters.']}
In case of multiple errors for a single field, we will get the following output on doing serializer.custom_full_errors:
# serializer.custom_full_errors
{'error': [u'Name: Error1', u'Name: Error2', u'Password: Error1', u'Password: Error2' ]}

Answer 2

I read this blog article, am yet to test it out
https://masnun.com/2015/11/06/django-rest-framework-custom-exception-handler.html
You create a custom error handler that concatenates the key-value pairs, after passing the exception through the default error handler method
from rest_framework.views import exception_handler
def custom_exception_handler(exc, context):
# Call REST framework's default exception handler first,
# to get the standard error response.
response = exception_handler(exc, context)
if response is not None:
data = response.data
response.data = {}
errors = []
for field, value in data.items():
errors.append("{} : {}".format(field, " ".join(value)))
response.data['errors'] = errors
response.data['status'] = False
response.data['exception'] = str(exc)
return response
Then you point DRF to your custom error handler
REST_FRAMEWORK = {
'DEFAULT_AUTHENTICATION_CLASSES': (
'rest_framework.authentication.BasicAuthentication',
'rest_framework.authentication.SessionAuthentication',
'rest_framework.authentication.TokenAuthentication',
),
'EXCEPTION_HANDLER': 'api.utils.custom_exception_handler'
}

Answer 3

In case somebody is still looking for a solution, here is a great library that helps a lot in dealing with custom error validation: https://pypi.org/project/drf-standardized-errors/, here is the GitHub repo: https://github.com/ghazi-git/drf-standardized-errors
The response will be flat and instead of this:
{
"shipping_address":
{
"non_field_errors": [ErrorDetail("We do not support shipping to the provided address.", code="unsupported")]
}
}
You'll get this:
{
"code": "unsupported",
"detail": "We do not support shipping to the provided address.",
"attr": "shipping_address.non_field_errors"
}
It deals pretty good with List Serializers, changing such response:
{
"recipients": [
{"name": [ErrorDetail("This field is required.", code="required")]},
{"email": [ErrorDetail("Enter a valid email address.", code="invalid")]},
]
}
To this:
{
"type": "validation_error",
"errors": [
{
"code": "required",
"detail": "This field is required.",
"attr": "recipients.0.name"
},
{
"code": "invalid",
"detail": "Enter a valid email address.",
"attr": "recipients.1.email"
}
]
}
Based on this flat response it's much easier to form one string message to represent all the errors if you need to.
Basically, it handles the hardest part - getting a flat response out of a complex Django structure (all the nested fields, dicts, lists, etc.)

Answer 4

I know it is a bit late but I made this one using backtracking in case you have a lot of embedded nested serialisers.
from rest_framework.views import exception_handler
def custom_exception_handler(exc, context):
# Call REST framework's default exception handler first,
# to get the standard error response.
response = exception_handler(exc, context)
stack = [response.data]
response.data = {}
errors = {}
if response is not None:
while len(stack) > 0:
data = stack.pop()
for field, value in data.items():
if type(value) is dict:
stack.append(value)
else:
if value is not list:
errors[field] = value
continue
for val in value:
code = val.code
message = str(val)
errors[field] = {
'message': message,
'code': code
}
response.data['errors'] = errors
return response