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The scenario is quite straight-forward:

I have a model with some fields that are required. Let's say one of them is a TextField which can't be blank. I also have a ModelSerializer (Django Rest Framework) that represents that model.

When an empty string is used to set that field through the serializer the error returned comes from the model itself (This field can't be blank).

I would like to override the error messages only in the serializer level, without the need to explicitly re-specifying every field in the serializer (which I believe is against the DRY principle), having to write a validate_ method for each field and raise my own ValidationError or having to change the error messages in the Model level (because sometimes the context of the error message matters to my use-case and the error message should be given accordingly).

In other words, is there a way to override error messages in the serializer level as easy as it is for a ModelForm:

class MyModelForm(ModelForm):
    class Meta:
        model = MyModel
        error_messages = {"field1": {"required": _("For some reason this is a custom error message overriding the model's default")}}

Answers

EDIT: I see that this question still receives some views, so it is important to note that there's another approach, much cleaner than the original answer I posted here.

You can just use the extra_kwargs attribute of the serializer's Meta class, like so:

class UserSerializer(ModelSerializer):

    class Meta:
        model = User
        extra_kwargs = {"username": {"error_messages": {"required": "Give yourself a username"}}}

Original answer:

Using @mariodev 's answer I created a new class in my project that does that:

from rest_framework.serializers import ModelSerializer, ModelSerializerOptions

class CustomErrorMessagesModelSerializerOptions(ModelSerializerOptions):
    """
    Meta class options for CustomErrorMessagesModelSerializerOptions
    """
    def __init__(self, meta):
        super(CustomErrorMessagesModelSerializerOptions, self).__init__(meta)
        self.error_messages = getattr(meta, 'error_messages', {})

class CustomErrorMessagesModelSerializer(ModelSerializer):
    _options_class = CustomErrorMessagesModelSerializerOptions

    def __init__(self, *args, **kwargs):
        super(CustomErrorMessagesModelSerializer, self).__init__(*args, **kwargs)

        # Run through all error messages provided in the Meta class and update
        for field_name, err_dict in self.opts.error_messages.iteritems():
            self.fields[field_name].error_messages.update(err_dict)

The first one gives the possibility to add a new Meta class attribute to the serializer as with the ModelForm. The second one inherits from ModelSerializer and uses @mariodev's technique to update the error messages.

All is left to do, is just inherit it, and do something like that:

class UserSerializer(CustomErrorMessagesModelSerializer):
    class Meta:
        model = User
        error_messages = {"username": {"required": "Give yourself a username"}}