"""Helpers methods for marshalling."""

from marshmallow.fields import Field


def create_custom_field(base_class, custom_validations=(),
                        error_messages=None):
    """Build a custom field extending a Marshmallow built-in."""
    if not issubclass(base_class, Field):
        raise ValueError(
            'base_class should be a subclass of marshmallow.Field')

    class _CustomField(base_class):
        def __init__(self, *args, **kwargs):
            """Init method."""
            extended_kwargs = dict(**kwargs)
            extended_kwargs['validate'] = \
                kwargs.get('validate', []) + list(custom_validations)
            super().__init__(*args, **extended_kwargs)

    if error_messages:
        setattr(_CustomField, 'default_error_messages', error_messages)

    return _CustomField