CompressedTextField Django Model Field

fields.py

# Simple wrapper over the BinaryField. Assumes it is given text and automatically compresses
# and decompresses upon access. Intended as a space saving mechanism on what
# would normally be bulk text that isn't accessed very often.
# Tested on Python3.4 / Django 1.7
from django.db import models
from django.utils.translation import ugettext_lazy as _
import lzma


class CompressedTextField(models.BinaryField, metaclass=models.SubfieldBase):
    description = _("LZMA Compressed Text for BinaryFields")
    empty_values = [None, b'']

    def get_db_prep_value(self, value, connection, prepared=False):
        if value is not None:
            return lzma.compress(value.encode())
        return value

    def value_to_string(self, obj):
        return lzma.decompress(self._get_val_from_obj(obj)).decode()

    def to_python(self, value):
        if value is not None:
            if not type(value) in (str,):
                return lzma.decompress(value).decode()
            else:
                return value
        return value