say we have a model like that:

class Data(models.Model):
    file_name = models.CharField(max_length=100)
    upload_file = models.FileField(upload_to='data')

we what create a file in back,but not save to filesystem. so use StringIO create string first

from cStringIO import StringIO

my_str = 'qwer\nasdf\m\n'

buff = StringIO(my_str)

buff.seek(0,2)

use InMemoryUploadFile to create the string file

from django.core.files.uploadedfile import InMemoryUploadedFile
from app.models import Data
file_name = 'myfile.txt'
file_data = InMemoryUploadedFile(buf,'file',file_name,None,buff.tell(),None)

d = Data(file_name = file_name)
d.upload_file.save(file_name,file_data)
d.save() 

untile now we create an InMemoryUploadedFile object and save it to our model.

by the way we can use simpler way to create the file

from django.core.files.base import ContentFile

file_data = ContentFile(buff.getvalue())