Processing file uploads before object is saved - django

Question

I've got a model like this:
class Talk(BaseModel):
title = models.CharField(max_length=200)
mp3 = models.FileField(upload_to = u'talks/', max_length=200)
seconds = models.IntegerField(blank = True, null = True)
I want to validate before saving that the uploaded file is an MP3, like this:
def is_mp3(path_to_file):
from mutagen.mp3 import MP3
audio = MP3(path_to_file)
return not audio.info.sketchy
Once I'm sure I've got an MP3, I want to save the length of the talk in the seconds attribute, like this:
audio = MP3(path_to_file)
self.seconds = audio.info.length
The problem is, before saving, the uploaded file doesn't have a path (see this ticket, closed as wontfix), so I can't process the MP3.
I'd like to raise a nice validation error so that ModelForms can display a helpful error ("You idiot, you didn't upload an MP3" or something).
Any idea how I can go about accessing the file before it's saved?
p.s. If anyone knows a better way of validating files are MP3s I'm all ears - I also want to be able to mess around with ID3 data (set the artist, album, title and probably album art, so I need it to be processable by mutagen).

Answer 1

You can access the file data in request.FILES while in your view.
I think that best way is to bind uploaded files to a form, override the forms clean method, get the UploadedFile object from cleaned_data, validate it anyway you like, then override the save method and populate your models instance with information about the file and then save it.

Answer 2

a cleaner way to get the file before be saved is like this:
from django.core.exceptions import ValidationError
#this go in your class Model
def clean(self):
try:
f = self.mp3.file #the file in Memory
except ValueError:
raise ValidationError("A File is needed")
f.__class__ #this prints <class 'django.core.files.uploadedfile.InMemoryUploadedFile'>
processfile(f)
and if we need a path, ther answer is in this other question

Answer 3

You could follow the technique used by ImageField where it validates the file header and then seeks back to the start of the file.
class ImageField(FileField):
# ...
def to_python(self, data):
f = super(ImageField, self).to_python(data)
# ...
# We need to get a file object for Pillow. We might have a path or we might
# have to read the data into memory.
if hasattr(data, 'temporary_file_path'):
file = data.temporary_file_path()
else:
if hasattr(data, 'read'):
file = BytesIO(data.read())
else:
file = BytesIO(data['content'])
try:
# ...
except Exception:
# Pillow doesn't recognize it as an image.
six.reraise(ValidationError, ValidationError(
self.error_messages['invalid_image'],
code='invalid_image',
), sys.exc_info()[2])
if hasattr(f, 'seek') and callable(f.seek):
f.seek(0)
return f