Django FileField（或ImageField）open()方法为有效文件返回None？

Question

let me put it like this:

model.py:

class Task(models.Model):
    ...
    seq_file = models.FileField(upload_to='files/', blank=True, null=True)
    ...

ajax.py (I'm using dajaxice but it doesn't matter):

...
def startTask(request, name):
    task = Task.objects.get(task_name=name)
    data = task.seq_file.open()
    filename = os.path.join(settings.MEDIA_ROOT ,task.seq_file.name)
    if not os.path.isfile(filename):
        raise Exception, "file " + filename + " not found."
    sequences = parser.parse(data.read())
    ...

this returns:

File "/home/mnowotka/Dokumenty/MgrFuncAdnot/app/django-gui/src/gui/ajax.py", line 43, in startTask
sequences = parser.parse(data.read())

AttributeError: 'NoneType' object has no attribute 'read'

but:

...
def startTask(request, name):
    task = Task.objects.get(task_name=name)
    filename = os.path.join(settings.MEDIA_ROOT ,task.seq_file.name)
    if not os.path.isfile(filename):
        raise Exception, "file " + filename + " not found."
    data = open(filename)  
    sequences = parser.parse(data.read())
    ...

works perfectly! Why?

(I'm using django 1.3)

Answer 1

because open method of models.FileField doesn't return anything

you can just use:

task.seq_file.read()

and you don't need calculate path of file for checking if file exist. you can use task.seq_file.path:

if not os.path.isfile(task.seq_file.path):
    ....

Answer 2

A FileField will give you a file-like object and there is no need to call open() on it. In your example, just call task.seq_file.file .

Why is that? There are many storage backends for FileField , and many of them are not backed by a file in disk (think of S3 storage, for example). I guess that is why the documentation says it returns a file-like object, not a file. For some kinds of storage the "open" method makes no sense.

Answer 3

When in doubt, check the code. Here's an excerpt from django.db.models.fields.files :

def open(self, mode='rb'):
    self._require_file()
    self.file.open(mode)
# open() doesn't alter the file's contents, but it does reset the pointer
open.alters_data = True

So, in the case of a FileField , open reopens the file using the specified mode. Then, once you call open , you can continue to use methods like read using the newly applied mode.

Answer 4

Surprisingly but django.db.models.fields.files does not utilize file.storage.exists() method so I had to implement my own small function to have cross-storage compatible check for actual physical file existence:

# Check whether actual file of FileField exists (is not deleted / moved out).
def file_exists(obj):
    return obj.storage.exists(obj.name)