How to make a Django form retain a file after failing validation

The problem with what you want to do is that, for security reasons, browsers will not allow a file input box to have a pre-selected value on page load. This is true even if it is simply preserving the value from a previous instance of the same page. There is nothing Django can do to change this.

If you want to avoid asking the user to re-select and re-upload the file, you will need to save the uploaded file even when validation fails, and then replace the file input field with something to indicate that you already have the data. You would also probably also want a button that runs some JavaScript to re-enable the file field if the user wants to put in a different file. I don't think Django comes with any machinery for this, so you'll have to code it yourself.

Try django-file-resubmit

installation

pip install django-file-resubmit

settings.py

INSTALLED_APPS = {
    ...
    'sorl.thumbnail',
    'file_resubmit',
    ...
}

CACHES = {
    'default': {
        'BACKEND': 'django.core.cache.backends.locmem.LocMemCache',
    },
    "file_resubmit": {
        'BACKEND': 'django.core.cache.backends.filebased.FileBasedCache',
        "LOCATION": project_path('data/cache/file_resubmit')
    },
}

usage

from django.contrib import admin
from file_resubmit.admin import AdminResubmitMixin

class ModelAdmin(AdminResubmitMixin, ModelAdmin):
    pass

or

from django.forms import ModelForm
from file_resubmit.admin import AdminResubmitImageWidget, AdminResubmitFileWidget

class MyModelForm(forms.ModelForm)

    class Meta:
        model = MyModel
        widgets = {
            'picture': AdminResubmitImageWidget,
            'file': AdminResubmitFileWidget, 
        }

Ok, so I first upvoted and implemented Narendra's solution before realizing it couldn't work as javascript can't set input type="file" values. See: http://www.w3schools.com/jsref/prop_fileupload_value.asp

But here's a solution that works:

1. Separate the rest of the form, from the files needing to be uploaded.
2. Always save the file (or run the validation to see if the file should be saved) and save to temporary model.
3. In the template, have a hidden field saying the id of the instance of the temporary model so if you change the file the change propagates. You may get extra files saved on your server, but you can clean up externally.
4. When the form minus any files can be saved, save it, bind the saved files to the originally model and delete the intermediate uploaded file model.

Ok, here's a sketch of the procedure, which works for me of an example where you are trying to save a pdf of a book with an email address (chosen as emails sometimes do not validate) of an author.

models.py

class Book(models.Model):
    pdf = models.FileField("PDF", upload_to="books/")
    author_email = models.EmailField("Author Email")

class TempPDF(models.Model):
    pdf = models.FileField("PDF", upload_to="books/")

forms.py

from project_name.app_name.models import Book, TempPDF
from django.forms import ModelForm
from django.contrib.admin.widgets import AdminFileWidget

class BookForm(ModelForm):
    class Meta:
        model = Book
        exclude = ['pdf',]

class TempPDFForm(ModelForm):
    class Meta:
        model = TempPDF
        widgets = dict(pdf = AdminFileWidget) 
        # The AdminFileWidget will give a link to the saved file as well as give a prompt to change the file.
        # Note: be safe and don't let malicious users upload php/cgi scripts that your webserver may try running.

views.py

def new_book_form(request):
    if request.method == 'POST': 
        ## Always try saving file.  
        try: 
            temp_pdf_inst = TempPDF.objects.get(id=request.POST.has_key('temp_pdf_id'))
        except:  ## should really catch specific errors, but being quick
            temp_pdf_inst = None
        temp_pdf_id = None
        temp_pdf_form = TempPDFForm(request.POST, request.FILES, instance=temp_pdf_inst, prefix='temp_pdf')
        if temp_pdf_form.is_valid() and len(request.FILES) > 0:
            temp_pdf_inst = temp_pdf_form.save()
            temp_pdf_id = temp_pdf_inst.id
        book_form = BookForm(request.POST, prefix='book')

        if book_form.is_valid(): # All validation rules pass
            book = book_form.save(commit=False)
            book.pdf = temp_pdf_inst.pdf
            book.save()
            if temp_pdf_inst != None:
                temp_pdf_inst.delete()
            return HttpResponseRedirect('/thanks/') # Redirect after POST
    else:
        book_form = BookForm() # An unbound form
        temp_pdf_form = TempPDFForm()
        temp_pdf_id = None 
    return render_to_response('bookform_template.html',
                              dict(book_form = book_form,
                                   temp_pdf_form = temp_pdf_form,
                                   temp_pdf_id = temp_pdf_id)
                              )

bookform_template.html

<table>
  {{ book_form }}
  {{ temp_pdf_form }}
  <input type="hidden" name="temp_pdf_id" value="{{ temp_pdf_id }}">
</table>