Retaining uploaded file (FileField) when form is not valid - django

Question

Given a Model with a required field called number and a ClearableFileInput FileField called upload_file:
class ExampleModel(models.Model):
number = models.IntegerField()
upload_file = models.FileField(blank=True, null=True)
In my view, on POST, if the form is_valid then I can populate the clearable part of the FileField when returning to the same page.
def example_view(request):
context = RequestContext(request)
if request.method == 'POST':
form = ExampleForm(request.POST, request.FILES)
if form.is_valid():
form_instance = form.save()
form = ExampleForm(instance=form_instance)
# or alternatively, for just the upload file field
form = ExampleForm(initial={'upload_file': form_instance.upload_file})
else:
form_instance = form.save(commit=False)
form = ExampleForm(initial={'upload_file': form_instance.upload_file})
# unfortunately, this also does not work:
form = ExampleForm(initial={'upload_file': form.fields['upload_file']})
else:
form = ExampleForm()
return render_to_response('enterrecords/example.html', {'form': form}, context)
This is how it looks:
However, if the form is not valid (see first else case), I can not form.save(commit=False) and therefore cannot populate the clearable part of the FileField.
The form.save(commit=False) gives the following error:
ValueError at /en/myapp/example/
The ExampleModel could not be created because the data didn't validate.
Is there a workaround for this problem?
For completeness...
ModelForm
class ExampleForm(forms.ModelForm):
class Meta:
model = ExampleModel
Template
<form enctype="multipart/form-data" method="POST" action="{% url 'eg' %}">
{% csrf_token %}
{{ form.as_p }}
<button type="submit" name="next" value="Next" />Next</button>
</form>

Answer 1

Use the form.clean_xxx with xxx being the field to clean in your form. It is called before the call to is_valid() of your view.
Here's a sample of what I do when I get an Excel file in my form (my UploadedFileHandler class is useless here, it's just to show you the principle).
The idea is that, even if the form is not valid, the Excel file is always saved and thus I keep a trace of what happened:
def clean_excel_file(self):
uploaded_file = self.files.get('excel_file')
if uploaded_file:
try:
nom = UploadedFileHandler.generate_filename(
splitext(basename(uploaded_file.name))[1])
dst = UploadedFileHandler.get_url(nom, 'imports/excel/')
# sauver le fichier
dst_full = UploadedFileHandler.full_filename(dst)
UploadedFileHandler.make_dir(dst_full)
UploadedFileHandler.save(uploaded_file, dst_full)
retour = ExcelFile.objects.create(
# description = nom de fichier sans l'extension :
description=path.splitext(basename(str(uploaded_file)))[0],
fichier_origine=uploaded_file,
excel_file=dst)
retour.save()
return retour
except IOError:
self.errors['excel_file'] = ErrorList([_("Unknown file type")])
return None