from django import newforms as forms

class SellForm(forms.Form):
	barcode = forms.IntegerField(label='Articolo')
	amount = forms.IntegerField(label='Quantita\'')


views.py

def sell(request):
	if request.method == 'POST':
		form = SellForm(request.POST)
		if form.is_valid():
			try:
				a = Article.objects.get(id = request.POST['barcode'])
			except Article.DoesNotExist:
				error_message = 'L\'articolo selezionato non esiste'
				return render_to_response('warehouse/sell.html', {'form': form, 'error_message': error_message})
			if (a.quantity - int(request.POST['amount']) < 0):
				error_message = 'La quantita\' selezionata supera quella disponibile in magazzino: %s' % str(a.quantity)
				return render_to_response('warehouse/sell.html', {'form': form, 'error_message': error_message})
			else:
				a.quantity -= int(request.POST['amount'])
				a.save()
				return render_to_response('warehouse/sell.html', {'article' : a, 'form': form})
		else:
			return render_to_response('warehouse/sell.html', {'form': form})
	else:
		form = SellForm()
		return render_to_response('warehouse/sell.html', {'form': form})