带有格式检查的Django可选字段

Question

我想在注册时将键字段设为可选。 但我不知道为什么我不能在这里使用== None。表单总是响应它期望键字段的数据。 如果没有提供密钥，我如何跳过此检查？仅应在用户提供密钥的情况下进行检查。

def signup(request):
    if request.method == 'POST':
        form = RegistrationForm(request.POST)
        key = Key.from_blob(request.POST['key'].rstrip("\r\n"))[0]
        if form.is_valid():
            if key == None:
                form.save()
                messages.add_message(request, messages.INFO, "Thanks for you Registration, you are now able to login.")
                return redirect(reverse('login'))
            if key.key_algorithm == KeyAlgorithm.RSAEncryptOrSign:
                form.save()
                messages.add_message(request, messages.INFO, "Thanks for you Registration, you are now able to login.")
                return redirect(reverse('login'))
            else:
                messages.add_message(request, messages.INFO, "Only RSA keys are allowed.")
        else:
            return render(request, 'signup.html', {'form': form})
    else:
        form = RegistrationForm()
        args = {'form': form}
        return render(request, 'signup.html', args)