在 django/python 中，什么不是 None 代表？

Question

I want to display a not valid message when my html form field is empty. But the code below triggers the "if block" when the field is empty and the "else block" when the field is not empty.

I wrote it this way because this seems to do the job. But I'm confused. Shouldn't it be the other way around?

if(request.POST['amount'] is not None):

   os.remove(os.path.join(settings.MEDIA_ROOT,str(uploaded_file.name) ))
            messages.success(request,('Invalid Field....'))
            return redirect('assignFine')
else:
   form = Fine(amount=request.POST['amount'],numberPlate=num,policeUsername=request.user)
            form.save()

Answer 1

This condition:

if request.POST['amount'] is not None:

Means: if the parameter exists (is not None ), do the block below, otherwise do the else part. For this to make sense, if the amount exists we should create a Fine , otherwise we signal an error. Your conditions appear to be switched!

Depending on the expected values of the amount parameter, I'd also consider using this version:

if request.POST['amount']:
  # create fine
else:
  # signal an error

It's shorter, and means: if the amount is not None , or '' , or False , or 0 or any other falsey values in Python, do the block below, otherwise do the else part.

Answer 2

You can use the phrase below to check if amount parameter exists in request.POST :

if 'amount' in request.POST:
    # Your code
else:
    # Else case

You can use not in operator to have negative if/else phrase:

if 'amount' not in request.POST:
    # No 'amount' available
else:
    # There is 'amount' parameter

[Edit]

And to check if the value is not empty , False , or None you can check it in this way:

if 'amount' in request.POST and request.POST['amount']:
    # 'amount' exists, and is not 'empty', 'False', or 'None'