Django/Python中is not None含义及表单判断逻辑疑问

Hey there! Let's break down your questions one by one to clear up the confusion:

In Python (and by extension Django), is not None is a check to verify that a variable or value is not the None object.

None is Python's way of representing "no value" or "a missing value". The key thing to note here is that this check doesn't care if the value is "empty" (like an empty string "", zero 0, or an empty list []) — those are still valid values, just empty ones. So:

• "" is not None returns True (empty string is not None)
• 0 is not None returns True (zero is not None)
• None is not None returns False

This is different from a simple truthiness check like if my_variable, which would treat empty strings, zero, and empty collections as "falsy" (i.e., the check would fail).

Let's unpack why your current code works in reverse of what you expected:

First, in Django, when a user submits an HTML form without entering anything into a field, request.POST['amount'] will return an empty string "" — not None. The only time request.POST['amount'] would be None is if the field wasn't included in the form at all (and even then, accessing it directly with [] would throw a KeyError; you'd use request.POST.get('amount') to safely get None if the key is missing).

Your current code checks:

if(request.POST['amount'] is not None): 
    # This runs when the value is NOT None (which is always true here!)
    os.remove(os.path.join(settings.MEDIA_ROOT,str(uploaded_file.name) )) 
    messages.success(request,('Invalid Field....')) 
    return redirect('assignFine') 
else: 
    # This only runs if the value IS None (which almost never happens)
    form = Fine(amount=request.POST['amount'],numberPlate=num,policeUsername=request.user) 
    form.save()

Wait, that doesn't align with what you said about it working — so there's probably a coincidence here, or maybe you meant to check if the value is empty instead of checking for None.

The correct way to check if the field is empty (i.e., the user didn't enter anything) is to check if the value is falsy (after stripping whitespace, to catch cases where the user just entered spaces):

amount = request.POST.get('amount', '').strip()
if not amount:
    # Field is empty — show error
    os.remove(os.path.join(settings.MEDIA_ROOT, str(uploaded_file.name)))
    messages.error(request, 'Invalid Field....')  # Pro tip: Use error instead of success for error messages!
    return redirect('assignFine')
else:
    # Field has content — save the form
    form = Fine(amount=amount, numberPlate=num, policeUsername=request.user)
    form.save()

The reason your original code might seem to work is likely a fluke (maybe in your testing, the field was missing from the POST when empty, leading to a None via some edge case). But using the falsy check with .strip() is the reliable way to handle empty form fields.

内容的提问来源于stack exchange，提问作者sayeedk06