Django Model returning NoneType

Question

I have a model Product

it has two fields size & colours among others

colours = models.CharField(blank=True, null=True, max_length=500)
size = models.CharField(blank=True, null=True, max_length=500)

In my view I have

current_product = Product.objects.get(slug=title)
if len(current_product.size) != 0 :
    current_product.size = current_product.size.split(",")

and get this error:

object of type 'NoneType' has no len()

What is NoneType and how can I test for it?

Solution 1

NoneType is the type that the None value has. You want to change the second snippet to

if current_product.size: # This will evaluate as false if size is None or len(size) == 0.
  blah blah

Solution 2

NoneType is Pythons NULL-Type, meaning "nothing", "undefined". It has only one value: "None". When creating a new model object, its attributes are usually initialized to None, you can check that by comparing:

if someobject.someattr is None:
    # Not set yet

Comments

[fishu]

Cheers, I thought I'd tried that. That's why I was trying testing the length. Ah well. You get this on the big jobs

[Ferdinand Beyer]

In fact, Django will raise an exception in this case (Product.NotFound).

[Ignacio Vazquez-Abrams]

Calling it "undefined" is a bit facetious. The variable/name is defined, but it has no useful value.

[ruds]

You do at that. I like this idiom -- it eliminates an "or" that makes the code less readable, and that's easy to forget (as illustrated here :) )

[user1066101]

Please use "is not None" rather than assuming that None is like False -- it make the if statement perfectly clear.

[Carl Meyer]

@S.Lott: I don't see any reason to test for None specifically in this case. The purpose is to test whether current_product.size has any contents, and this does that more clearly than a complicated double-check for "is not None" and len().

[ruds]

@Carl Meyer: Exactly. "is not None" is the correct idiom when you're testing for None-ness, but if you're checking whether a list is both non-None and has contents, use the list as a boolean value.