Refactor this Python code to iterate over a container

Question

Surely there is a better way to do this?

results = []
if not queryset is None:
    for obj in queryset:
        results.append((getattr(obj,field.attname),obj.pk))

The problem is that sometimes queryset is None which causes an exception when I try to iterate over it. In this case, I just want result to be set to an empty list. This code is from a Django view, but I don't think that matters--this seems like a more general Python question.

EDIT: I turns out that it was my code that was turning an empty queryset into a "None" instead of returning an empty list. Being able to assume that queryset is always iteratable simplifies the code by allowing the "if" statement to be removed. The answers below maybe useful to others who have the same problem, but cannot modify their code to guarantee that queryset is not "None".

Answer 1

you can use list comprehensions, but other than that I don't see what you can improve

result = []
 if queryset:
     result = [(getattr(obj, field.attname), obj.pk) for obj in queryset]

Answer 2

results = [(getattr(obj, field.attname), obj.pk) for obj in queryset or []]

Answer 3

How about

for obj in (queryset or []):
    # Do your stuff

It is the same as J.F Sebastians suggestion, only not implemented as a list comprehension.

Answer 4

For what it's worth, Django managers have a "none" queryset that you can use to avoid gratuitous None-checking. Using it to ensure you don't have a null queryset may simplify your code.

if queryset is None:
    queryset = MyModel.objects.none()

References:

• Django documentation on none()
• Null Object Pattern