Question

I had a bit of conditional code that looked like this:

if self.above and self.above.author and self.above.author.username!=self.author.username:
    "notify above.author that someone has replied to their comment"

But if there is no self.above or if either of the comments has no author then I get an error like:

 AttributeError: 'NoneType' object has no attribute 'author'

or

AttributeError: 'NoneType' object has no attribute 'username'

so I can check first before looking for those attributes

if self.above:
    if self.above.author:
        if self.author:
            if self.author.username!=self.above.author.username:
                "notify about response"
        else:
            "notify about response"

but with all the extra ifs and the duplication of the “notify about response” code, the number of lines is more than doubled. There are a lot of times when this problem arises, so if I use the above solution it’s going to mean fifty extra lines of code to check a few simple conditions.

Is there a better way to handle this?

Answer 1

Factor all the object attribute lookups out into a function that handles the missing attributes gracefully. If self.above.author.username is missing, that’s the same situation as self.above being missing as far as you’re concerned — you don’t care where you ran out of runway, just that you did at some point.

def resolve(obj, path):
    """Resolves an attribute path on an object, returning `None` 
       if any attribute is not found"""
    for name in path.split("."):
        obj = getattr(obj, name, None)
        if obj is None:
            break
    return obj

selfauthor  = resolve(self, "author.username")
aboveauthor = resolve(self, "above.author.username")

if not selfauthor or (selfauthor and selfauthor != aboveauthor):
    "notify about response"

You can even put the resolve() method somewhere on a base class, then it’s a nicer:

self.resolve("above.author.username")   # etc