Which style of return is "better" for a method that might return None?

Question

I have a method that will either return an object or None if the lookup fails. Which style of the following is better?

def get_foo(needle):
    haystack = object_dict()
    if needle not in haystack: return None
    return haystack[needle]

or,

def get_foo(needle):
    haystack = object_dict()
    try:
        return haystack[needle]
    except KeyError:
        # Needle not found
        return None

I'm undecided as to which is more more desirable myself. Another choice would be return haystack[needle] if needle in haystack else None, but I'm not sure that's any better.

Answer 1

For this particular example, it looks like the dict method get is the most concise:

def get_foo(needle):
    haystack = object_dict()
    return haystack.get(needle)

In general, in Python, people tend to prefer try/except than checking something first - see the EAFP entry in the glossary. Note that many "test for membership" functions use exceptions behind the scenes.

I won't start a flamewar on the relative merits of multiple returns or the alternatives :)

Answer 2

if both do exactly the same, choose that one that you like the most and thats more readable - i like the second one, but both are good.

(maybe one if this is faster than the other one - so if preformance is realy important, test it and choose the faster one)

Answer 3

In every language I've used the first version is perferable. Exceptions are basically goto's in disguise with many of the same problems, so I don't use them if I can avoid it.

There is also a possible performance cost. I don't know about Python, but in many other languages there is a heavy cost for creating the exception that you are going to throw away on the next line.