On the actual behavior, there is no difference. They all return None and that's it. However, there is a time and place for all of these. The following instructions are basically how the different methods should be used (or at least how I was taught they should be used), but they are not absolute rules so you can mix them up if you feel necessary to.

Using return None

This tells that the function is indeed meant to return a value for later use, and in this case it returns None. This value None can then be used elsewhere. return None is never used if there are no other possible return values from the function.

In the following example, we return person's mother if the person given is a human. If it's not a human, we return None since the person doesn't have a mother (let's suppose it's not an animal or something).

def get_mother(person):
    if is_human(person):
        return person.mother
    else:
        return None

Using return

This is used for the same reason as break in loops. The return value doesn't matter and you only want to exit the whole function. It's extremely useful in some places, even though you don't need it that often.

We've got 15 prisoners and we know one of them has a knife. We loop through each prisoner one by one to check if they have a knife. If we hit the person with a knife, we can just exit the function because we know there's only one knife and no reason the check rest of the prisoners. If we don't find the prisoner with a knife, we raise an alert. This could be done in many different ways and using return is probably not even the best way, but it's just an example to show how to use return for exiting a function.

def find_prisoner_with_knife(prisoners):
    for prisoner in prisoners:
        if "knife" in prisoner.items:
            prisoner.move_to_inquisition()
            return # no need to check rest of the prisoners nor raise an alert
    raise_alert()

Note: You should never do var = find_prisoner_with_knife(), since the return value is not meant to be caught.

Using no return at all

This will also return None, but that value is not meant to be used or caught. It simply means that the function ended successfully. It's basically the same as return in void functions in languages such as C++ or Java.

In the following example, we set person's mother's name and then the function exits after completing successfully.

def set_mother(person, mother):
    if is_human(person):
        person.mother = mother

Note: You should never do var = set_mother(my_person, my_mother), since the return value is not meant to be caught.

There is no such thing as "returning nothing" in Python. Every function returns some value (unless it raises an exception). If no explicit return statement is used, Python treats it as returning None.

So, you need to think about what is most appropriate for your function. Either you should return None (or some other sentinel value) and add appropriate logic to your calling code to detect this, or you should raise an exception (which the calling code can catch, if it wants to).

Functions always return something (at least None, when no return-statement was reached during execution and the end of the function is reached).

Another case is when they are interrupted by exceptions. In this case exception handling will "dominate over the stack" and you will return to the appropriate except or get some nasty error :)

Regarding your problem I must say there are two possibilities: Either you have something to return or you do not have.

• If you have something to return then do so, if not then don't.
• If you rely on something being returned that has a certain type but you cannot return anything meaningful of this type then None will tell the caller that this was the case ( There is no better way to tell the caller that "nothing" is returned than by None, so check for it and you will be fine)

Might not be exactly on topic but I wish more people would code their functions to NOT return None and instead return expected type.

BeautifulSoup is the biggest perpetrator of this. Whenever you parse the tree/soup you'll either get a list of results or a single result, now if no results are found instead of getting an empty list you get None returned, which in turn requires you to make checks on every single lookup which really messes up your code up and makes everything look ridiculous.

switched to lxml/xpath and never looked back.

Like you said, return None is (almost) never needed.

But you should consider that the intention of your code is much clearer with an explicit return None. Remember: a piece of code also needs to be readable by human-beings, and being explicit usually helps.

If a function doesn't specify a return value, it returns None.

In an if/then conditional statement, None evaluates to False. So in theory you could check the return value of this function for success/failure. I say "in theory" because for the code in this question, the function does not catch or handle exceptions and may require additional hardening.