Consider three functions:
def my_func1(): print "Hello World" return None def my_func2(): print "Hello World" return def my_func3(): print "Hello World" They all appear to return None. Are there any differences between how the returned value of these functions behave? Are there any reasons to prefer one versus the other?
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 atleast 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.
return NoneThis 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 so).
def get_mother(person): if is_human(person): return person.mother else: return None returnThis 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 tho you don't need it that often.
We 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 cause 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 the 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.
return at allThis 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.
Yes, they are all the same.
We can review the interpreted machine code to confirm that that they're all doing the exact same thing.
import dis def f1(): print "Hello World" return None def f2(): print "Hello World" return def f3(): print "Hello World" dis.dis(f1) 4 0 LOAD_CONST 1 ('Hello World') 3 PRINT_ITEM 4 PRINT_NEWLINE 5 5 LOAD_CONST 0 (None) 8 RETURN_VALUE dis.dis(f2) 9 0 LOAD_CONST 1 ('Hello World') 3 PRINT_ITEM 4 PRINT_NEWLINE 10 5 LOAD_CONST 0 (None) 8 RETURN_VALUE dis.dis(f3) 14 0 LOAD_CONST 1 ('Hello World') 3 PRINT_ITEM 4 PRINT_NEWLINE 5 LOAD_CONST 0 (None) 8 RETURN_VALUE They each return the same singleton None -- There is no functional difference.
I think that it is reasonably idiomatic to leave off the return statement unless you need it to break out of the function early (in which case a bare return is more common), or return something other than None. It also makes sense and seems to be idiomatic to write return None when it is in a function that has another path that returns something other than None. Writing return None out explicitly is a visual cue to the reader that there's another branch which returns something more interesting (and that calling code will probably need to handle both types of return values).
Often in Python, functions which return None are used like void functions in C -- Their purpose is generally to operate on the input arguments in place (unless you're using global data (shudders)). Returning None usually makes it more explicit that the arguments were mutated. This makes it a little more clear why it makes sense to leave off the return statement from a "language conventions" standpoint.
That said, if you're working in a code base that already has pre-set conventions around these things, I'd definitely follow suit to help the code base stay uniform...