what is the correct way to return a value inside of a method? in python

Question

This function makes a range of dates (this function is related to a class):

def _makeRangeDates(self, desde, ultima_carga):
    dates = []
    for fechaRequest in rrule(DAILY, dtstart=desde, until=ultima_carga):
        dates.append(fechaRequest)
    return self.dates

What is the best way to return the list of dates inside of this function?

is mandatory to return self.dates ?

Regards. Nelson.

Answer 1

You can't return self.dates here. You are appending your dates in dates variable and that should be returned inside the method. In your case, self.dates might not even exist.

Edit: I think this is what you are trying to implement

class abc():
    def __init__(self):
        self.dates = []

    def _makeRangeDates(self,desde,ultima_carga):
        for fechaRequest in rrule(DAILY, dtstart=desde, until=ultima_carga):
            self.dates.append(fechaRequest)
        return self.dates

Answer 2

I'm assuming that you've got this function in some sort of class foo like so:

class foo(object):
    def __init__(self):
        self.dates = []

    def _makeRangeDates(self,desde,ultima_carga):
        dates = []
        for fechaRequest in rrule(DAILY, dtstart=desde, until=ultima_carga):
            dates.append(fechaRequest)
        return self.dates

In this case returning dates and self.dates are two totally different things. Since dates is created within the _makeRangeDates function, it will be garbage collected soon after the function is called. If you're looking to just return dates and not save it in some state then that's perfect fine.

If you're hoping to save the state of dates then do the following:

class foo(object):
    def __init__(self):
        self.dates = []

    def _makeRangeDates(self,desde,ultima_carga):

        for fechaRequest in rrule(DAILY, dtstart=desde, until=ultima_carga):
            self.dates.append(fechaRequest)
        return self.dates

Answer 3

You're confusing local variables and data members. And _makeRangeDates isn't even a method, just a function , since it doesn't read or write the object (you never use the self parameter).

dates is just some local variable you created inside your function with:

dates = []
    for fechaRequest in rrule(DAILY, dtstart=desde, until=ultima_carga):
        dates.append(fechaRequest)

dates is not a data member, ie it's not self.dates .

If you just want a function which returns that result, then return dates . That function can stand outside the class hierarchy.

If you want to put a helper function like this inside a class, but which doesn't actually access the object at all, that sort of function is called a staticmethod / classmethod (suggest you read up about the difference between the two).

Answer 4

The dates variable and self.dates variable are two different variables (unless you do some assignment such as self.date = date , after this assignment they both are referring to the same object (but are still different names/variables pointing to same reference) .

And yes you can return any of them python should not complain (unless you are trying to return a variable that has not been defined/initialized , like you are doing in your example)

In your example you are creating dates variable and appending values to it, but at the end you are returning self.dates (which may cause issues since self.dates may not have been defined when the function is called).

I believe you want to return dates from your function , since that is the variable which you define in the function and to which you append the dates .