Python：return语句仍未从函数返回任何内容

Question

I've looked at all of the other "Returns none" questions on here and none of them seem to solve my problem.

rates = []
for date in unformatted_returns: # Please ignore undefined variables, it is redundant in this context
    if date[0] >= cutoff_date:
        date_i = unformatted_returns.index(date)
        r = date_initialize(date[0], date_i)
        print "r is returned as:", r
        rates.append(r)
        print date[0]
    else:
        continue

def date_initialize(date, date_i):
        print " initializing date configuration"
        # Does a bunch of junk
        rate_of_return_calc(date_new_i, date_i)

def rate_of_return_calc(date_new_i, date_i):
        r_new = unformatted_returns[int(date_i)] # Reverse naming, I know
        r_old = unformatted_returns[int(date_new_i)] # Reverse naming, I know
        if not r_new or not r_old:
            raise ValueError('r_new or r_old are not defined!!')
            # This should never be true and I don't want anything returned from here anyhow
        else:
            ror = (float(r_new[1])-float(r_old[1]))/float(r_old[1])
            print "ror is calculated as", ror
            return ror

The functions them selves work fine, the output is like so:

initializing date configuration
('2014-2-28', u'93.52')
ror is calculated as -0.142643284859
r is returned as: None
2015-2-2
>>> 

ror is the correct value, but why does it not get returned when I have it written right there return ror ?? Doesn't make any sense to me

Answer 1

You need to return it here too

def date_initialize(date, date_i):
        print " initializing date configuration"
        # Does a bunch of junk
        return rate_of_return_calc(date_new_i, date_i)

Answer 2

In date_initialize , you need to return the function that is returning the value that you want. Explicitly, change your call from

rate_of_return_calc(date_new_i, date_i)

to

return rate_of_return_calc(date_new_i, date_i)

Your first call, to date_initialize , does not return anything. Therefore, when you call rate_of_return_calc , you receive the value and then throw it away. You need to return it to pass the value along to your main function.

Answer 3

You need to return the value in date_initialize too:

def date_initialize(date, date_i):
    print " initializing date configuration"
    # Does a bunch of junk
    return rate_of_return_calc(date_new_i, date_i)