my mapper function doesn't unpack all the values in python

Question

I got a file that it has lines like this :

Name_of_country,somedata,Max or min degree,degree,other data

so it goes like this :

France,xxx,TMAX,30,....
Germany,xxx,TMIN,40,....
France,xxx,TMIN,10,.....
.
.
.

now i tried this code i have wrote :

from mrjob.job import MRJob
class MRWordFrequencyCount(MRJob):
def mapper(self,_,line):
    x=line.split(",")
    d={}
    if x[2]=="TMAX":
        if x[0] not in d.keys() :
            d[x[0]]=list(x[3])
        else :
            d[x[0]]=d[x[0]].append(x[3])
    yield d
def reducer(self,d):
    for x,y in d.items():
        yield x,max(y)
if __name__=='__main__':
     MRWordFrequencyCount.run()

now whenever i try to run it using :

!python work.py file.CSV

it gives me this error :

for k, v in mapper(key, value) or ():
ValueError: not enough values to unpack (expected 2, got 1)

which i couldn't understand .. any help ?