write unit test case in python for below algorithm

I have written code but would not understand how to write test cases in python for below program. Can someone provide an example test case?

Thanks in advance. I have tried some samples but I do not understand what parts of the code should be tested as it only pick the company name related with highest price.

with open('/home/company_data.csv') as f:
    reader = csv.reader(f)
    tup = namedtuple('tup', ['price','year', 'month'])
    d = OrderedDict()
    names = next(reader)[2:]
    for name in names:
    d[name] = tup(0,'year', 'month')
    for row in reader:
    year, month = row[:2]        
    for name, price in zip(names, map(int, row[2:])):    
        if d[name].price < price:     
        d[name] = tup(price, year, month)