Python PicklingError无法腌制类...：与对象不同

Question

运行此代码时，我收到pickle.picklingerror，但不知道为什么。我已经检查过以确保名称没有冲突，并且已经检查了类似的帖子，但出现了此错误，但似乎无法解决问题。

这是3部分程序的部分“ a”，其中outfile将用于部分“ b”和“ c”。

要解决此错误需要更改什么？

如果需要对此信息进行编辑以包含其他信息，我很乐意这样做。

Traceback (most recent call last):
  File "/home/tarnette/assessment7question1a.py", line 80, in <module>
    main()
  File "/home/tarnette/assessment7question1a.py", line 15, in main
    dictionaryNations()
 File "/home/tarnette/assessment7question1a.py", line 32, in dictionaryNations
    pickle.dump(nationDictionary, outfile)
_pickle.PicklingError: Can't pickle <class '__main__.Nation'>: it's not the same object as __main__.Nation

 
import pickle

file = "UN.txt"

infile = open("/home/tarnette/UN/UN.txt", 'r')

def main():
    dictionaryNations()

def dictionaryNations():

    nationDictionary = {}

    for line in infile:
        data = line.split(',')

        country = Nation()
        country.setCountryName(data[0])
        country.setContinent(data[1])
        country.setPopulation(float(data[2]))
        country.setArea(float(data[3]))

        nationDictionary[country.getCountryName()] = country

    outfile = open("nationsDict.dat", 'wb')
    pickle.dump(nationDictionary, outfile)
    outfile.close()

    return nationDictionary

class Nation(): ##class named Nation as defined by textbook, THIS WILL ALSO BE SAVED IN A FILE CALLED "nation.py"
    def __init__(self): ##first method, is an initializer, first parameter is self
        self_countryName = "" ##These four variables are the four instance variables that must be created per the project instructions
        self_continent = ""
        self_population = 0.0
        self_area = 0.0

    ##This is the mutator method (change value of instance variable)
    def setCountryName(self, countryName):
        self._countryName = countryName

    ##This is the accessor method (retrieve value of instance variable)
    def getCountryName(self):
        return self._countryName

    ##This is the mutator method (change value of instance variable)
    def setContinent(self, continent):
        self._continent = continent

    ##This is the accessor method (retrieve value of instance variable)
    def getContinent(self):
        return self._continent

    ##This is the mutator method (change value of instance variable)
    def setPopulation(self, population):
        self._population = population

    ##This is the accessor method (retrieve value of instance variable)
    def getPopulation(self):
        return self._population

    ##This is the mutator method (change value of instance variable)
    def setArea(self, area):
        self._area = area

    ##This is the accessor method (retrieve value of instance variable)
    def getArea(self):
        return self._area

    ##This accessor method retrieves value of the calculation made for population density. Titled popDensity per project instructions.
    def popDensity(self):
        return (self._population / self._area)

main()