Python methods from csv

Question

I am working on an assignment where I create "instances" of cities using rows in a .csv, then use these instances in methods to calculate distance and population change. Creating the instances works fine (using steps 1-4 below), until I try to call printDistance:

##Step 1. Open and read CityPop.csv
with open('CityPop.csv', 'r', newline='') as f:
try:
    reader = csv.DictReader(f)
    ##Step 2. Create "City" class
    class City:
        ##Step 3. Use _init method to assign attribute values
        def __init__(self, row, header):
            self.__dict__ = dict(zip(header, row))

            ##Step 4. Create "Cities" list
            data = list(csv.reader(open('CityPop.csv')))
            instances = [City(i, data[0]) for i in data[1:]]

            ##Step 5. Create printDistance method within "Cities" class  
            def printDistance(self, othercity, instances):
                dist=math.acos((math.sin(math.radians(self.lat)))*(math.sin(math.radians(othercity.lat)))+(math.cos(math.radians(self.lat)))*(math.cos(math.radians(othercity.lat)))*(math.cos(math.radians(self.lon-othercity.lon)))) * 6300 (self.lat, self.lon, othercity.lat, othercity.lon)

When I enter instances[0].printDistance(instances1) in the shell, I get the error:

 `NameError: name 'instances' is not defined`

Is this an indentation problem? Should I be calling the function from within the code, not the shell?

Answer 1

Nested functions must not contain self as parameter because they are not member functions. Class cannot pass instance variables to them. You are infact passing the same self from parent to child function.

Also you must not nest constructor, this is only for initiation purpose. Create a separate method indeed.

And try creating instance variable inside the constructor, and that is what init for !

self.instances = [self.getInstance(i, data[0]) for i in data[1:]]

Also create seperate function for instantiation

@classmethod
def getInstance(cls,d1,d2):
    return cls(d1,d2)