python：在迭代中处理 csv object 时出现问题

Question

First, sorry if the title is not clear. I (noob) am baffled by this...

Here's my code:

import csv
from random import random
from collections import Counter

def rn(dic, p):
    for ptry in parties:
        if p < float(dic[ptry]):
            return ptry
        else:
            p -= float(dic[ptry])


def scotland(r):
    r['SNP'] = 48
    r['Con'] += 5
    r['Lab'] += 1
    r['LibDem'] += 5


def n_ireland(r):
    r['DUP'] = 9
    r['Alliance'] = 1
    # SF = 7


def election():
    results = Counter([rn(row, random()) for row in data])
    scotland(results)
    n_ireland(results)
    return results


parties = ['Con', 'Lab', 'LibDem', 'Green', 'BXP', 'Plaid', 'Other']

with open('/Users/andrew/Downloads/msp.csv', newline='') as f:
    data = csv.DictReader(f)
    for i in range(1000):
        print(election())

What happens is that in every iteration after the first one, the variable data seems to have vanished: the function election() creates a Counter object from a list obtained by processing data , but on every pass after the first one, this object is empty, so the function just returns the hard coded data from scotland() and n_ireland() . ( msp.csv is a csv file containing detailed polling data). I'm sure I'm doing something stupid but would welcome anyone gently pointing out where...

Answer 1

I'm going to place a bet on your definition of newline . Are you sure you don't want newline = “\n”? Otherwise it will interpret the entire file as a single line, which explains what you're seeing.

EDIT

I now see another issue. The file object in python acts as a generator for each line. The problem is once the generator is finished (you hit the end of the file), you have no more data generated . To solve this: reset your file pointer to the beginning of the file like so:

with open('/Users/andrew/Downloads/msp.csv') as f:
    data = csv.DictReader(f)
    for i in range(1000):
        print(election())
        f.seek(0)

Here the call to f.seek(0) will reset the file pointer to the beginning of your file. You are correct that data is a global object given the way you've defined it at the module level, there's no need to pass it as a parameter.

Answer 2

I agree with @smassey, you might need to change the code to

with open('/Users/andrew/Downloads/msp.csv', newline='\n') as f:

or simply try not use that argument

with open('/Users/andrew/Downloads/msp.csv') as f: