Python3: What's wrong with my Singleton class and why can't I instantiate it? - python-3.x

Question

Trying to create a Singleton class to fetch and manage data like following but I keep getting RunTimeError, not sure if cls.stock_data is correct here either.
class StockData:
__instance = None
def __init__(self):
raise RuntimeError("Call instance()")
#classmethod
def instance(cls):
if cls.__instance is None:
cls.__instance = cls()
cls.__instance.__dict__ = {}
cls.stock_data = {}
return cls.__instance
def save_stock_data_from_yahoo(self, stock_symbol):
import urllib.request
import json
url = "http://finance.yahoo.com/d/quotes.csv?s=" + stock_symbol + "&f=sl1d1t1c1ohgv&e=.csv"
response = urllib.request.urlopen(url)
data = response.read().decode("utf-8")
data = data.split(',')
self.stock_data = {
"symbol": data[0],
"last_trade_price": data[1],
"date": data[2],
"time": data[3],
"change": data[4],
"change_in_percent": data[5],
"open": data[6],
"high": data[7],
"low": data[8],
"volume": data[9],
"average_daily_volume": data[10],
"previous_close": data[11],
"price_per_sales": data[12],
"price_per_book": data[13],
"pe_ratio": data[14],
"dividend_per_share": data[15],
"dividend_yield": data[16],
"earnings_per_share": data[17],
"ebitda": data[18],
"market_cap": data[19],
"one_year_target_price": data[20],
"price_earnings_ratio": data[21],
"price_earnings_growth_ratio": data[22],
"price_sales_ratio": data[23],
"price_book_ratio": data[24],
"short_ratio": data[25],
"day_range": data[26],
"day_range_realtime": data[27],
"year_range": data[28]
}
def get_stock_data(self):
print(self.stock_data)
return self.stock_data
StockData.instance()