How to get a response with data from CoinAPI?

Question

I have been working on a data science project and wrote these two scripts to write crypto data from api or update data if already present, I wrote one script to write and update data and one scripts full of helper functions, I tried de-bugging using print statements and traced back to get_lists() in helper script, when I am running my main scripts I am getting empty response from server, response.status_code returns 200 but response.text is empty. Please help, I am not posting any links to api website(coinapi) but if you want I can provide those in comments. Also I am trying to write a function to make api calls with different keys if limit exceeds with current key, any suggestion on that.

download_update_csv.py

#!/usr/bin/env python3

"""

Download or Update data

"""

import os
import sys
import traceback

from datetime import date, timedelta
from preprocess import Preprocess

preprocess = Preprocess()

date_today = date.today()
start_date = date_today - timedelta(days=180)

filepath = './unprocessed_data/'

coins = {'btcusd': ['BTC/USD', './unprocessed_data/btcusd.csv'], 'ethusd': ['ETH/USD', './unprocessed_data/ethusd.csv']}

key = "get your key from coinapi it's free"


headers = {'X-CoinAPI-Key' : key}


def download_csv(coin: list, start: date, end: date) -> None:
    """
        def download_csv(coin: list, start: date, end: date) -> None

            writes coin.csv files in uprocessed_data folder
    """
    url = f"https://rest.coinapi.io/v1/exchangerate/{coin[0]}/history?period_id=1HRS&time_start={start}T00:00:00&time_end={end}T00:00:00&limit=100000"
    lists = preprocess.get_lists(url, headers)
    time_step, rate_close = preprocess.list_from_dict(lists)
    print(f"Writing {coin[0]}.csv file")
    preprocess.write_csv(filepath=coin[1], time_steps=time_step, rate_close=rate_close)
    
def update_csv(coin: list, start: date, end: date) -> None:
    """
        def update_csv(coin: list) -> None

            updates coin.csv with new data in uprocessed_data folder
    """
    url = f"https://rest.coinapi.io/v1/exchangerate/{coin[0]}/history?period_id=1HRS&time_start={start}T00:00:00&time_end={end}T00:00:00&limit=100000"
    lists = preprocess.get_lists(url, headers)
    time_steps, rate_close = preprocess.list_from_dict(lists)
    preprocess.write_csv(filepath=coin[1], time_steps=time_steps, rate_close=rate_close)

def main() -> None:
    """
        Driver Function
    """
    try:
        if not os.path.exists(filepath):
            print(f'Creating folder {filepath}')
            os.mkdir(filepath)
        
        for coin in coins.values():
            if os.path.exists(coin[1]):
                last_date = preprocess.get_last_date(coin[1])
                update_csv(coin=coin, start=date_today, end=last_date)
            else:
                download_csv(coin=coin, start=date_today, end=start_date)

    except Exception:
        print(traceback.format_exc())

if __name__ == "__main__":
    sys.exit(main())

preprocess.py

import requests
import json
import csv
import matplotlib.pyplot as plt
import os

from datetime import datetime

class Preprocess:
    """Helper functions for preprocessing and saving and retrieving crypto-exchangerate data"""
    def __init__(self):
        pass

    def get_lists(self, url: str, headers: dict) -> list:
        """
            def get_lists(self, url: str, headers: dict) -> list

                returns list_of_dictionaries
        """
        response = requests.get(url=url, headers=headers)
        lists = json.loads(response.text)
        return lists

    def list_from_dict(self, lists: list) -> tuple:
        """
            def list_from_dict(self, lists: list) -> tuple

                returns (time_steps, rate_close)
        """
        time_steps = ['time']
        rate_close = ['close']
        for dictionary in lists:
            date_string = dictionary.get('time_close')[:-9]
            datetimeObj = datetime.strptime(date_string, "%Y-%m-%dT%H:%M:%S")
            time_steps.append(datetimeObj.timestamp())
            rate_close.append(dictionary.get('rate_close'))

        return (time_steps, rate_close)

    def write_csv(self, filepath: str, time_steps: list, rate_close: list) -> None:
        """
            def write_csv(self, filename: str, time_steps: list, rate_close: list) -> None

                writes time_steps and rate_low to a csv file
        """
        
        if len(time_steps) == 0 or len(rate_close) == 0:
            raise Exception('Empty lists, no useful data returned.')

        with open(filepath, 'a+') as csvfile:
            writer = csv.writer(csvfile)
            for i in range(len(time_steps)):
                writer.writerow([time_steps[i], rate_close[i]])

    def read_csv(self, filepath: str) -> tuple:
        """
            def read_csv(self, filepath: str) -> tuple

                returns time_steps and rate_low lists
        """
        time_steps = []
        rate_close = []
        with open(filepath) as csvfile:
            reader = csv.reader(csvfile, delimiter=',')
            next(reader)
            for column in reader:
                time_steps.append(float(column[0]))
                rate_close.append(float(column[1]))
        return (time_steps, rate_close)

    def plot_series(time: list, series: list, format="-", start=0, end=None, name='fig.jpg', figsize=(10, 6)) -> None:
        """
            def plot_series(time: list, series: list, format="-", start=0, end=None, name='fig.jpg', figsize=(10, 6))

                plots figure for given parameters
        """
        plt.figure(figsize=figsize)
        plt.plot(time[start:end], series[start:end], format)
        plt.xlabel("Time")
        plt.ylabel("Value")
        plt.grid(True)
        plt.savefig(name)
        

    def get_last_date(self, filepath: str) -> datetime:
        """
            def get_last_date(coin: list

                returns last date when coin.csv file was updated
        """
        time_steps_coin, _ = self.read_csv(filepath)
        last_date_coin = datetime.fromtimestamp(time_steps_coin[-1])

        return last_date_coin