Questions about Python CSV... ValueError: I/O operation on closed file.

'''Traceback (most recent call last):
  File "test.py", line 47, in <module>
    csv_output.writerow(row)
ValueError: I/O operation on closed file.'''

import RPi.GPIO as GPIO
from adxl345 import ADXL345
import time
from sys import version_info
from datetime import datetime
import csv
import os

adxl345 = ADXL345()

relayPin = 17
GPIO.setmode(GPIO.BCM)
GPIO.setup(relayPin, GPIO.OUT, initial=GPIO.HIGH)

print("Relay open...")
GPIO.output(relayPin, GPIO.HIGH)

print("Program is running...")
print("Program ")
junk = input("Press Enter to begin\n")
print("Waiting 20 seconds before activating relay")

count = 667  # increase this number by 10 for every second extra to want to record data for.
time.sleep(20)

filename = '/home/pi/logfiles/data_log.csv'
with open(filename, 'w', newline="") as f_output:
    csv_output = csv.writer(f_output)
    csv_output.writerow(["Time", "Accel"])

print("Relay close...")
GPIO.output(relayPin, GPIO.LOW)
time.sleep(20)

while count > 0:
    axes = adxl345.getAxes(True)
    print("ADXL345 on address 0x%x:") % (adxl345.address)
    x = (axes['x'])
    y = (axes['y'])
    z = (axes['z'])

    print("   x = ", x)
    print("   y = ", y)
    print("   z = ", z)

    row = [datetime.now().strftime("%Y-%m-%d %H:%M:%S"), x, y, z]
    csv_output.writerow(row)
    count = count - 1
    time.sleep(0.05)

print("End program")
GPIO.output(relayPin, GPIO.HIGH)
GPIO.cleanup()