Python • Failure of successive file.writes in callback routine

Code:

def lightcallback(channel): # night time function    global flagnight # do this to use variable within function    time.sleep(0.1) # DEBOUNCE delay to remove false positives    if GPIO.input(18)==0: # DEBOUNCE recheck thatlight is low        print("debounce check complete - light trigger valid")        now = datetime.now()        formatted_time = now.strftime('%Y/%m/%d %H:%M:%S')        linetoinsert = formatted_time + "   LIGHT ON" + "\n"        file1 = open(r'logfile_cam.txt', 'a', newline='') #open file for LIGHT ON time        file1.write (linetoinsert)        file1.close()        print (linetoinsert)        if (flagnight == False): # prior status = daytime            picam2.set_controls({"ExposureTime":10000, "AnalogueGain": 16.0})            picam2.set_controls({"FrameDurationLimits": (100000,100000)}) # this overrides the configuration setting but is not shown when config setting is printed            print ("Change to Night exposure")            flagnight = 1        else: # do nothing already set to night exposure            print ("Continue Night exposure")        recordandsave () # call the function to record and save file        while GPIO.input(18) == 0: # do more recordings until the light goes off            print("light still on - next successive recording")            now = datetime.now()            formatted_time = now.strftime('%Y/%m/%d %H:%M:%S')            linetoinsert = formatted_time + "   LIGHT ON SUCCESSIVE" + "\n"            file1 = open(r'logfile_cam.txt', 'a', newline='') #open file for SUCCESSIVE LIGHT ON time            file1.write (linetoinsert) # ****** THIS LINE DOES NOT WORK********            file1.close()            print (linetoinsert)            recordandsave () # call the function to record and save file        print ("waiting for trigger")