code after fixes

from pynput import keyboard
import u6
from time import sleep
import matplotlib.pyplot as plt


d = u6.U6()  # initialize the interface; assumes a single U6 is plugged in to a USB port
d.configU6()  # set default configuration
d.configIO()  # ask for analog inputs

d.streamConfig(
    NumChannels=3,
    ChannelOptions=[7, 0, 0],
    ChannelNumbers=[0, 1, 2],
    ResolutionIndex=0,
    ScanFrequency=1000)

d.packetsPerRequest = 1  # you can adjust this value to get more or less data

#set DAC0 to 4.0 voltage
DAC0 = 5000
d.writeRegister(DAC0, 4.0)
break_program = False
x = []
y = []
def show_result(i, ain0):
    print("Iteration: " + str(i))
    print("AIN0(Diff): " + str(round( ain0, 3)))
    print("")


def on_press(key):
    global break_program
    if key == keyboard.Key.esc:
        break_program = True
        return False


# Try to measure a data set.
def measure():
    try:
        d.streamStart()
        for r in d.streamData():
            if r is not None:
                if r['errors'] or r['numPackets'] != d.packetsPerRequest or r['missed']:
                    print("error")
                break
    finally:
        d.streamStop()
    return r


# Write a set of data to a file "data.txt"
def writeData(r, i):
    print(d.getAIN(2))
    result = d.getAIN(positiveChannel=0,  resolutionIndex=12, gainIndex=2, settlingFactor=0, differential=True)
    show_result(i, result)
    f2.write(str(result) + '\t\n')
    f2.close()
    f2.close()




with keyboard.Listener(on_press=on_press) as listener:
    iterator = 0
    while not break_program:
        writeData(measure(), iterator)
        iterator = iterator + 1
    listener.join()