Windows command prompt in Python

I have the following code which captures traffic packets, write them into a traffic.pcap file, then convert the pcap to text file using the tshark. If I enter the full path and name of the files in the last line, the code works perfectly, however it is always overwriting the files, so I added the lines to add time to each file so I can save all, the sniff and write commands work as expected, they create a new name each time I run it and save the file with the new name, however, when the code reaches the last line, it does not recognize the name of the file (source and destination) to read and convert, any help would be appreciated.

from scapy.all import *
import time
import os
# Create a new unique name
if os.path.exists('D:/folder/traffic.pcap'):
    file_name = ('D:/folder/traffic_{}.pcap'.format(int(time.time())))
# Create a destination file
txt_file = file_name + '.txt'
# Sniff traffic and write to a file *.pcap
x = sniff(count=10)
wrpcap(file_name,x)
# Convert pcap file to txt file usign tshark command
#os.system('cmd /c "tshark -i - < "D:/folder/traffic.pcap" > "D:/folder/traffic.txt""')# working line
os.system('cmd /c "tshark -i - < %file_name% > %txt_file%"')#not working line

The output is generated by the last line is The system cannot find the file specified.