content missing when a file is written into another file Python -

im entry level python developer working on requirement need merge many text files in forder 1 file in folder b. wrote below code working not writing entire content files in folder a.

 import os  = open(r"c:\temp\datsoutput\new folder\output.txt", "w")  path = r'c:\temp\dats'  filename in os.listdir(path):      fullpath = path+"\\"+filename      open(fullpath,'r',encoding="utf_8_sig", errors="ignore") ins:             line in ins:                  a.write(line) 

the around 6000 lines below line missing.

 rout:=  996.6mm          angl:=  95degree          orrf:=  =0/0        tmrref:=  =0/0        repcou:=  0     end     new cylinder 26 of subequipment /fa-3101/main_m1_davit            name:=  =805324448/24319            type:=  cyli            lock:=  false           owner:=  /fa-3101/main_m1_davit            purp:=  unset             pos:=  w 537.345mm s 46.083mm u 0mm             ori:=  y s , z u            leve:=  0 10            obst:=  2            diam:=  76.2mm            heig:=  304.8mm            orrf:=  =0/0          tmrref:=  =0/0          repcou:=  0        :pstatus:=  unset     end     new cylinder 27 of subequipment /fa-3101/main_m1_davit            name:=  =805324448/24320            type:=  cyli            lock:=  false           owner:=  /fa-3101/main_m1_davit            purp:=  unset             pos:=  w 537.345mm s 46.083mm u 6.125mm             ori:=  y s , z u            leve:=  0 10            obst:=  2 

for testing purpose folder has 1 file has 204034 lines , script has written 203799 lines, problem of encoding? cant go direction here small appreciated , saves job, thank you