os.write(fd, str)

import os, sys

# Open a file
fd = os.open( "f1.txt", os.O_RDWR|os.O_CREAT )

# Write one string
line="this is test"

# string needs to be converted byte object
b=str.encode(line)
ret=os.write(fd, b)

# ret consists of number of bytes written to f1.txt
print ("the number of bytes written: ", ret)

# Close opened file
os.close( fd)
print ("Closed the file successfully!!")

the number of bytes written: 12
Closed the file successfully!!