os.ftruncate(fd, length)

import os, sys

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

# Write one string
os.write(fd, "This is test - This is test")

# Now you can use ftruncate() method.
os.ftruncate(fd, 10)

# Now read this file from the beginning.
os.lseek(fd, 0, 0)
str = os.read(fd, 100)
print ("Read String is : ", str)

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

Read String is : This is te
Closed the file successfully!!