How to redirect ‘print’ output to a file?

I want to redirect the print to a .txt file using Python. I have a for loop, which will print the output for each of my .bam file while I want to redirect all output to one file. So I tried to put:

f = open('output.txt','w')
sys.stdout = f

at the beginning of my script. However I get nothing in the .txt file.
My script is:

#!/usr/bin/python

import os,sys
import subprocess
import glob
from os import path

f = open('output.txt','w')
sys.stdout = f

path="/home/xxx/nearline/bamfiles"
bamfiles = glob.glob(path + '/*.bam')

for bamfile in bamfiles:
    filename = bamfile.split("https://stackoverflow.com/")[-1]
    print 'Filename:', filename
    samtoolsin = subprocess.Popen(["/share/bin/samtools/samtools","view",bamfile],
                                  stdout=subprocess.PIPE,bufsize=1)
    linelist= samtoolsin.stdout.readlines()
    print 'Readlines finished!'

So what’s the problem? Any other way besides this sys.stdout?

I need my result look like:

Filename: ERR001268.bam
Readlines finished!
Mean: 233
SD: 10
Interval is: (213, 252)