How to output or create the requirements.txt file which is a list of all the needed software?

https://stackoverflow.com/questions/29715249/is-there-any-way-to-output-requirements-txt-automatically

pip freeze > requirements.txt

How to redirect 'print' output to a file using python?

 How to redirect 'print' output to a file using python?

import sys

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

for i in range(2):
    print 'i = ', i

sys.stdout = orig_stdout
f.close()