Insert string at the beginning of each line

Question

How can i insert a string at the beginning of each line in a text file, i have the following code:

f = open('./ampo.txt', 'r+')
with open('./ampo.txt') as infile:
    for line in infile:
        f.insert(0, 'EDF ')
f.close

i get the following error:

"'file' object has no attribute 'insert'"

Please note that im a complete programming n00b.

Answer 1

Python comes with batteries included:

import fileinput
import sys

for line in fileinput.input(['./ampo.txt'], inplace=True):
    sys.stdout.write('EDF {l}'.format(l=line))

Unlike the solutions already posted, this also preserves file permissions.