f = open(filename, 'r')
filestr = f.read()    # reads the whole file into a string
lines = f.readlines() # reads the whole file into a list of lines

for line in f:        # read line by line
    <process line>

while True:           # old style, more flexible reading
    line = f.readline()
    if not line: break
    <process line>

f.close()

f = open(filename, 'w')
f.write(somestring)
f.writelines(list_of_lines)
print >> f, somestring