seek()

seek(0)代表从文件头开始，1表示从当前位置开始，2代表从末尾开始

from sys import argv

script, input_file = argv

def print_all(f):
    print f.read()


def rewind(f):
    f.seek(0)


def print_a_line(line_count, f):
    print line_count, f.readline()

current_file = open(input_file)

print "First let's print the whole file: \n"

print_all(current_file)

print "Now let's rewind, kind of like a tape."

rewind(current_file)

print "Let's print three lines:"

current_line = 1
print_a_line(current_line, current_file)
current_line = current_line + 1
print_a_line(current_line, current_file)

current_line = current_line + 1
print_a_line(current_line, current_file)
print "4", current_file.readline()
print "5", current_file.readline()
current_file.close()

Xavier@Xavier-PC ~/python_test
$ python ex20.py test_20.txt
First let’s print the whole file:

gege
i
love
u

Now let’s rewind, kind of like a tape.
Let’s print three lines:
1

2 gege

3 i

4 love

5 u