如何使用python编程获取文件中特定行数？

Question

This is my code for getting 20 lines every time but f1.tell() gives last position of the file. So i cannot get 20 lines for the next time. Can anyone help me to do this? please

f1=open("sample.txt","r")
last_pos=0
while true:
    f1.seek(last_pos)
    for line,i in enumerate(f1.readlines()):
        if line == 20:
            last_pos=f1.tell()
            break
        else:
            print i
sample.txt file contains below data
1
2
3
4
.
.
.
.
40
I want Output like
1
2
3
4
.
.
.
20
20
21
22
23
24
25
.
.
.
.
39
39
40

Answer 1

Using readlines reads all the file: you reach the end, hence what you are experiencing.

Using a loop with readline works, though, and gives you the position of the end of the 20th line (well, rather the start of the 21st)

Code (I removed the infinite loop BTW):

f1=open("sample.txt","r")
last_pos=0
line=0
while True:
    l=f1.readline()
    if l=="":
        break
    line+=1
    if line == 20:
        last_pos=f1.tell()
        print(last_pos)
        break
f1.close()

You could iterate with for i,l in enumerate(f1): but iterators & ftell are not compatible (you get: OSError: telling position disabled by next() call ).

Then, to seek to a given position, just f1.seek(last_pos)

EDIT: if you need to print the line twice eveny 20 lines, you actually don't even need seek , just print the last line when you count 20 lines.

BUT if you really want to do that this way, here's the way:

f1=open("sample.txt","r")
line=0
rew=False

while True:
    start_line_pos=f1.tell()
    l=f1.readline()
    if l=="":
        break
    print(l.strip())
    line+=1
    if rew:
        rew = False   # avoid re-printing the 20th line again and again
    elif line % 20 == 0:
        f1.seek(start_line_pos)
        rew = True
        line-=1   # pre-correct line counter
f1.close()

You notice a bit of logic to avoid getting stuck on line 20. It works fine: when reaching line 20, 40, ... it seeks back to the previously stored file position and reads 1 line again. The rew flag prevents to do that more than once.

Answer 2

Jean-François Fabre already explained that readline could read the whole file.

But anyway you should never mix line level input ( readline ) and byte level input ( tell , seek or read ). As the low level OS system calls can only read a byte count, readline actually reads a buffer and searches for a newline in it. But the file pointer given by tell is positionned at the end of the buffer and not at the end of the line.

There is no way to set the file pointer at the end of a line except by processing the file one char at a time and manually detecting the end of line.