如何使用Python跳过文件中的2行？

Question

I have a series of files and I want to extract a specific number from each of them. In each of the files I have this line:

name, registration num

and exactly two lines after that there is the registration number. I would like to extract this number from each file. and put it as a value of a dictionary.Anyone have any idea how it is possible ?

my current code that does not actually work is like below:

matches=[]
for root, dirnames, filenames in os.walk('D:/Dataset2'):  
    for filename in fnmatch.filter(filenames, '*.txt'):   
        matches.append([root, filename])

filenames_list={}       
for root,filename in matches:
    filename_key = (os.path.join(filename).strip()).split('.',1)[0]

    fullfilename = os.path.join(root, filename)
    f= open(fullfilename, 'r')
    for line in f:
        if "<name, registration num'" in line:
            key=filename_key
            line+=2
            val=line

Answer 1

I usually use next() when I want to skip a single line, usually a header for a file.

with open(file_path) as f:
    next(f) # skip 1 line
    next(f) # skip another one.
    for line in f:
        pass # now you can keep reading as if there was no first or second line.

Note: In Python 2.6 or earlier you must use f.next()

Answer 2

One way would be to load the whole line into an array, and then read the line(s) you want from it. Example

A file called testfile contains the following:

A1
B2
C3
D4
E5

A program test.py:

#!/usr/bin/env python

file = open('testfile')
lines = file.readlines()[2:]
file.close()

for line in lines:
    print(line.strip())

Output:

$./test.py
C3
D4
E5

EDIT: I read the question again, and noticed you just want a single line. Then you could just remove the : , and use f.getlines()[2] to get the third line in a file

• Or you could use f.getline() three times, and just ignore the first two

• Or you could use a for line in f type loop, and just ignore the first two line (have an incrementing counter)

Answer 3

I suppose something like that would work...

f= open(fullfilename, 'r')
for line in f:
    if "name, registration num" in line:
        key=filename_key
        break
f.readline()
res = f.readline()[:-1] #removed trailin newline

Answer 4

from itertools import islice
with open('data.txt') as f:
    for line in islice(f, 2, None):
        print line

Answer 5

Generally speaking, if you want to do something to a python iterator in-loop, like look two ahead, I find a good first place to look is to import itertools and look here . In your case, you might benefit from their implementation of consume .

Worth having a look to see if this issue hasn't been covered on SO before. Edit: Indeed- look here , which includes a good discussion of python iterators.