找到一行时如何从文件中获取下 n 行

Question

While reading a file in python, I was wondering how to get the next n lines when we encounter a line that meets my condition.

Say there is a file like this

mangoes:
1 2 3 4 
5 6 7 8
8 9 0 7
7 6 8 0
apples:
1 2 3 4
8 9 0 9

Now whenever we find a line starting with mangoes, I want to be able to read all the next 4 lines.

I was able to find out how to do the next immediate line but not next n immediate lines

if (line.startswith("mangoes:")):
            print(next(ifile))  #where ifile is the input file being iterated over 

Answer 1

just repeat what you did

if (line.startswith("mangoes:")):
    for i in range(n):
        print(next(ifile)) 

Answer 2

Unless it's a huge file and you don't want to read all lines into memory at once you could do something like this

n = 4

with open(fn) as f:
    lines = f.readlines()

for idx, ln in enumerate(lines):
    if ln.startswith("mangoes"):
        break

mangoes = lines[idx:idx+n]

This would give you a list of the n number of lines, including the word mangoes . if you did idx=idx+1 then you'd skip the title too.

Answer 3

With itertools.islice feature:

from itertools import islice

with open('yourfile') as ifile:
    n = 4
    for line in ifile:
        if line.startswith('mangoes:'):
            mango_lines = list(islice(ifile, n))

From your input sample the resulting mango_lines list would be:

['1 2 3 4 \n', '5 6 7 8\n', '8 9 0 7\n', '7 6 8 0\n']