在文件中迭代 for 时向后移动

Question

imagine this situation:

a file with 1000 lines. the name of the file is file.txt

file = file.txt
word = 'error'
for line in file:
    if word in line:
        execute things

if I want the 8 lines BEFORE the line with the word "error", how I get it?

Answer 1

Read the file and save the lines in a deque of a fixed size

from collections import deque

file = "file.txt"
word = 'error'

lines = deque(maxlen=8)
with open(file) as f:
    for line in f:
        if word in line:
            break
        lines.append(line)

print(lines)

Answer 2

You can use combination of collections.deque() with fixed length and itertools.takewhile() .

from collections import deque
from itertools import takewhile

with open("file.txt") as f:
    lines = deque(takewhile(lambda l: "error" not in l, f), maxlen=8)
print(*lines, sep="", end="")

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