What is the code equivalent to grep -B in python?

Question

How do I print n lines before a matched string from a file using python?

man grep

-B NUM, --before-context=NUM

Print NUM lines of leading context before matching lines. Places a line containing a group separator (--) between contiguous groups of matches. With the -o or --only-matching option, this has no effect and a warning is given.

I have code for grep -A:

def grepA(word,file,num=0):
    with open(file) as f:
        for line in f:
            if word in line:
                print(line + ''.join(islice(file, num)))
                print('---')

@vishes_shell: interesting link. How does that solution work for `-A` and `-B`? — serv-inc, Jul 19 '18 at 08:08

score 4 · Accepted Answer · answered Jul 19 '18 at 08:10

4

You'll just have to keep a buffer (a list) of the N last lines, then print them when you encounter a match.

context_lines = []
for line in f:
    context_lines.append(line)
    if len(context_lines) > 5:  # Too many lines?
        context_lines.pop(0)  # Pop the first one off.
    if word in line:
        # context_lines already includes the `line` we're looking at
        for ctx_line in context_lines:  
             print(ctx_line)

answered Jul 19 '18 at 08:10

AKX

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I would have used a `deque` with a history of 5 (much more efficient) – Jean-François Fabre Jul 19 '18 at 08:11
Sure, that'll work too. :) – AKX Jul 19 '18 at 08:12
it`s work,but the output in the console adds spaces that are not in the source file – Vladimir Smith Jul 19 '18 at 08:39
@VladimirSmith If `line`s already contain newlines, then you'll want to `print(ctx_line, end='')`, yeah :) – AKX Jul 19 '18 at 09:44

score 2 · Answer 2 · edited Jul 19 '18 at 08:16

2

You need to cache the lines:

def grepb(word,file, num=0):
    if num == 0:
        print(re.findall(re.compile(word), line)[0])  # https://stackoverflow.com/questions/26659142
    else:
        cache = collections.deque([], num)
        with open(file) as f:
            for line in f:
                cache.append(line)
                if word in line:
                    print('\n'.join(cache))
                    print('---')

edited Jul 19 '18 at 08:16

Jean-François Fabre

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answered Jul 19 '18 at 08:12

serv-inc

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the best answer so far – Jean-François Fabre Jul 19 '18 at 13:26

What is the code equivalent to grep -B in python?

2 Answers2