Print 4 lines before and after a "match" found 1

[Tester_V]

Greeting!
I’m trying to print 4 lines before and after a “match” found in a line.
I do not understand how to do it with my current knowledge of Python Confused but I found 2 snippets,
One finds lines Before the match and one After the match. Need help putting them together.
Thank you.

from itertools import islice
 
with open(myfile, "r") as f:
    lines = f.readlines() 
    for index, line in enumerate(lines):  
        if "FIND" in line:  
#            print(line.rstrip())  
            print("".join(lines[max(0,index-4):index]))  # print 4 lines preceeding it
 
with open(myfile, "r") as f:
    for line in f:
        #print (line)
        if "FIND" in line:
            #print (line)
            #print("".join(line))
            print ("".join(islice(f,4)))   ### 4 Lines after match  ###

 

[feherke]

Hi

The first half of your code is using [tt]file.readlines()[/tt] to slurp the entire file into memory. That allows you to easily access any line, so just change the slice's 2[sup]nd[/sup] value :

with [COLOR=orange]open[/color][teal]([/teal]myfile[teal],[/teal] [i][green]"r"[/green][/i][teal])[/teal] as f[teal]:[/teal]
    lines [teal]=[/teal] f[teal].[/teal][COLOR=orange]readlines[/color][teal]()[/teal]
    [b]for[/b] index[teal],[/teal] line [b]in[/b] [COLOR=orange]enumerate[/color][teal]([/teal]lines[teal]):[/teal]
        [b]if[/b] [i][green]"FIND"[/green][/i] [b]in[/b] line[teal]:[/teal]
            [b]print[/b][teal]([/teal][i][green]""[/green][/i][teal].[/teal][COLOR=orange]join[/color][teal]([/teal]lines[teal][[/teal][COLOR=orange]max[/color][teal]([/teal][purple]0[/purple][teal],[/teal] index [teal]-[/teal] [purple]4[/purple][teal]):[/teal]index [teal]+[/teal] [purple]5[/purple][teal]]))[/teal]

While that is simple and readable, work probably not work fine on bigger files. For those the second half of your code is more suitable, but unfortunately [tt]islice()[/tt] can not handle negative indexes, so you have to handle the before lines yourself :

[red]from[/red] itertools [red]import[/red] islice

before [teal]= [][/teal]
with [COLOR=orange]open[/color][teal]([/teal]myfile[teal],[/teal] [i][green]"r"[/green][/i][teal])[/teal] as f[teal]:[/teal]
    [b]for[/b] line [b]in[/b] f[teal]:[/teal]
        before[teal].[/teal][COLOR=orange]append[/color][teal]([/teal]line[teal])[/teal]
        [b]if[/b] [COLOR=orange]len[/color][teal]([/teal]before[teal]) >[/teal] [purple]5[/purple][teal]:[/teal]
            before[teal].[/teal][COLOR=orange]pop[/color][teal]([/teal][purple]0[/purple][teal])[/teal]
        [b]if[/b] [i][green]"FIND"[/green][/i] [b]in[/b] line[teal]:[/teal]
            [b]print[/b][teal]([/teal][i][green]""[/green][/i][teal].[/teal][COLOR=orange]join[/color][teal]([/teal]before[teal]) +[/teal] [i][green]""[/green][/i][teal].[/teal][COLOR=orange]join[/color][teal]([/teal][COLOR=orange]islice[/color][teal]([/teal]f[teal],[/teal] [purple]4[/purple][teal])))[/teal]

The above is the simplified version, but all those [tt]list.pop()[/tt] will slow it down, so you better avoid it :

[red]from[/red] itertools [red]import[/red] islice

before [teal]= [][/teal]
before_index [teal]=[/teal] [purple]0[/purple]
with [COLOR=orange]open[/color][teal]([/teal]myfile[teal],[/teal] [i][green]"r"[/green][/i][teal])[/teal] as f[teal]:[/teal]
    [b]for[/b] line [b]in[/b] f[teal]:[/teal]
        [b]if[/b] [teal]([/teal][COLOR=orange]len[/color][teal]([/teal]before[teal]) <[/teal] [purple]5[/purple][teal]):[/teal]
            before[teal].[/teal][COLOR=orange]append[/color][teal]([/teal]line[teal])[/teal]
        [b]else[/b][teal]:[/teal]
            before[teal][[/teal]before_index[teal]] =[/teal] line
            before_index [teal]+=[/teal] [purple]1[/purple]
            [b]if[/b] before_index [teal]>[/teal] [COLOR=orange]len[/color][teal]([/teal]before[teal]) -[/teal] [purple]1[/purple][teal]:[/teal]
                before_index [teal]=[/teal] [purple]0[/purple]
        [b]if[/b] [i][green]"FIND"[/green][/i] [b]in[/b] line[teal]:[/teal]
            [b]print[/b][teal]([/teal][i][green]""[/green][/i][teal].[/teal][COLOR=orange]join[/color][teal]([/teal]before[teal][[/teal]before_index[teal]:]) +[/teal] [i][green]""[/green][/i][teal].[/teal][COLOR=orange]join[/color][teal]([/teal]before[teal][:[/teal]before_index[teal]]) +[/teal] [i][green]""[/green][/i][teal].[/teal][COLOR=orange]join[/color][teal]([/teal][COLOR=orange]islice[/color][teal]([/teal]f[teal],[/teal] [purple]4[/purple][teal])))[/teal]

Just for completeness, there would be another way using [tt]islice()[/tt], but due to its inability to handle negative indexes, we rewind the file, so this one does not handle multiple matches :

[red]from[/red] itertools [red]import[/red] islice

index [teal]=[/teal] [purple]0[/purple]
with [COLOR=orange]open[/color][teal]([/teal]myfile[teal],[/teal] [i][green]"r"[/green][/i][teal])[/teal] as f[teal]:[/teal]
    [b]for[/b] line [b]in[/b] f[teal]:[/teal]
        index [teal]+=[/teal] [purple]1[/purple]
        [b]if[/b] [i][green]"FIND"[/green][/i] [b]in[/b] line[teal]:[/teal]
            f[teal].[/teal][COLOR=orange]seek[/color][teal]([/teal][purple]0[/purple][teal])[/teal]
            [b]print[/b][teal]([/teal][i][green]""[/green][/i][teal].[/teal][COLOR=orange]join[/color][teal]([/teal][COLOR=orange]islice[/color][teal]([/teal]f[teal],[/teal] index [teal]-[/teal] [purple]5[/purple][teal],[/teal] index [teal]+[/teal] [purple]4[/purple][teal])))[/teal]
            [b]break[/b]

Feherke.
feherke.github.io

 

[Tester_V]

That is awesome!
Now I need to understand it. Thank you for your coaching!

 

[Tester_V]

To "feherke".

I'm trying to understand your code but it is too much for me for now.
Why this If block is the last block in the script?

if "FIND" in line:

And this part of the script is totally confusing

        if (len(before) < 5):
            before.append(line)
        else:
            before[before_index] = line
            before_index += 1

from itertools import islice

before = []
before_index = 0
with open(myfile, "r") as f:
    for line in f:
        if (len(before) < 5):
            before.append(line)
        else:
            before[before_index] = line
            before_index += 1
            if before_index > len(before) - 1:
                before_index = 0
        if "FIND" in line:
            print("".join(before[before_index:]) + "".join(before[:before_index]) + "".join(islice(f, 4)))

Also, I cannot print lines "before" and lines "after" to a file.
Could you help me please?
Thank you.

 

[Tester_V]

And the same story with this snippet.
It prints what I need but I need to print it to aa different file.
Have no idea how to print the lines to a file.

index = 0
with open(myfile, "r") as f:
    for line in f:
        index += 1
        if "FIND" in line:
            f.seek(0)
            print("".join(islice(f, index - 5, index + 4)))
            
            break