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At the moment I have a block of code that allows me to find an exact line in a notepad file and the add it to GTINlist. However I also want to add the line below that, and the line below that as well. I do not however want to import the rest of the file as a list. This is my code at the moment:

GTINlist=[]
GTIN=input("Please enter your GTIN code. ")
GTINcodes = [line for line in open('GTINcodes.txt') if GTIN in line]
stringGTINcode = str(GTINcodes)
GTINlist.append(stringGTINcode)*
william188325
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3 Answers3

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Here is what I did:

GTIN=input("Please enter your GTIN code. ")
with open('GTINcodes.txt', 'r') as file:
    GTINcodes = file.readlines()  #this seperates the lines of the file into the list called GTINcodes
GTINlist = GTINcodes[GTINcodes.index(GTIN):GTINcodes.index(GTIN) + 3]  #create the list GTINlist starting from the index where GTIN is found and add next two lines
Mark Skelton
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  • Ah yes, this is also a good solution if `GTINcodes.txt` is not too big and if you are interrested only on the first occurence of `GTIN`.You can store `GTINcodes.index(GTIN)` in a variable to prevent double computation. – Cyrille Pontvieux Feb 29 '16 at 12:58
  • Maybe OP edited the question, but it now says "I do not however want to import the rest of the file as a list." – gil Feb 29 '16 at 15:42
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You cannot use list comprehension in that case. But you can do this:

GTINlist=[]
GTIN=input("Please enter your GTIN code. ")
GTINcodes = []
read_ahead = 0
for line in open('GTINcodes.txt'):
    if GTIN in line:
        GTINcodes.append(line)
        read_ahead = 2
    elif read_ahead > 0:
        GTINcodes.append(line)
        read_ahead -= 1
stringGTINcode = str(GTINcodes)
GTINlist.append(stringGTINcode)*
Cyrille Pontvieux
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0

The builtin next() advances an iterator by one step. So in your case:

# setup
GTIN = input("Please enter your GTIN code. ")
GTINcodes = []


extra = 2  # number of extra lines to be appended

with open('GTINcodes.txt') as f:
    for line in f:
        if GTIN in line:
            GTINcodes.append(line)
            for _ in range(extra):
                GTINcodes.append(next(f))
            # if need to loop through the rest of the file, comment out break
            break

This can be further simplified by using itertools.dropwhile to easily skip lines without GTIN in them. dropwhile takes a predicate and an iterable, and returns an iterator which yields values from the iterable starting from the first value for which the predicate is false. So:

from itertools import dropwhile

# setup
GTIN = input("Please enter your GTIN code. ")

lines_to_take = 3  # first line with GTIN in it, and 2 lines after it
lines = dropwhile(lambda line: GTIN not in line, open('GTINcodes.txt'))
GTINcodes = [next(lines) for _ in range(lines_to_take)]
gil
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