Question

I have the following text file:

This is my text file
NUM,123
FRUIT
DRINK
FOOD,BACON
CAR
NUM,456
FRUIT
DRINK
FOOD,BURGER
CAR
NUM,789
FRUIT
DRINK
FOOD,SAUSAGE
CAR
NUM,012
FRUIT
DRINK
FOOD,MEATBALL
CAR

And I have the following list called ‘wanted’:

['123', '789']

What I’m trying to do is if the numbers after NUM is not in the list called ‘wanted’, then that line along with 4 lines below it gets deleted. So the output file will looks like:

This is my text file
NUM,123
FRUIT
DRINK
FOOD,BACON
CAR
NUM,789
FRUIT
DRINK
FOOD,SAUSAGE
CAR

My code so far is:

infile = open("inputfile.txt",'r')
data = infile.readlines()

for beginning_line, ube_line in enumerate(data):
    UNIT = data[beginning_line].split(',')[1]
    if UNIT not in wanted:
        del data_list[beginning_line:beginning_line+4]

Answer 1

You shouldn’t modify a list while you are looping over it.

What you could try is to just advance the iterator on the file object when needed:

wanted = set(['123', '789'])

with open("inputfile.txt",'r') as infile, open("outfile.txt",'w') as outfile: 
    for line in infile:
        if line.startswith('NUM,'):
            UNIT = line.strip().split(',')[1] 
            if UNIT not in wanted:
                for _ in xrange(4):
                    infile.next()
                continue

        outfile.write(line)

And use a set. It is faster for constantly checking the membership.

This approach doesn’t make you read in the entire file at once to process it in a list form. It goes line by line, reading from the file, advancing, and writing to the new file. If you want, you can replace the outfile with a list that you are appending to.