从打开的文件中获取行号python

Question

I wrote this little Python 2.7 prototype script to try and read specified lines (in this example lines 3,4,5) from a formatted input file. I am going to be later parsing data from this and operating on the input to construct other files.

from sys import argv

def comparator (term, inputlist):
    for i in inputlist:
        if (term==i):
            return True
    print "fail"
    return False

readthese = [3,4,5]

for filename in argv[1:]:
    with open(filename) as file:
        for line in file:
            linenum=#some kind of way to get line number from file
            if comparator(linenum, readthese):
                print(line)

I fixed all the errors I had found with the script but currently I don't see anyway to get a line number from file. It's a bit different than pulling the line number from a file object since file is a class not an object if I'm not mistakened. Is there someway I can pull the the line number for my input file?

I think a lot of my confusion probably stems from what I did with my with statement so if someone could also explain what exactly I have done with that line that would be great.

Answer 1

You could just enumerate the file object since enumerate works with anything iterable...

for line_number, line in enumerate(file):
    if comparator(line_number, line):
        print line

Note, this indexes starting at 0 -- If you want the first line to be 1, just tell enumerate that's where you want to start:

for line_number, line in enumerate(file, 1):
    ...

^{Note, I'd recommend not using the name file -- On python2.x, file is a type so you're effectively shadowing a builtin (albeit a rarely used one...).}

Answer 2

You could also use the list structure's index itself like so:

with open('a_file.txt','r') as f:
    lines = f.readlines()
readthese = [3,4,5]
for lineno in readthese:
    print(lines[1+lineno])

Since the list of lines already implicitly contains the line numbers based on index+1

If the file is too large to hold in memory you could also use:

readthese = [3,4,5]
f = open('a_file.txt','r')
for lineno in readthese:
    print(f.readline(lineno+1))
f.close()