Python 3 - Looking up line in large text file by line number

Question

Title pretty much says it all: I've spent over a day figuring out how to fetch a single line of text out of a large text file through inputting its line number, without success. So far, it seems like the only solutions people are talking about online load the entire text file into a list, but it's simply too large to do so. The structure of the .txt file I'm working with is literally just a big listing of urls, one per line.

I've tried using handle.readline() , but that didn't help the pinpointing of the specific line. Because the file is so large, I can't really justify loading all of its lines into memory using the handle.readlines() method, so that's a bust as well. I tried to write a function using a for index,line in enumerate(handle) as found online, but that weirdly returns None . Any help is appreciated.

EDIT: Some code below that does not work:

fh = open("file.txt","a+")

def offsetfunc(handle,lineNum):
    line_offset = []
    offset = 0
    for line in handle:
        line_offset.append(offset)
        offset += len(line)
    handle.seek(line_offset[lineNum-1],0)

offsetfunc(fh,1)        #Returns IndexError
print(fh.readline())    #Presumably would be viable, if the previous statement worked?

Also, using the linecache technique loads the file into memory, so yeah, that's non-viable, too.

Answer 1

This program might do what you want:

def fetch_one_line(filename, linenumber):
    with open(filename) as handle:
        for n, line in enumerate(handle, 1):
            if n == linenumber:
                return line
    print("OOPS! There aren't enough lines in the file")
    return None

my_line = fetch_one_line('input.txt', 5)
print(repr(my_line))