Question

New to python and trying to learn the ropes of file i/o.

Working with pulling lines from a large (2 million line) file in this format:

56fr4
4543d
4343d
hirh3

I’ve been reading that readline() is best because it doesn’t pull the whole file into memory. But when I try to read the documentation on it, it seems to be Unix only? And I’m on a Mac.

Can I use readline on the Mac without loading the whole file into memory? What would the syntax be to simply readline number 3 in the file? The examples in the docs are a bit over my head.

Edit

Here is the function to return a code:

def getCode(i):
    with open("test.txt") as file:
        for index, line in enumerate(f):
            if index == i:
                code = # what does it equal?
                break
    return code

Answer 1

You don’t need readline:

with open("data.txt") as file:
    for line in file:
        # do stuff with line

This will read the entire file line-by-line, but not all at once (so you don’t need all the memory). If you want to abort reading the file, because you found the line you want, use break to terminate the loop. If you know the index of the line you want, use this:

with open("data.txt") as file:
    for index, line in enumerate(file):
        if index == 2: # looking for third line (0-based indexes)
            # do stuff with this line
            break # no need to go on