In Python, how to test whether a line is the last one?

Question

Suppose I want to process each line of a file, but the last line needs special treatment:

with open('my_file.txt') as f:
    for line in f:
        if <line is the last line>:
            handle_last_line(line)
        else:
            handle_line(line)

The question is, how does one implement ? There seems to be no function for detecting end-of-file in Python.

Is there another solution than read the lines into a list (with f.readlines() or similar)?

Answer 1

Process the previous line:

with open('my_file.txt') as f:
    line = None
    previous = next(f, None)
    for line in f:
        handle_line(previous)
        previous = line

    if previous is not None:
        handle_last_line(previous)

When the loop terminates, you know that the last line was just read.

A generic version, letting you process the N last lines separately, use a collections.deque() object:

from collections import deque
from itertools import islice

with open('my_file.txt') as f:
    prev = deque(islice(f, n), n)
    for line in f:
        handle_line(prev.popleft())
        prev.append(line)

    for remaining in prev:
        handle_last_line(remaining)

Answer 2

You can use itertools.tee to iterate on two copies of an iterable:

next_lines, lines = itertools.tee(file_object)
next(next_lines)
for next_line, line in zip(next_lines, lines):
    handle_line(line)
last_line = next(lines, None)
if last_line is not None:
    handle_last_line(last_line)