read a number at the end of a file

Question

I have a series of textfiles. They all end with a float, without a preceding whitespace ...foo123.456. The float has an unbounded number of digits.

the files are large so I would like to avoid reading them completely in memory. They have also different sizes.

How to avoid readgin the entire file?

You can use `file.seek`, hints available here: https://stackoverflow.com/questions/2301789/read-a-file-in-reverse-order-using-python — Blorgbeard, Oct 18 '17 at 20:59
@Liborio arbitrary length shouldn't matter as long as you know the *maximum* length. Just read a bunch of characters at the end of the file, then search backwards for the point where the number begins. — Mark Ransom, Oct 18 '17 at 21:01
uh, how arbitrary? There's a maximum, right? I guess you could just keep reading chunks until you hit something that's not digits or a `.`. — Blorgbeard, Oct 18 '17 at 21:02
at very large scale it is a waste to read even 1 byte extra per file... — 00__00__00, Oct 18 '17 at 21:04
That is not true. However, nothing stops you from using a chunk size of 1. — Blorgbeard, Oct 18 '17 at 21:09
It is likely to be false, I/O is buffered and 1 extra byte probably won't matter as much as you think because the OS already reads in larger chunks. — Paulo Scardine, Oct 18 '17 at 21:09

Paulo Scardine · Accepted Answer · 2017-10-18T21:13:15.187

2

Just read the last few bytes and use a regular expression in order to extract the float number.

Untested:

import re

with open('/path/to/file.txt') as input_file:
    input_file.seek(-100, 2)
    last_100_bytes = input_file.read()
    match = re.search(r'\D(\d+\.\d+)$', last_100_bytes)
    if match:
        print('The float is {}'.format(match.group(0)))
    else:
        print('no float found at the end of the file')

edited Oct 18 '17 at 21:13

answered Oct 18 '17 at 21:07

Paulo Scardine

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I think this is the answer after all...the over end of reading 100 bytes is insignificant as suggested by @Paulo Scardine – 00__00__00 Oct 19 '17 at 20:48

read a number at the end of a file

1 Answers1