How to save only 5 lines in file with python code?

Question

I have this function def to save some variable value I have add it manually by remote control of some E2 device

def saveKey(key):
     try:
         f=open("/etc/savekeys","w")
         f.write(str(key.replace("|", "")))
         f.close()
     except:
         pass

The value that save is like

7777776577777765

or

777AACD77777AACD

But every time I save a new value, the old value is deleted. I nedd to modify the def to make it save only 5 values, and when I save value number 6 it takes the place of number 1 and value number 7 takes the place of 2 and value number 8 takes the place of 3 ... etc ?!!

[`open`](https://docs.python.org/3.8/library/functions.html#open) with mode `w` will truncate the file. You may want to use `a` instead. — Ignatius Reilly, Mar 09 '23 at 21:45
[Why is "except: pass" a bad programming practice?](https://stackoverflow.com/questions/21553327/why-is-except-pass-a-bad-programming-practice) — Ignatius Reilly, Mar 09 '23 at 21:46
Does this answer your question? [Open a text file without clearing everything in it?](https://stackoverflow.com/questions/6334382) — Jorge Luis, Mar 09 '23 at 21:47
But how I can make like like loop ?!! only 5 values and after that the new value place line number 1 and second new value place with line number 2 ...etc — Fair Bird, Mar 09 '23 at 21:59
Just to be *crystal clear*, you are intending to create a file that only contains five lines of it, with the idea that your method here will rotate out the oldest line(s) for the newest line(s)? — Makoto, Mar 09 '23 at 22:00
How are you keeping track of what number value you are adding? If it's just based on the file itself, once it has five lines, you don't know which line was the most recently modified. — Michael Cao, Mar 09 '23 at 22:27

Makoto · Accepted Answer · 2023-03-09T23:10:14.370

2

Some assumptions to be declared:

Age in the file is determined from the top down. The first line is the oldest, and the last line is the newest. Any new lines are always placed at the end of the file.
You're only writing one line at a time.
No threading or multiple processes are involved in this.

In this case, the flow is simple.

Open the file to get all lines.
Add your line.
If the number of lines exceeds your cap, then remove the first line.
Write all lines out.

The code looks like this:

def save_key(key, max_lines=5):
    lines = []
    with open("file", "r") as f:
        lines = f.readlines()

    lines.append(key)

    lines = lines[-max_lines:]

    with open("file", "w") as f:
        f.writelines(lines)

edited Mar 09 '23 at 23:10

answered Mar 09 '23 at 22:53

Makoto

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I would use `lines = lines[-max_lines:]`. You don't even need the `if` statement before it if you do it that way. – Mark Ransom Mar 09 '23 at 22:58
1

This behavior makes more sense than what I think the asker described, but you should probably point out explicitly that it may be different. There's some ambiguity for sure, but it seems to me like the asker wanted the sixth value saved to go at the to of the file, taking the place of #1. – CrazyChucky Mar 09 '23 at 23:06
@CrazyChucky: I feel like this is stated in the assumption made; the first line is the oldest and the last line is the newest. I can make that a bit more explicit, though. – Makoto Mar 09 '23 at 23:09

How to save only 5 lines in file with python code?

1 Answers1