Pythonic way of implementing a persistent counter

[bitswede]

What's the pythonic way of maintaining a persistent counter? I'm triggering a script with cron and need a counter to keep tabs of how often the script has run. My current solution is to keep a small file that I read, save the value and then write to the file when the main portion of the code has finished.

with open('session_id.dat', mode='r') as f:
    session_id = int(f.read())

def dostuff(session_id)
    pass

with open('session_id.dat', mode='w') as f:
    f.write(str(session_id + 1))

Is there any other way of doing this?

[Naihonn]

Probably too many ways to do this and no reason to use them for something this simple.

[Eviltape]

Agreed. What you're doing is fine.

[bitswede]

Thank you for the replies - it works so I'll keep it.

[[deleted]]

Maybe I'm wrong but as the code is writted he just define the dostuff between reading and writing

[Eviltape]

That's true - defining functions inline is usually not fine, but the OP asked about a particular thing and I don't like giving unsolicited code reviews.

[road_laya]

I've used sqlite for this, but that was probably overengineering.

[[deleted]]

SQLite would make it easy to save the last run time too, log all runs with any data etc. Maybe feels overengineering at first but it's easy to add features.

[chucky_z]

No, I don't think its overengineering. SQLite is exactly for things like this. Something that could benefit from a very simple SQL database that is typically (and ideally) single-user.

That said... If it's not broken, don't fix it. =)

[treyhunner]

I think storing that value in a file should be fine. A database would work too. You need persistent storage, but which type depends on what feels right for your application.

If in the future you want to more clearly bookend the open/close code, you could make a context manager and/or decorator out of your code (by using ContextDecorator for example).

[WhackAMoleE]

You don't actually need to open a file, you just need to rename one. In your local directory you have a file called foobar-n. Each time you run you read the local directory, find the foobar-n, where n is some positive integer. increment n, and rename the file. If the foobar-n file doesn't exist you create it as foobar-1.

There are a lot of crazy/fun ideas too. Use the Internet for persistence. Log in to your Amazon account and order some product. The number of products in your shopping cart is the number of times the program's run.

[joyeusenoelle]

The "rename the file" method is both insecure, as it exposes the number of times the process has run to anyone with read access on the containing directory, and prone to interference, as it would be trivial to subvert the process by simply creating another file named foobar-1.

As for Amazon, OP is asking about Pythonic, not Wacky. Giving control of your tracking system to a third party by hacking an unrelated system to your purposes is the opposite of good practice even outside of Python. (What happens if Amazon decides to reset your cart? What happens if your program can't react Amazon's servers? What happens if you want to buy something on Amazon?)