🔒 please help me understand reading/writing from a txt file

[supple sedge]

I now know how to read and write to a file but need to understand how to save the file after I'm done..

with open('C:\\Users\\K\\Documents\\Python\\Protox\\Assets\\config.txt', 'r+') as configread:
    line = configread.readlines()
    line[0] = "light_mode = 0"
    print(line[0])

with open('C:\\Users\\K\\Documents\\Python\\Protox\\Assets\\config.txt', 'r+') as configread:
    line = configread.readlines()
    print(line[0])

output is:

light_mode = 0
light_mode = 1

Meaning it will change it and then close it but when I re-open the file and try to read the line it will give me the previous line.

[quiet aspen]

Hey @supple sedge!

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[quiet aspen]

@supple sedge

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[supple sedge]

Can someone help me figure out how to save it?

[knotty finch]

Your issue is that you're not changing the file. You're reading the file as a list of lines and then changing one of the elements of that list - that doesn't concern the file.

[supple sedge]

so im not writing to it

[knotty finch]

You'll want to write that list back to the file, e.g. by doing configread.seek(0) and configread.write("\n".join(line))

hmm, actually that's not quite right, because you also need to truncate it by configread.truncate()

[supple sedge]

could you maybe do that in a for loop if you had multiple lines of things?

[knotty finch]

You only need to rewrite the file once, no matter how many edits you do.

[supple sedge]

well this txt file has the one line could you maybe show me how

with configread.write could you specify line your writing to?

[knotty finch]

So consider:

with open('C:\\Users\\K\\Documents\\Python\\Protox\\Assets\\config.txt', 'r+') as configread:
    lines = configread.readlines()
    lines[0] = "light_mode = 0"
    configread.seek(0)
    configread.truncate()
    configread.write("\n".join(lines))

or even just do two opens, instead of carefully seeking etc:

with open('C:\\Users\\K\\Documents\\Python\\Protox\\Assets\\config.txt') as fo:
    lines = fo.readlines()
    lines[0] = "light_mode = 0"
with open('C:\\Users\\K\\Documents\\Python\\Protox\\Assets\\config.txt', "w") as fo:
    # 'w' mode truncates automatically, so we just write the new contents
    fo.write("\n".join(lines))

[supple sedge]

what is .seek doing?

[knotty finch]

Generally speaking, no. That's not how files work. Text files are internally just a sequence of characters, with linebreaks indicated by a special character. So to change a certain "line", you generally need to overwrite the entirety of the file past that point, because all of those contents need to be shifted.

[supple sedge]

das so annoying and like stupid aha

[knotty finch]

here's an example:

# before the file is:
"a = 'yay'\nb = 'nay'"
# we change yay to 'wooow!'. afterwards the file is:
"a = 'wooow!'\nb = 'nay'"
# as you can see, the second line got shifted - so even though we "only changed a bit of the first line", we need to overwrite the entire second line

[knotty finch]

this is most of the point of databases - in them, you can change a single row of a multiple-gigabyte database without rewriting it all.

[supple sedge]

i dont wanna create a database just for my gui.. ;-;

[knotty finch]

in most formats, you can't. a notable exception is FWF ("fixed width files" (or "format")). the idea is that you keep each line a constant width by having padding in it. That means you can immediately tell where in the file a certain line starts (if each line is 50 bytes, then line 30 starts at byte 1500, so you can seek right there without even reading the previous lines), and can edit a specific line without touching the rest of the file, as long as you don't exceed the limit. It's not very common in the modern world, however - it's a big hassle and you can just use a database instead.

[supple sedge]

too much info brain hurts

so with a FWF you can change a specific line as long as limit is not exceeded?

[knotty finch]

yeah.

[supple sedge]

what would you suggest i do then gimme sec

my problem being everytime i open this app everything that was altered theme and button colours wise is defaulted

i wanna make a lil config file of sorts that saves the users choices made in the session that the app was open, Suggestions?

[knotty finch]

just use json, really

there's nothing wrong with rewriting the entire file to make changes. it's just a config, it's small.

[supple sedge]

whats json sorry?

well if i have to rewrite the entirety of it again whats ,join

[knotty finch]

an extremely common serialization format. Check out https://realpython.com/python-json/ (and, if you aren't familiar with dicts and lists, something on them)

[supple sedge]

idk if im ready for dat..

[silver belfry]

if u use pickle to it looks ugly and unreadible in the file it can save even custom Classes

json is only for dict and list

[quiet aspen]

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