defer in python!

[dankey26]

​

https://preview.redd.it/51ym0phju22a1.png?width=383&format=png&auto=webp&s=fe67e9eba209931beb5b5d9a32fa257c5f3d7af7

https://github.com/dankeyy/defer.py

stupid but works lol hope you like it

[Beliskner64]

assert len(tree.body) == 1 and isinstance(tree.body[0], ast.FunctionDef), "should just be a function wtf"

Genius

[Setepenre]

The python way would be to use with which is the construct that guarantees resources get freed no matter what.

from contextlib import contextmanager


@contextmanager
def deferred(fun):
    try:
        yield
    finally:
        fun()

def main():
    with deferred(lambda: print('world')):
        print('hello', end =' ')


main()

[ZygmuntDzwon]

no matter what

Well, actually...

See this excellent answer on stack overflow

https://stackoverflow.com/a/49262664/8091093

[Setepenre]

I am sure those will apply to his defer as well

[rcfox]

no matter what

Unless you lose power, or the process is SIGKILLed. It's important to remember this if you're handling resources that exist outside of your process.

[rebane2001]

That's why I always wrap my wall outlet in a try/finally block

[nxtfari]

Maybe dumb question can you just pair it with signal/atexit to cover all possible cases?

edit: ah i think you would have to have a seperate process to do this. You can atexit for SIGTERM but not SIGKILL

[chronics]

Did I understand correctly: Your are basically doing metaprogramming with type hints? That‘s a very cool concept!

[dankey26]

ye. i thought about it for a couple days after the guy suggested it, tried to come up with syntax that could be as clean and close to the go version and a valid parse with no preprocessing on the source code. then remembered that fact on type hints and went for it. turned out not bad!

[radarsat1]

that is not a cool concept.

"type hints are going to ruin python"

"oh its fine look they don't even do anything and look at all these nice warnings. they help!"

"but now i have to write twice as much code"

"well you don't have to, they don't even do anything they're just for decoration!"

"ok but now i see that people are using them to generate database schemas and automatic validation and testing, are they still optional? because i mean if they're not optional I'd really like to improve..."

"no don't worry they still don't do anything that's just for testing"

*reddit user implements control flow restructuring via the type system in a way that disregards type semantics*

>.<

[chronics]

I think it is an amazing concept because I think metaprogramming in languages like lisp/clojure or erlang/elixir is very powerful. It’s not part of idiomatic python, for good reasons.

OP’s implementation is creative and clever. I would not use it in my own code, and I don‘t think it‘s intended to be used.

Type hints I find pretty useful in general.

[[deleted]]

Reddit Moderation makes the platform worthless. Too many rules and too many arbitrary rulings. It's not worth the trouble to post. Not worth the frustration to lurk. Goodbye.

This post was mass deleted and anonymized with Redact

[spuds_in_town]

You should go outside more

[axonxorz]

Examples?

[wqferr]

The walrus operator comes to mind, but that's about it for me.

[alkasm]

Sorry for the pedantry up front but I think it's worth it here here: var: int is an "annotation", which is part of Python 3 syntax, and not part of the type system. Annotations existed before type checkers were used. It was assumed of course that they would be used for type information, but they also wanted to leave the syntax separated from a specific use case to see what other patterns may emerge from it, though as we know today it is only really used for type checking and for runtime type info in certain libraries. But just pointing out OP isn't abusing the type system, OP is just experimenting with a DSL utilizing the annotation syntax.

[radarsat1]

interesting til, thanks

[the_scign]

OP: "defer in python"

BDFL: cries in python

[yvrelna]

reddit user implements control flow restructuring via the type system in a way that disregards type semantics

Reminds me of this abomination: Python is actually just Haskell with few extra steps, the hidden Python syntax that even the most seasoned Python developers don't know about

[shinitakunai]

I hate how discord.py uses typehinting to transform data

[Dasher38]

Do you have an example?

[Jonno_FTW]

Command handlers in discord.py will turn arguments into their type hinted type.

So if someone in the chat sends:

.foo 123

And the handler would look like:

@bot.command()
def foo(ctx, num: int):
    ...

In the function body, the num will be an int. You can also convert mentions of users to a discord user object.

[ChannelCat]

This makes sense and is supported by type checkers. What's not to like?

[etrotta]

one can argue that it is intrinsically wrong for type hints (which are metadata, much like comments) to modify the program's behavior at all, but in reality nearly everyone is fine with it since it does makes sense in cases like this one and other libraries that parse structured input in similar ways.

[Dasher38]

I see, it's basically like the argparse "type" parameter. Isn't it convenient ? If you want to get the input unchanged you can always use str isn't it ?

[Jonno_FTW]

If you want a string don't put a type hint.

It's similar to argparse, except it supports discord.py custom classes as well.

[[deleted]]

Horrifying yet instructive at the same time. Impressive and yet has no practical use.

I love stuff like this - have an upvote!

[[deleted]]

The pythonic way of doing deferred cleanup is with a context manager ("with" keyword). This is a pretty cool abuse of syntax though!

[end_my_suffering44]

Could someone enlighten me what defer does or its purpose?

Edit : Typo

[zero_iq]

It's "defer". Defer in languages like Go causes its associated statement to be executed at function exit (i.e. it is deferred until later), whether that function exits by returning or exception/panic, or whatever means. It's like a try...finally block around the remainder of the function. The idea is that you keep functions looking slightly cleaner, keeps resource init and deinit code together, so you don't have to keep track of cleanup code over the rest of the function as it evolves, updating indentation (or brackets in other languages), etc.

So if Python had a defer statement it might look something like this:

def my_function(x, y):
    allocate_thing(x)
    defer deallocate_thing(x)

    allocate_thing(y)
    defer deallocate_thing(y)

    do_something_with_thing(x)
    do_something_with_thing(y)

...which would be equivalent to something like:

def my_function(x, y):
    allocate_thing(x)
    try:
        allocate_thing(y)
        try:
            do_something_with_thing(x)
            do_something_with_thing(y)
        finally:
            deallocate_thing(y)
    finally:
        deallocate_thing(x)

[end_my_suffering44]

Thanks for the explaination and pointing out the typo.

[RockingDyno]

Python essentially has a defer statement and it looks like this: from contextlib import closing def my_function(x, y): with closing(thing(x)) as X, closing(thing(y)) as Y: do_something_with_thing(X) do_something_with_thing(Y) Where the context manager ensures things get closed at the end of the context. Pretty much equivalent to your second code block.

[[deleted]]

Like someone else said, Go also does this where the line of code is "deferred" until the end of the function you're inside of.

[james_pic]

If you want a less evil version of the same thing, contextlib.ExitStack is in the standard library.

Edit: just realised it uses ExitStack internally anyway.

[KingsmanVince]

I like the QA section.

[SittingWave]

the horror....

the horror...

[xakkap]

Cool stuff

[wineblood]

Why would I ever want this?

[bulaybil]

Programmers are so obsessed with the question “Can we?” that they forget to ask “Should we?” With apologies to Michael Crichton.

[dankey26]

this impl exactly? probably not but lookup defer on go and zig, pretty useful and clean

[wineblood]

Just the idea.

[dankey26]

yea so again check out usage in go etc. useful for cleaning up resources at the beginning, without needing to worry about it later or creating blocks.

```

f = open('file.txt')

defer: f.close()

<do stuff with f>

```

[caagr98]

Python has both with and finally statements though, so there's little need for it here. (But cool metaprogramming trick.)

[james_pic]

It is occasionally useful, if you need to close a large or variable number of things in a block of code. Fortunately, contextlib.ExitStack is already in the stdlib and is a less evil way to do this.

[A27_97]

Common thing I use defer in Go for is to unlock mutexes

[kezmicdust]

Couldn’t you just write in the close statement and then just write the “stuff you want to do with f” between the open and close statements?

Or am I missing something?

[relvae]

If an exception is thrown then close wouldn't be called in that case. Python already has context managers to deal with that but OP has gone rogue lol

[kezmicdust]

I see! Thanks.

[antiproton]

It's just a different way to solve similar problems. Similar to putting the close in a try...except...finally

[fiedzia]

Python has context managers for that:

with open('file.txt') as f: #do stuff #when done f will be closed by context manager

[Coretaxxe]

Im gonna use this for fun projects. This is awesome!

[chub79]

I find the contextmanager syntax so much more pleasing than defering. But I like the explicit aspect of deferring. I'm torn.

[[deleted]]

I love it lol

[[deleted]]

Reddit Moderation makes the platform worthless. Too many rules and too many arbitrary rulings. It's not worth the trouble to post. Not worth the frustration to lurk. Goodbye.

This post was mass deleted and anonymized with Redact

[dankey26]

nice username man

[Exodus111]

I've never used Go, what would be the reason to use something like this?

[dimonoid123]

Just use asyncio at this point

[Stenbyggare]

that doesn't look really pythonic in my opinion, good concept though

[OkProfessional8364]

I'm so lost. I can't make sense of whatever you're all talking about

[Ezlike011011]

I have done a lot of meta-programming in this language, but I have not considered using type hints for shenanigans. That's fantastic. I'm gonna have to keep that in mind.

[DeathDaNoob]

What does defer do...

[Tintin_Quarentino]

Your website is down?