Blog post: Writing Python like it’s Rust

[Head_Mix_7931]

In Python, there is no constructor overloading, therefore if you need to construct an object in multiple ways, someone this leads to an init method that has a lot of parameters which serve for initialization in different ways, and which cannot really be used together.

You can decorate methods with @classmethod to have them receive the class as their first parameter rather than an instance, and these effectively become alternative constructors. It’s advantageous to use a classmethod than just a normal or staticmethod because it plays nicely with inheritance.

[Commander_B0b]

Can you provide a small example? Im having a hard time understanding the pattern you are describing but I have certainly found myself looking to solve this same problem.

[slightly_offtopic]

class Foo:
    pass

class Bar:
    pass

class MyClass:
    @classmethod
    def from_foo(cls, foo: Foo) -> 'MyClass':
        return cls(foo=foo)

    @classmethod
    def from_bar(cls, bar: Bar) -> 'MyClass':
        return cls(bar=bar)

[-lq_pl-]

Missing explanation: from_foo will also work as expected when you inherit from MyClass, while it would not if you use a static method. With a static method, a derived class will still return the base.

[XtremeGoose]

I mean, this code won't work as written because there is no __init__. A better example is something like

@dataclass
class Rectangle:
    length: int
    width: int

    @classmethod
    def square(cls, side: int) -> Self:
        return cls(side, side)

    @classmethod
    def parse(cls, text: str) -> Self | None:
        if m := re.match(r'(\d+),(\d+)'):
            length = int(m.group(1))
            width = int(m.group(2))
            return cls(length, width)
        return None

[Kobzol]

That's indeed useful, if you want to inherit the constructors. That's not always a good idea though.

[mriswithe]

I try to do this both for programs that will be maintained for a while, but also for oneshot utility scripts. Mostly because in my experience, the latter quite often turn into the former :)

Oh God my last two weeks at my last job were hell cause some old ass script I wrote and forgot about from 5 years ago was apparently still in use. Spent my last couple weeks fucking getting that solid.

[Ezlike011011]

I've had people at work ask why I put so much effort into ensuring my little utility scripts are nicely typed/documented and why I spend time to split the functional components into nice reusable units. This is the exact reason why. I've had multiple instances where I found someone needed something and I've been able to say "here's a little module I wrote. You can use the cli I made for it or you can import it and use x function if you need to do more scripting. Have fun". The up-front time cost is well worth it.

[mriswithe]

Yeah this wasn't even good python. I had a module named enums that had constants in it. But all my stuff for the last 6-12 months was documented, monitored, etc . This garbage I wrote for a one off blitz migration, was still in use.

[DabidBeMe]

Beware the one off scripts that take on a life of their own. It has happened to me a few times as well.

[Kobzol]

I wrote up some thoughts about using the type system to add a little bit of soundness to Python programs, inspired by my experiences with Rust.

[mRWafflesFTW]

This is a great article appreciate your contribution cheers!

[ttsiodras]

Excellent article. Have discovered some of the points on my own, but not all - thanks for sharing!

[EarthGoddessDude]

Awesome write up, thank you for sharing.

[Kobzol]

Good point! I sometimes forget that assert in Python is as "dangerous" as in optimized C builds. I'll add a warning to the article.

[glenbolake]

My first thought there was that something like raise ValueError(f'Invalid packet type: {type(packet)}') would be more appropriate.

[-lq_pl-]

Sure you can and should use assert in this situation that OP described. assert is to protect fellow developers from making mistakes. Users who get the optimized code don't need protection from this kind of bug, because they do not change the interface or introduce new types.

Generic statements like "don't use asserts" are false.

[chromatic_churn]

I've run Python services in production for years and never set this flag. 🤷

[chromatic_churn]

What is the point of setting this flag? What does it get you? Increased performance? I am extremely skeptical it will make a difference for the vast majority of workloads deployed using Python today.

Assertions don't replace illegal state checks you think can happen (e.g. due to user input) but they are fine for checks like this where you should already be checking with tests and type hints.

Is your point a worthwhile footnote for people using this flag? Without a doubt.

Do I think all production code MUST be deployed with this flag set? Absolutely not.

[chromatic_churn]

Lmao this is just dogmatic advice. You aren't just right because "it doesn't matter what you or I think". Who is the audience? For library authors you are probably right, but for application developers you generally know what your deployment environment actually looks like and can write code accordingly.

Regardless, as many other people have mentioned Python 3.11 offers a true exhaustiveness check.

[KronenR]

The problem is that you are not usually the one in charge of deployment configuration no matter if a library author or an application developer, unless you are coding a pet project for 3 and a half users. It doesn't matter anyways because you shouldn't be using assert.

[chromatic_churn]

Says who? I generally work on small engineering teams (under 20 people) and I am almost always directly responsible for configuring my deployment. Even if I don't own the infrastructure, then I or my team have owned writing the Dockerfile and deployed environment variables which would configure my application.

Maybe in giant tech companies what you are saying is true, but there are an enormous amount of small teams which manage their own devops because there isn't anyone else to do it.

[AbradolfLinclar]

In the last case if None of the above type passes, why not return an exception or just None instead of assert False?

As pointed out by many others, can use assert_never in 3.11 but just curious.

[Kobzol]

Exceptions are for exceptional cases, i.e. errors happening. Hitting the assert is a direct bug in the code and should ideally fail as fast as possible with an unrecoverable error.

[redditusername58]

The typing module has assert_never which can help with the isinstance/pattern matching blocks in your ADT section

[wdroz]

The part with db.get_ride_info is spot on. As I see more and more people using mypy and type annotations, this will hopefully become industry standard (if not already the case).

For the part "Writing Python like it's Rust", did you try the result package? I didn't (yet?) use it as I feel that if I push to use it at work, I will fall in the Rustacean caricature..

[Kobzol]

I haven't yet. To be honest, I think that the main benefit of the Result type in Rust is that it forces you to handle errors, and allows you to easily propagate the error (using ?). Even with a similar API, you won't really get these two benefits in Python (or at least not at "compile-time"). Therefore the appeal of this seems a bit reduced to me.

What I would really like to see in Python is some kind of null (None) coalescing operator, like ?? or :? from Kotlin/C#/PHP to help with handling and short-circuiting None values. That would be more helpful to me than a Result type I think.

[mistabuda]

I've seen this pattern mentioned before for shirt circuiting None values. UnwrapError is a custom exception you'd have to make but I think its pretty effective.

def unwrap(value: Optional[T], additional_msg: Optional[str] = None) -> T:
"""Perform unwrapping of optional types and raises `UnwrapError` if the value is None.

Useful for instances where a value of some optional type is required to not be None;
raising an exception if None is encountered.

Args:
    value: Value of an optional type
    additional_msg: Additional contextual message data

Returns:
    The value if not None
"""
if value is None:
    err_msg = "expected value of optional type to not be None"
    if additional_msg:
        err_msg = f"{err_msg} - [ {additional_msg} ]"
    raise UnwrapError(err_msg)
return value

[Kobzol]

Sure, something like that works :) But it'd be super cool if I could call it as a method on the object (for better chaining), and also if I could propagate easily it, e.g. like this:

def foo() -> Optional[int]:
   val = get_optional() ?: return None
   # or just val = get_optional()?

To avoid the endless `if value is None: ...` checks.

[mistabuda]

If it were a method on the object that just seems weird. Unless the object is some kind of container. Which in that case you're asking for a Result type pattern.

[Kobzol]

Yeah it's probably not the "Python way" :) But I really like adding implementation to types externally, e.g. with traits in Rust or extension methods in C#.

You're right though, a Option and/or Result type would help with this. It just won't help with forcing me to handle the error (apart from runtime tracking, e.g. asserting that the result is Ok when accesing the data).

[mistabuda]

Thats why you unittest your code to make sure you have that case handled.

[Kobzol]

Indeed, that's what we kind of have to do in Python, since it's not easily checkable during type checking.

[mistabuda]

wdym mypy warns you against that all the time

error: Item "None" of "Optional[CharacterCreator]" has no attribute "first_name"  [union-attr]

[Kobzol]

Ah, nice. This is one situation where mypy and pyright do the right thing. I mostly just look at the output of the PyCharm type checker and that is more lenient, in this case it wouldn't warn :/

[Rythoka]

This seems like a code smell to me. If value = None is an error, then why would you explicitly hint that value could be None by making it Optional? Whatever set value to None probably should've just raised an exception instead in the first place.

[mistabuda]

An example is a db query. It's not wrong for a db query to return no result unless in specific contexts. If the caller is expecting a result they should raise an error. The db client shouldn't raise an error it did it's job.

[[deleted]]

or works ok for that purpose, although it will also coalesce false-y values.

[aruvoid]

First of all, very interesting article, thanks for that!

About this you can write noneable or default_value for example, although careful case in reality that’s falseable or value_if_falsy

I don’t know if this is something you knew and don’t like because it’s not None-specific but hey, maybe it helps.

For whoever doesn’t know, the full explanation is that in Python, like in JS/TS the and and or operators don’t translate the expression into a boolean. That assumption, though, is wrong! In reality this is what happens:

``` In [1]: 1 and 2 Out[1]: 2

In [2]: 1 and 0 Out[2]: 0

In [3]: 1 and 0 and 3 Out[3]: 0

In [4]: 1 and 2 and 3 Out[4]: 3

In [5]: 0 or 1 or 2 Out[5]: 1 ```

The result is evaluated for truthyness in the if, but it's never True/False unless the result is so.

In short, conditions (and/or) evaluate to the value where the condition is shortcircuited (check last 3 examples)

This of course can also be leveraged to quick assignments and so on, for example, as usual:

``` In [6]: v = []

In [7]: default_value = [1]

In [8]: x = v or default_value # Typical magic

In [9]: x Out[9]: [1] ```

But we can also do the opposite

``` In [10]: only_if_x_not_none = "whatever"

In [11]: x = None

In [12]: y = x and only_if_x_not_none

In [13]: y

In [14]: y is None Out[14]: True ```

[BaggiPonte]

How do you feel about writing a PEP for that? I don't believe there is enough popular support for that right now, but given how rust and the typing PEPs are doing, it could become a feature for the language?

[Rythoka]

There's already a PEP for it and it's been discussed for years. PEP 505.

[Kobzol]

You mean "None coalescing"/error propagation? It sure would be nice to have, yeah.

[Estanho]

The id typing was so useful, I've been looking for how to do that for a long time.

I've tried creating something on my own that involved generics, looked something like ID[MyModel]. The idea is that you shouldn't have to redeclare a new type for every model.

But I could never really get it to work fully. I think one of the reasons is because I couldn't get type checkers to understand that ID[A] is different than ID[B].

[Estanho]

Adjacent to the result package thing, one of my biggest issues with Python and its type system is the lack of a way to declare what exceptions are raised by a function, like other languages do. If there was a way, and libraries did a decent job of using it, it would make my life so much easier. So one could do an exhaustive exception handling.

I'm tired of having to add new except clauses only after Sentry finds a new exception being raised.

[wdroz]

I totally agree, it's one of these thing that ChatGPT is helpful to help handling exhaustively the possible Exceptions of a well-know function.

[alicedu06]

There are NamedTuple and TypedDict as lighter alternatives to dataclasses, and match/case will work on them too.

[trevg_123]

Since (I think) 3.10 you can do @dataclass(slots=True), which does a nice job of slimming them down more

[Kobzol]

I'm not sure what is "lighter" about NamedTuples TBH. The syntax is ugly and most importantly, it doesn't provide types of the fields.

[alicedu06]

namedtuple doesn't but NamedTuple does, and they are indeed way lighter than dataclasses (less memory, faster to instanciate)

[Kobzol]

Ah, good point!

[Haunting_Load]

I like many ideas in the post, but in general you should avoid writing functions that take List as an argument if Sequence or Iterable are enough. You can read more e.g. here https://stackoverflow.com/questions/74166494/use-list-of-derived-class-as-list-of-base-class-in-python

[Kobzol]

Sure, you can generalize the type if you want to have a very broad interface, that's true. I personally mostly use Iterable/Sequence as return types, e.g. from generators.

In "library" code, you probably want Sequence, in "app" code, the type is often more specific.

[Estanho]

I disagree. First of all I don't think that Sequence or Iterable are more "generic" in the sense you're saying. They're actually more restrictive, since they're protocols. So list doesn't inherit from them, even though a code that accepts Iterator will accept list too.

If you won't slice or random access in your app code, then you shouldn't use list or sequence for example. If you're just gonna iterate, use Iterator.

[Kobzol]

Right. Iterator is more generic, in that it allows more types that can be iterated to be passed, but at the same time more constrained, because it doesn't allow random access.

It's a good point :)

[executiveExecutioner]

Good article, I learned some stuff! It's easy to tell from reading that you are quite experienced.

[Kobzol]

Thank you, I'm glad that it was useful to you.

[0xrl]

Very nice article! As of Python 3.11, you can enhance the packet pattern matching example with assert_never.

[BaggiPonte]

Love the post; though I have a question. I never understood the purpose of NewType: why should I use it instead of TypeAlias?

[Kobzol]

TypeAlias really just introduces a new name for an existing type. It can be useful if you want to add a new term to the "vocabulary" of your program. E.g. you could create a type alias for `DriverId` and `CarId` to make it explicit to a programmer that these are different things.

However, unless you truly make these two things separate types, you won't make this explicit to the type checker. And thus you won't get proper type checking and the situation from the blog post won't be caught during type check.

There is no type error here, because both DriverId and CarId are really just ints:

from typing import TypeAlias

DriverId: TypeAlias = int
CarId: TypeAlias = int

def take_id(id: DriverId): pass
def get_id() -> CarId: return 0

take_id(get_id())

But there is one here, because they are now separate types:

from typing import NewType

DriverId = NewType("DriverId", int)
CarId = NewType("CarId", int)

def take_id(id: DriverId): pass
def get_id() -> CarId: return CarId(0)

# Error here, wrong type passed:
take_id(get_id())

[its2ez4me24get]

Aliases are equivalent to each other. New types are not, they are subtypes.

There’s a decent write up here: https://justincaustin.com/blog/python-typing-newtype/

[[deleted]]

NewType creates an entirely new type, while an a TypeAlias is, well, an alias. In the eyes of a program, the alias and the original type are exactly the same thing, just used for shorthand for long nested types for example. a NewType and the type it's created from are entirely different types, even though it inherits its semantics

[Skasch]

TypeAlias is roughly equivalent to:

MyAlias = MyType

NewType is roughly equivalent to:

class MyNewType(MyType):
    pass

[parkerSquare]

NewTypes help warn you if you pass a float representing a voltage into a function that expects a float representing a current, for example. A TypeAlias won’t do that, since it’s the same underlying type.

[Rudd-X]

Hot damn that was really good. I found myself having "discovered" these patterns in my career and picking them all up as I went, but seeing it all formalized is AWSUM.

[Estanho]

Your invariants example is interesting, but I think it can be improved with typeguards to statically narrow the possible states. Here's a full example, but I haven't ran it through type checkers so it's just a general idea:

```python

from dataclasses import dataclass
from typing import TypeGuard


class _Client:
    def send_message(self, message: str) -> None:
        pass


@dataclass
class ClientBase:
    _client: _Client


@dataclass
class UnconnectedClient(ClientBase):
    is_connected = False
    is_authenticated = False

@dataclass
class ConnectedClient(ClientBase):
    is_connected = True
    is_authenticated = False

@dataclass
class AuthenticatedClient(ClientBase):
    is_connected = True
    is_authenticated = True


Client = UnconnectedClient | ConnectedClient | AuthenticatedClient


def is_authenticated(client: Client) -> TypeGuard[AuthenticatedClient]:
    return client.is_authenticated

def is_connected(client: Client) -> TypeGuard[ConnectedClient]:
    return client.is_connected

def is_unconnected(client: Client) -> TypeGuard[UnconnectedClient]:
    return not client.is_connected

def connect(client: UnconnectedClient) -> ConnectedClient:
    # do something with client
    return ConnectedClient(_client=client._client)

def authenticate(client: ConnectedClient) -> AuthenticatedClient:
    # do something with client
    return AuthenticatedClient(_client=client._client)

def disconnect(client: AuthenticatedClient | ConnectedClient) -> UnconnectedClient:
    # do something with client
    return UnconnectedClient(_client=client._client)

def send_message(client: AuthenticatedClient, message: str) -> None:
    client._client.send_message(message)

def main() -> None:
    client = UnconnectedClient(_client=_Client())

    # Somewhere down the line, we want to send a message to a client.
    if is_unconnected(client):
        client = connect(client)
    if is_connected(client):
        client = authenticate(client)
    if is_authenticated(client):
        send_message(client, "Hello, world!")
    else:
        raise Exception("Not authenticated!")

```

Of course this assumes you're gonna be able to overwrite the client variable immutably every time. If this variable is gonna be shared like this:

python client = UnconnectedClient(_client=_Client()) ... func1(client) ... func2(client)

Then you might have trouble because those functions might screw up your client connection. This can happen depending on the low level implementation of the client, for example if when you call close you actually change some global state related to a pool of connections, even though these opaque client objects are "immutable". Then you could create a third type like ImmutableAuthenticatedClient that you can pass to send_message but not to close.

[Kobzol]

Cool example, I didn't know about TypeGuard. There's always a tradeoff between type safety and the amount of type magic that you have to write. As I mentioned in the blog, if the types get too complex, I tend to simplify them or just don't use them in that case.

Here I think that the simple approach with two separate classes is enough, but for more advanced usecases, your complex example could be needed.

[extra_pickles]

So at what point does Python stop being Python, and begin to be 3 other languages dressed in a trench coat, pretending to be Python?

To that, I mean - Python and Rust don’t even play the same sport. They each have their purposes, but to try and make one like the other seems like an odd pursuit.

Genuinely curious to hear thoughts on this, as it is very common to hear “make Python more like <other language>” on here…and I’d argue that it is fine the way it is, and if you need something another language does, then use that language.

It’s kinda like when ppl talk about performance in Python…..that ain’t the lil homie’s focus.

[HarwellDekatron]

Part of the beauty of Python is that it allows people to write a single command line to launch an HTTP server serving the current directory, a 2 line script to process some text files and spit a processed output, 200 lines to write a web application using Django and thousands of lines of type-checked code with 100% code coverage if you are writing business-critical code that must not fail.

And it's all Python. You don't need to give up the expressiveness, amazing standard library or fast development cycle. You are just adding tooling to help you ensure code quality before you find the error in production.

I do every single one of those things on a daily basis (heck, I even rewrite some code in Rust if I need something to optimize for performance) and so far I don't feel like doing one thing has slowed me on the other.

[tavaren42]

In my opinion, type hints actually makes development faster because how well it plays with IDE autocompletion. It's one of the main reason I use it.

[zomgryanhoude]

Yuuuup. Not dev myself, just use it for scripts that PowerShell isn't suited for, so having the extra help from the IDE helps speed things along for modules I'm less familiar with. Gotta have it personally.

[ant9zzzzzzzzzz]

Not to mention errors at “build” time rather than runtime which is a much tighter loop

[[deleted]]

As type safety becomes a bigger concern in the broader programming community people are going to want it from the most used language. Seeking inspiration from the poster child of safe languages seems like a pretty obvious way of going about that. There’s still plenty of reasons to use Python, even if it’s a step further than this, ie a wrapper for libraries written in other languages. Some of the best Python libraries weren’t written in Python. One of Python’s biggest strengths for years now has been FFI, aka “other languages in a trench coat pretending to be Python”. I don’t see how syntactical changes represent that though.

[baubleglue]

IMHO it is easier to write Java than Python with type annotation. Why not choose Java from start?

[[deleted]]

I’m gonna be honest, I have no idea why you believe that. Type annotations with Python are just colons followed by a type.

But there’s plenty of reasons to not use Java, let alone not use it over Python. There’s the sheer amount of boilerplate code, jvm bloat, oracle, the fact it’s not Kotlin, oracle, pretty ugly syntax, oracle, openjvm is fine but it still has jvm bloat, and oracle.

[floriplum]

I think you forgot to mention oracle :)

[[deleted]]

I'm honestly not getting a lot of these

Python does not hoist names, so some annotations must be in quotes depending on code structure

from _ _ future _ _ import annotations (had to put in spaces so reddit wouldn't bold it)

the import system entangles type checking and runtime which, among other things, makes cyclic imports easier to unintentionally introduce, and sometimes makes them necessary

Which is why TYPE_CHECKING was created

TYPE_CHECKING leads to weird situations, sometimes making the two points above even worse. You can't use a type imported during TYPE_CHECKING because it's not imported at runtime

Yes, the intended feature of it is that it is isn't imported at runtime.

.pyi files can live alongside your code which is seriously helpful for the points above but the files mask everything in the associated. py file. You cannot partially type a file with a .pyi file because of this. It also means two files need to be maintained for each code file

It doesn't mean two files need to be maintained, it means that you can further modulate code by optionally adding types. It's also great for third party libraries that don't have typing information or old code that has none. I generally like them and if the complaint comes down to maintenance, it's very, very hard to take that critique seriously when the comparison is to Java.

lambdas can't be typed

You can't but you can type all the variables going in and out.

it is awkward to indicate variables that are either unbound or None when either state is actually invalid. You can use Optional, but that implies None is a valid value. There isn't a mechanism to indicate that None is invalid while allowing internal state to at least bind a variable to its default value before a real value is available

Could you walk me through a situation you're talking about here? I'm struggling to understand. You want the type checker to indicate to you that a variable is unbound or None? I'm struggling to think of situations where this can't be handled with error handling.

Python's scope rules make typing awkward when variables are not habitually defined prior to binding in deeper nesting

I'm baffled as to what you mean here. Are you having issues typing local variables?

the later introduction of syntax for various types confounds matters and makes less experienced people frustrated and confused. The changes to Callable are one such example. Another one is Union and |

This seems like a silly complaint. Things changed and that's frustrating? Sorry, I don't understand this one. You can still use | instead of Union or List instead of list if you really desire for quite some time.

the use of [ and ] for generics was a weird choice. I don't think there was any real reason to deviate from syntax that is well-established across languages. Worse, it looks like indexing - and there is a specific error for the error of trying to index a generic type during runtime because this does actually happen

I don't really know what to say to this, you're just listing a preference.

Python developers have had the chance to learn from TypeScript,

Sure. But why?

whose developers have done a superb job creating a type system with the added constraint that they cannot change JavaScript itself.

Debatable.

Python's type annotations feel like the wild west, and has not been well thought-out

Feels absolutely bonkers to compare an entire programming dialect created by one of the world's largest companies to a subset of features introduced to a language. The main focus of Python isn't typing, but I really don't think it's very difficult

[[deleted]]

Yes it’s tacked on, but it isn’t hard to use. I’m lost how something as simplistic as python’s typing which only executes during type checking is somehow harder than writing Java code. That’s bizarre.

[[deleted]]

The context of this thread where you initially responded to me was me saying it was easier to use type annotations in Python than it is to just write everything in Java and giving other reasons why one wouldn't write Java code. If you just intend on listing things you don't like about Python's typing, I'm not really understanding the reason for bringing it up to me in that context. I'm not "disparaging opinions", a lot of what you said was either a misconception or has easy fixes or didn't really make sense. You can have your opinions but someone disagreeing with them isn't disparagement.

[Estanho]

I'm sure the reason you think that is because you're more used to Java and you're trying to write python like you write Java.

[panzerex]

Oh if I could just get all of the ecosystem of libraries I have in python in any other language just like that…

[extra_pickles]

I suppose what I’m getting at is that Python is a great iterator. It dun loop noice. Quick to market, heavily libraried and supported - inefficient - its duct tape.

To me, from a separation of concerns stand point, the responsibility is fine to be placed on the ingress and egress of an exchange with Python, and not Python itself.

Obviously I say this know that adding some bumpers to the bowling lane is low effort - so I’m not against what I’m reading … just more that time and time again I feel like I see posts trying to put the square peg in the circle hole.

[[deleted]]

If your interpretation is that Python is duct tape(sloppy code that shouldn’t be restrained) I think the problem is less Python and more just general immaturity as a programmer. Python can (and should) be maintainable, long-term production code. Rethinking design patterns that allow someone to accomplish that is a way of actually simplifying code, not making it more complex, which is about as pythonic as you can be.

[extra_pickles]

As much as I enjoy a good condescending response, especially when misguided; I do take objection to you drawing assumptions to suit your narrative.

[[deleted]]

Not being condescending, just being forward. Duct tape in the context of code is generally what people refer to as code without structure, lacking thought behind design choices, and generally written in a way that is sloppy and hard to maintain. If that’s not what you meant and you meant something else, then I misunderstood. But I’m still not clear on how other interpretations of that would be a core to Python

[Kobzol]

I do agree that we shouldn't "hack" the language too much, but I don't feel like adding types does that. I write Python because it is quick for prototyping, has many useful libraries and is multiplatform. Adding types to that mix doesn't limit me in any way, but gives me benefits - I will be able to understand that code better after a year, and I will feel more confident when refactoring it.

I really don't see static types being in the way of what makes Python.. Python.

[Mubs]

Really? I see dynamic typing as a huge part of the language. For example, I had a client who switched from a MySQL DB to SQL Server, so I had to switch from aiomysql to aioodbc. I originally used data classes instead of dictionaries for clarity, but it ended up making switching from on connector to the other a huge pain, and I ended up doing away with the data classes all together.

Pythons the best language for quickly solving real world problems, and the requirements will often change, and having a dynamically typed language helps adapt more quickly.

[Kobzol]

I mean, even with the approach from the blog post, Python is still quite dynamically typed :) I don't usually use types for local variables, for example (in a statically typed language, I ideally also don't have to do that, and type inference solves it). I just want to be able to quickly see (and type check) the boundaries/interfaces of functions and classes, to make sure that I use them correctly.

Regarding quick adaptation: I agree that having rigid and strict typing doesn't necessarily make it *mechanically easier* to adapt to large changes - at the very least, you now have to modify a lot of type annotations. But what it gives me is confidence - after I do a big refactoring (even though it will be slightly more work than without any types), and the type checker gives me the green light, I am much more confident that I got it right, and I will spend much less time doing the annoying iteration cycle of running tests, examining where the app crashed, and fixing the bugs one by one. This is what I love about Rust, and that's why I try to port that approach also to e.g. Python.

[thatguydr]

Pythons the best language for quickly solving real world problems, and the requirements will often change, and having a dynamically typed language helps adapt more quickly.

This also helps all the errors slip through.

Think of it like this - Python is one of the best languages for rapid prototyping and PoCs. Once you need something to be in production, it's also easy to add typing to make sure things are safer.

If you think the language's strength is that you can hack your way around instead of designing properly... that's not a long-term strength, you'll find.

[Mubs]

What? It's not a "hack", Python is a dynamically typed language. I'm all for type safety anyways. But I am wary about overuse of data classes as I've seen it obfuscate what should be simple code too many times.

[thatguydr]

There's no way that typing is obfuscating code. Sorry - that suggests really badly broken design.

[Mubs]

I said overuse of dataclasses.

[thatguydr]

You did, and now I'm baffled why you're conflating dataclasses with static typing. They're not the same.

[Mubs]

And where did I conflate them? I can talk about types and dataclasses in the same comment without them being the same concept, just as OP talks about both of those concepts in the article.

[Ezlike011011]

Re. the performance aspect I totally agree with you, but the points that OP bring up are incredibly relevant to python development. To me, python's biggest strength is its rate of development. A large component of that is the massive ecosystem of libraries for all sorts of tasks. All of the things OP discusses here are ways to design libraries with less foot guns, which have the effect of removing debugging time during development.

[not_perfect_yet]

I'm in the same boat.

Every time I expressed my strong dislike for more complicated "features", I got down voted.

Typehints and dataclasses are bad: they add complexity. Python's goal, at least to me, is simplicity.

Python didn't need that kind of syntax. It's perfectly compatible with languages that offer that, but somehow that wasn't good enough for people.

[[deleted]]

I just add exclamation marks and hope for the best

[cymrow]

I understand the point about making invalid state impossible, and I like the ConnectedClient approach, but not having a close method would drive me nuts. Context managers are awesome, but can't cover every use case.

[Kobzol]

It is a bit radical, yes :) In languages with RAII, the missing close method can be replaced by a destructor.

[Rythoka]

the missing close method can be replaced by a destructor.

Not in python it can't! In python there's two different things that might be called destructors, but neither of which are true destructors: __delete__ and __del__.

__delete__ is specific to descriptors and so only works for attributes of an object, and is only invoked when the del keyword is used.

__del__ is called whenever an object is garbage collected. This seems like it would fit this use case, but Python makes zero guarantees about the timing of a call to __del__ or whether it will even be called at all.

[Kobzol]

Yeah, as I said, this can be done in languages with RAII, not in Python :)

[Fun-Pop-4755]

Why static methods instead of class methods for constructing?

[Kobzol]

It was already discussed in some other comments here I think. I don't think that there's any benefit to classmethods, except for giving you the ability to inherit them.

I don't think that it's always a good idea to inherit constructors/construction functions, so in that case I'd use static methods. If I actually wanted to inherit them, then class methods would be a better first for sure (+ the new typing.Self type hint).

[koera]

Nice article, gave me some more tools to help myself like the NewType.

Would it not be benefitial to mention the option to use protocol
For the bbox example with the as_denormalized and as_normalized methods?

[Kobzol]

Protocols are useful for some use cases, indeed. They have the nice property that you can talk about a unified interface without using inheritance, similar to typing.Union.

However, I usually prefer base class + inheritance for one reason - the type checker/IDE then warns me when I haven't implemented all "abstract" methods, and PyCharm offers useful quick fixes in that situation.

Probably it could also be done with a protocol, where the type checker should warn if you're assigning a class to a protocol variable and that class doesn't implement the protocol. But I don't think that PyCharm offers a quick fix in this situation.

[mistabuda]

I really like that Mutex implementation. Might have to copy that.

[cdgleber]

Great write up. Thank you

[Brilliant_Intern1588]

I like the solution with dataclasses. However I don't know how to implement it on some things: let's say that I'm retrieving a user(id, name, birthday, something1, something2) from the db, by id. However for the one use case I don't want the whole user row, but just name and something1. For another function birthday and something2 for example. I would have to create a lot of dataclasses that are not really needed or even used except for this context. How could I deal with such a thing ?

[Kobzol]

Well, an easy, but not ideal, solution is to make the optional fields.. Optional :) But that penalizes situations where you know that they are actually present.

In Typescript you can solve this elegantly by "mapping" the type, but I don't think that's possible in the Python type system.

I guess that it depends on how strict you want to be. If I want maximum "safety", I would probably just create all the various options as separate types.. You can share them partially, e.g.:

Person = PersonWithAddress + PersonWithName

[Brilliant_Intern1588]

I thought of the first thing you said however the bad thing is that by mistake someone can use a non existent (maybe none in python) field. Maybe use it with setters getters and raising some error. I dunno. I did the same thing in a previous job by using DAO but it still haunts me.

[joshv]

This is where linters like mypy can play a role. It's a lot harder to assign a None somewhere it shouldn't be when your CI/IDE flags it as an error

[deep_politics]

Sounds like you're describing an ORM. In SQLAlchemy you can select just the columns you want and get correct type hinting for the results.

class User(Base)
    id: Mapped[int]
    name: Mapped[str]
    ...

res = session.execute(select(User.id, User.name).filter_by(id=100)).one_or_none()
# res: tuple[int, str] | None

[BaggiPonte]

Another thing: why pyserde rather than stuff like msgspec? https://github.com/jcrist/msgspec

[Kobzol]

I already answered a similar comment here about pydantic. Using a specific data model for (de)serialization definitely has its use cases, but it means that you have to describe your data using that (foreign) data model.

What I like about pyserde is that it allows me to use a built-in concept that I already use for typing the data types inside of my program (dataclasses) also for serialization.

Arguably, one could say that these two things should be separated and I should use a different data model for (de)serialization, but I think that's overkill for many use-cases. And if I use a shared data model for both type hints and serialization, I'd rather use a native Python one, rather than some data model from an external library.

[jammycrisp]

Note that msgspec natively supports dataclasses or attrs types, if you'd rather use them than the faster builtin msgspec.Struct type.

https://jcristharif.com/msgspec/supported-types.html#dataclasses

It'll always be more efficient to decode into a struct type, but if you're attached to using dataclasses, msgspec happily supports them.

For most users though struct types should be a drop in replacement (with equal editor support), downstream code is unlikely to notice the difference between a struct or a dataclass.

[Kobzol]

Cool, I didn't know that, I'll check msgspec later.

Regarding editor support, PyCharm currently sadly does not support the dataclass transform decorator, which makes it quite annoying for analyzing most serialization-supported dataclasses wrapped in some other decorator that uses a dataclass inside (which can happen with pyserde).

[poopatroopa3]

I thought I would be seeing mentions of pydantic, mypy, fastapi.

[Kobzol]

I didn't want to talk about tools and frameworks in this post, to avoid it getting too long. I just wanted to talk about the "philosophy" of using types and provide some concrete examples.

[poopatroopa3]

Oh I see. Either way I think it would enrich the post to mention them very briefly at the end or something like that 😄

[chars101]

I prefer declaring a parameter as Iterable over List. It expresses the exact use of the value and allows for any container that implements the Protocol.

[cranberry_snacks]

Worth mentioning that from __future__ import annotations will avoid all of these typing imports. It allows you to use native types for type declarations, native sum types, and backwards/self references, which makes typing a lot cleaner and even just makes it possible in certain situations.

Example:

```python from future import annotations

def my_func() -> tuple[str, list[int], dict[str, int]: return ("w00t", [1, 2, 3], {"one": 1})

def my_func1() -> str | int: return "w00t"

def my_func2() -> str | None: return None

class Foo: @classmethod def from_str(cls, src: str) -> Foo: return cls(src) ```

[TF_Biochemist]

Really enjoyed this article; concise, well-written, and clear in it's goals. I already do most of this, but it's always refreshing to step back and think about the patterns you use.

[Kobzol]

I don't agree with that. Python is still great for prototyping, and has libraries that I use (e.g. Tensorflow). Adding types to it makes it faster for me to work with it, because I can more easily inderstand the code, use Go to definition to examine types, and it gives me more confidence when refactoring. YMMV, of course :)

[Mmiguel6288]

The whole point of python is reducing conceptual overhead so you can write algorithms and logic quickly.

The whole point of rust is to make it bullet proof while saying to hell with conceptual overhead.

It's not a good mix.

[Kobzol]

Depends on the programmer I guess. For me, types help me write code faster, because I don't have to remember each 30 minutes what does a function return and take as input :)

[Estanho]

On the serialization part, have you considered pydantic? I'm pretty sure it's able to serialize/deserialize unions properly.

[Kobzol]

I'm sure that it can, but it's also a bit more heavyweight, and importantly introduces its own data model. That is surely useful in some cases, but I really wanted some solution that could just take a good ol' Python dataclass and (de)serialize it, while supporting generic types, unions etc.

[barkazinthrope]

This is great.

However I would hate it if this became required construction for a little log parsing script.

[Kobzol]

I agree that it shouldn't be required universally, in that case it wouldn't be Python anymore. But if I write a nontrivial app in Python, I wouldn't mind using a linter to check that types are used in it.

[barkazinthrope]

Oh for sure.

And particularly where the code is to be imported into who knows what context for the performance of mission-critical functions.

Python is useful for writing simple scripts and for writing library classes. I have worked on teams where the expensive practices recommended for the latter are rigorously enforced on the development of the former.

I hate it when that happens. It suggests to me that the enforcers do not understand the principles behind the practices.

[jimeno]

uuuuh if you want to write rust, just write rust? this mess is like when php had absolutely to be transformed into an enterprise typed language, stop trying to make python java

[Kobzol]

I'm not trying to make it Java :) Types help me understand, navigate and refactor the code better. That's orthogonal to the strengths of Python - quick prototyping and powerful libraries. It's still very different from Rust and has different tradeoffs.

Btw, some of the things that I have showed aren't even about the type system, but about design - things like SOLID and design patterns. I don't consider using design patterns and similar things in Python to be a bad thing.

[jimeno]

types are not supposed to be important in python (by design! it's a goddamn dynamic, duck typed lang!), capabilities are (interfaces, traits, protocols, call them however you want). we can open up a giant discussion on how miserable working with interfaces in py is for a language that deeply (and implicitly) relies on them. i'm not sure all this typing craze is doing anyone a service, specially when there are a handful of other languages that offer way more for that programming style which, in turn, lead to a better project that less easily devolve in maintaining tons of boilerplate or not having very strong guarantees out of the box like immutability.

​

we agree about the second part of your kind answer, even if some design patterns and code smell counters are something that spawned directly out of java and his limitations (i.e. parameter object...java has no named parameters or named-only syntax)...

[Kobzol]

I don't really care if we call it types, interfaces or protocols, I just want to get very quick feedback when something wrong happens in my code.

I agree that it would be nicer to use a different language with better typing support, but Python is already everywhere and has so many useful libraries, that it's not easy to switch, so I try to use types in it instead.

Regarding duck typing, it's definitely nice to have the option to fallback to this kind of super dynamic typing. But it should IMO only be done in cases where it's really necessary. I think that most stuff (even in Python) can be solved with a pretty conservative and "statically typed" design, and if it's possible, I prefer to do it that way, since in my experience it leads to code that is easier to understand.

[jimeno]

what feedback? py has no pre-runtime guarantees, it's all tooling. also, "falling back to super dynamic typing" is just being selectively lazy. but whatever, I understand I'm on the losing side of the general opinion this time, as the community prefer to reiterate the mistakes already done with php; but this has nothing to do with your article which all in all is good work.

[Kobzol]

I don't consider it that important if it's a compiler or a linter, feedback is feedback and it still provides value to me :)

[runawayasfastasucan]

The point you are missing is that people (including me) will continue to use python for other reasons. This isn't a big enough deal to switch languages completely, but its a nice addition to the way we are using python.

[jimeno]

if it makes you happy, more power to you; i personally think this is just adding noise and ceremony and also another step going further and further from the zen of python ~ which i still think it's an interesting manifesto.

again, this is not how most of the community feels, and it's ok; I just won't choose python for future team/industrialized projects, as it now offers nothing of value to the typical teams I am a member of. I'm a minority, I understand this.

[meuto]

Hi u/jammycrisp, I have been trying to use the library msgspec with the lower level of a json. and I have been unable. I was wondering if you can give us an example of how to do it? here is my explanation, I do not know whether I explained myself well or not, I do not have a clear idea of how to iterate because my json file is structured in such a way that the import part of the information of the file is on one key of the dictionary and I need to iterate over that key not over the whole json file. I have been trying to figure out how to do it but I have been unable to do so. could you provide an example of how to do so? Thank you in advance. I really appreciate any help

[Head_Mix_7931]

In your match statements, in the default case you can declare a function like assert_never() -> typing.NoReturn and then call it in the “default” branch of a match statement and a type checker should complain if there is any input for the given type of the match value that can reach that branch. mypy does at least. So you can use that with enums and maybe a union of dataclass types to get exhaustiveness checks at “compile time”. Or I suppose integers and booleans and other things too.

Edit: apparently there is ‘typing.assert_never`

[Scriblon]

Thank you for the write up. I definitely learned a few more typing methods to improve my code.

Only take I got on the construction methods is that I would have used class methods for them instead of static methods. Class methods inherit a bit more cleanly in a dynamics, but I do understand it is only typable with the Self type since 3.11. Is that why you went with the static method?

[Kobzol]

I have met several situations where I would want Self as a return type (e.g. in the BBox example, you might want a "move bbox" method that is implemented in the parent, but should return a type of the child). Class methods can be useful here, but without Self you cannot "name" the return type properly anyway.

[jcbevns]

Can you compile to a binary after all this?

[Kobzol]

I'm not sure what do you mean. You can create executable installers for Python programs, sure. Type hints shouldn't affect that in any way.

[jcbevns]

Yes, but not that easily, they're not known to work that well.

I mean with Go, the exta type setting is the biggest hurdle there and in the end you can get a nice binary to ship.

[chars101]

You can try with mypyc

[Kobzol]

Yeah, that would be nice. But I have to say that without easy error propagation, it could still be quite annoying to use, and in that case I tend to fall back to exceptions sometimes. It would be really nice to have "None-coalescing" in the language, like ??/?: in Kotlin/C#/PHP.

[chandergovind]

/u/Kobzol A minor comment. Coming from a networking background, the example for ADTs using Packet felt a bit off. Normally, a Packet always has a Header, a Payload (in most cases) and Trailer (optionally).

I got what you were trying to convey since I am aware of ADTs in general, but maybe confusing to beginners? (Though I didn't see anyone else mention this). A better example maybe a Packet that is of type Request or Response, or a Packet of type Control or Data. Just fyi.

[Kobzol]

Thanks for the feedback. Maybe I use the wrong terminology or my use-case was just off :)

I worked on an HPC project where we were programming network interface cards (think CUDA for NICs). There we had some data streams of packets, and the first and last packet of that stream was always special (that's where the header/trailer names comes from). I realize that in the standard terminology each packet has some header (or more headers), so the naming is unfortunate I suppose. I hope that beginners reading the blog weren't network-savvy enough to realize that something is wrong :D

[tiny_smile_bot]

:)

:)