Type-safe "optional-nullable" fields

[nrbnlulu]

Preface

it is a common practice in strawberry that when your data layer have
an optional field i.e

class Person:
    name: str
    phone: str | None = None

and you want to update it you would use UNSET in the mutation input
in order to check whether this field provided by the client or not like so:

@strawberry.input
class UpdatePersonInput:
    id: strawberry.ID
    name: str| None
    phone: str | None = UNSET

@strawberry.mutation
def update_person(input: UpdatePersonInput) -> Person:
    inst = service.get_person(input.id)
    if name := input.name:
         inst.name = name
    if input.phone is not UNSET:
        inst.phone = input.phone  # ❌ not type safe
    
    service.save(inst)

Note that this is not an optimization rather a business requirement.
if the user wants to nullify the phone it won't be possible other wise
OTOH you might nullify the phone unintentionally.

This approach can cause lots of bugs since you need to remember that you have
used UNSET and to handle this correspondingly.

Since strawberry claims to

Strawberry leverages Python type hints to provide a great developer experience while creating GraphQL Libraries.

it is only natural for us to provide a typesafe way to mitigate this.

Proposal

The Option type.which will require only this minimal implementation

import dataclasses


@dataclasses.dataclass
class Some[T]:
    value: T
    
    def some(self) -> Some[T | None] | None:
      return self
    
@dataclasses.dataclass
class Nothing[T]:
    def some(self) -> Some[T | None] | None:
      return None 

Maybe[T] = Some[T] | Nothing[T]

and this is how you'd use it

@strawberry.input
class UpdatePersonInput:
    id: strawberry.ID
    name: str| None
    phone: Maybe[str | None]

@strawberry.mutation
def update_person(input: UpdatePersonInput) -> Person:
    inst = service.get_person(input.id)
    if name := input.name:
         inst.name = name
    if phone := input.phone.some(): 
        inst.phone = phone.value  # ✅  type safe
    
    service.save(inst)

Currently if you want to know if a field was provided

Backward compat

UNSET can remain as is for existing codebases.
Option would be handled separately.

which Option library should we use?

1. Don't use any library craft something minimal our own as suggested above.
2. ** use something existing**

The sad truth is that there are no well-maintained libs in the ecosystem.

Never the less it is not hard to maintain something just for strawberry since the implementation
is rather straight forward and not much features are needed. we can fork either

• https://github.com/rustedpy/maybe
• https://github.com/MaT1g3R/option

and just forget about it.

3. allow users to decide

# before anything

strawberry.register_option_type((MyOptionType, NOTHING))

then strawberry could use that and you could use whatever you want.

• Core functionality
• Alteration (enhancement/optimization) of existing feature(s)
• New behavior

[fruitymedley]

To clarify, is the following the use case you're interested in?

mutation Update {
    updatePerson(  ## this operation sets phone to null
        input: {
            id: ..., 
            name: ...,
            phone: null
        }
    ) {
        ...
    }
}

vs

mutation Update {
    updatePerson(  ## this operation does not modify phone number
        input: {
            id: ..., 
            name: ...
        }
    ) {
        ...
    }
}

[nrbnlulu]

yep

[Speedy1991]

I think this is a solid proposal. Both Option and Maybe are valid choices, though Maybe seems to be slightly better maintained. However, I’m not entirely sure if this justifies adding a third-party library as a dependency.

For an opt-in approach, I would prefer your solution:

strawberry.register_option_type((MyOptionType, NOTHING))

If this Type is meant to be included directly in Strawberry, I would avoid forking the repository. Instead, I’d suggest copying the relevant MIT-licensed code while ensuring proper attribution, such as adding credits in the form of code comments or including a reference to the original license.

[nrbnlulu]

@patrick91 👀 ?

[patrick91]

Code sample in pyright playground

from __future__ import annotations
import dataclasses
from typing import Self, reveal_type


@dataclasses.dataclass
class Some[T]:
    value: T
    
    def some(self) -> Self:
      return self
    
@dataclasses.dataclass
class Nothing[T]:
    def some(self) -> None:
      return None 

type Maybe[T] = Some[T | None] | Nothing[T]

def some_func() -> Maybe[str]:
    return Some("a")

a = some_func()

reveal_type(a)

if phone := a.some():
    reveal_type(phone)
    reveal_type(phone.value)
else:
    reveal_type(phone)

[Corentin-Bravo]

I have met the exact same problem, but I don't understand why you cannot solve it.

@strawberry.input
class UpdatePersonInput:
    id: strawberry.ID
    name: str| None
    phone: str | None | UnsetType = strawberry.field(
        graphql_type=str | None,
        default=UNSET
    )

@strawberry.mutation
def update_person(input: UpdatePersonInput) -> Person:
    inst = service.get_person(input.id)
    if name := input.name:
         inst.name = name
    if not instance(input.phone, UnsetType):
        inst.phone = phone.value
    
    service.save(inst)

The case where you do not want None but either a string or nothing cannot be handled cleanly due to the GraphQL specification (an optional value is a nullable value**), but if your API is used only internally, you can do the same thing:

@strawberry.input
class UpdatePersonInput:
    id: strawberry.ID
    name: str| None
    phone: str  | UnsetType = strawberry.field(
        graphql_type=str | None,
        default=UNSET
      description="This value is optional but not nullable, do not send a null!"
    )

(Arguably you could write custom code to ensure that sending a None would return an appropriate error instead of randomly breaking inside your code)

I find the proposition slightly contrived, as it turns scalar values into objects.

[nrbnlulu]

@Corentin-Bravo

phone: str | None | UnsetType

can actually solve this, I haven't thought about this really...

Though:

1. nothing stops you from using UNSET on anything.
   UNSET is UNSET: Any = UnsetType()
   because how we advertised the use of unset up until now which is

phone: str | None = strawberry.UNSET

You could say that we can change the signature to the real type but that would type-checkers would lint
for existing code and would require modifications.

2. using isinstance is not very pleasant to use IMO. (prob could avoid that with a typegaurd)

We could also think re-introducing the Unset usage which could look like this

type Maybe[T] = UnsetType | None | T

def is_unset[T](val: T | UnsetType) -> TypeGuard[UnsetType]:
    return isinstance(val, UnsetType)


@strawberry.input
class UpdatePersonInput:
    phone: Maybe[str]

def update_person(input_: UpdatePersonInput) -> ...:
       if not strawberry.is_unset(input_.phone)
           update_phone(input_.phone)  

I actually have no oppositions to this.

[ctorrington]

Could we achieve the same result with the Maybe class & changing the type of the Unset singleton? Then direct identity checks can continue to be supported & there is no need for helper methods?

from typing import Final

class UnsetType:
    __instance: Optional["UnsetType"] = None

    def __new__(cls: type["UnsetType"]) -> "UnsetType":
        if cls.__instance is None:
            ret = super().__new__(cls)
            cls.__instance = ret
            return ret
        return cls.__instance

    def __str__(self) -> str:
        return ""

    def __repr__(self) -> str:
        return "UNSET"

    def __bool__(self) -> bool:
        return False


~~UNSET: Any = UnsetType()~~
UNSET: Final[UnsetType] = UnsetType()

Would we need to remove the falsy behaviour as well? 🤔 Just thinking if 0 is passed & Unset is falsy, then we would miss that 0 value in some contexts?

This would return 42 instead of 0?

def foo(val: Maybe[int]) -> int:
         return val or 42

Existing code should only break if it was already relying on Any incorrectly?

[nrbnlulu]

Im not sure what is your suggestion. the approach I took is similar to what @Corentin-Bravo suggested.

1. AFAICT helper function is unavoidable.
2. you can still use is not strawberry.UNSET
3. no existing code will break.

see the Release notes in #3791

import strawberry


@strawberry.type
class User:
    name: str
    phone: str | None


@strawberry.input
class UpdateUserInput:
    name: str
    phone: strawberry.Maybe[str]


@strawberry.type
class Mutation:
    def update_user(self, info, input: UpdateUserInput) -> User:
        reveal_type(input.phone)  # str | None | UnsetType
        if strawberry.exists(input.phone):
            reveal_type(input.phone)  # str | None
            update_user_phone(input.phone)

        return User(name=input.name, phone=input.phone)

You can also use strawberry.Maybe as a field argument like so

import strawberry


@strawberry.field
def filter_users(self, phone: strawberry.Maybe[str] = strawberry.UNSET) -> list[User]:
    if strawberry.exists(phone):
        return filter_users_by_phone(phone)
    return get_all_users()

[ctorrington]

I do not understand why helper functions are required if it is possible to use direct identity checks with Python's is operator. The direct identity checks are already implemented & documented in the documentation.

@strawberry.field
def greet(self, name: Optional[str] = strawberry.UNSET) -> str:
    if name is strawberry.UNSET:
        return "Name was not set!"
    if name is None:
        return "Name was null!"
    return f"Hello {name}!"

Code will need to be changed to allow for type safety with the helper functions. I understand that changing strawberry's Unset singleton from Any to Final[UnsetType] will cause some existing code to be linted with errors. However, that will only occur if developers have been using Unset in a non-type-safe way. I see that as beneficial.

Including type None in type Maybe with generic type T & type UnsetType conflates the meaning of type Maybe. Type Maybe usually represents the absence or presence of a value. Adding type None creates an unnecessary third state. The field can be made optional if type None is required.

For optional nullable fields I would suggest:

type Maybe[T] = T | UnsetType

field: Maybe[str] | None

This allows type Maybe to handle whether a field was provided or not, while allowing the field to be nullable independently.

In another issue, we should look at the Falsy behaviour of type UnsetType, which could cause silent erros with Falsy values like 0.

[nrbnlulu]

Code will need to be changed to allow for type safety with the helper functions. I understand that changing strawberry's Unset singleton from Any to Final[UnsetType] will cause some existing code to be linted with errors. However, that will only occur if developers have been using Unset in a non-type-safe way. I see that as beneficial.

hmm are you certain about that? @patrick91 WDYT?

Including type None in type Maybe with generic type T & type UnsetType conflates the meaning of type Maybe. Type Maybe usually represents the absence or presence of a value. Adding type None creates an unnecessary third state. The field can be made optional if type None is required.

AFAICT there is no usecase for Maybe[T] without a None. It was added merely for convenience.

[Corentin-Bravo]

I think I have expressed all possible cases below

In python:

@strawberry.input()
class TestStringInput:
    string: str
    optional_string: str = "abc"
    optional_string_with_none_default: str | None = None
    nullable_string: str | None
    optional_string_nullable_with_default: str | None = "abc"
    optional_string_nullable_without_default: str | None | UnsetType = UNSET

In GraphQL:

input TestStringInput {
    string: String!
    optionalString: String! = "abc"
    optionalStringWithNoneDefault: String
    nullableString: String
    optionalStringNullableWithDefault: String = "abc"
    optionalStringNullableWithoutDefault: String
}

The graphqQL schema does not know None nor UNSET, so effectively:

• An optional nullable
• Nullable
• Optional (defaulting to None)

Are all equal for the schema.

Therefore, I believe we should not try to be smarter than the underlying GraphQL schema.

Maybe[T] should include None, because the schema will allow it.

While I agree that it would be better to be able to mark things as optional and nullable separately, GraphQL itself does not allow it.

The proposed PR is just syntaxic sugar, and I think it cannot be more than that.

[ctorrington]

I understand that GraphQL's schema cannot distinguish between the 3 types above, however, I do not believe that to be a reason to limit Strawberry's type safety when Python can distinguish between the 3. The above examples show the problem & the need for better type safety. I do not believe that limiting a capable type system is the correct approach.

A reason for Maybe[T] = UNSET | T:

@strawberry.input
class UpdateUserInput:
    id: strawberry.ID
    name: Maybe[str | None]        # Can be UNSET, str or None
    bio: Maybe[str]                # Can be UNSET, str, but CANNOT be None

@strawberry.mutation
def update_user(self, input: UpdateUserInput) -> User:
    user = get_user(input.id)

    if input.name is not UNSET:
        user.name = input.name     # Type checker knows type is str or None

    if input.bio is not UNSET:
        user.bio = input.bio       # Type checker knows type is str & cannot be None

    return user

A problem with Maybe[T] = UNSET | T | None:

@strawberry.input
class UpdateUserInput:
    id: strawberry.ID
    name: Maybe[str]                # Can be UNSET, str or None
    bio: Maybe[str]                 # Can be UNSET, str or None

@strawberry.mutation
def update_user(self, input: UpdateUserInput) -> User:
    user = get_user(input.id)

    if input.name is not UNSET:
        user.name = input.name      # Could be str or None

    if input.bio is not UNSET:
        user.bio = input.bio        # Could be str or None

    return user

Maybe[T] = UNSET | T allows for explicitly defining which fields can accept null values and which cannot. In the example above, the field Bio is optional (can be omitted), but cannot be set to null. This helps prevent accidentally nulling none-nullabe fields, which is not enforced by GraphQL at the schema level.

We can gradually adopt the type annotation for UNSET: Final[UnsetType] instead of [UNSET: Any]to allow for existing code to adapt, but I believe thatMaybe` should allow for separating presence/absence from nullability from the beginning.