Bradley Kirton's Blog

Published on Jan. 18, 2024

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Railway oriented programming in Django

As far as I can tell railway oriented programming is not a widely recognised concept. I think the first time I was introduced to the concept was reading through the dry-python returns documentation.

I really like this idea because it treats errors as data. In Python I often find myself using errors as a mechanism for control flow within a try except block. I have not used the returns package in any projects yet but have been experimenting with it a bit.

Below is an example of such an experiment (Note I have not even checked if this works, it is more conceptual at the moment).

Suppose we have a use case to create a user object in our database and on some external CRM.

This specification describes the user case

        Result[User, ValidationError | DatabaseError | CRMClientError]

        Validate Input:
            raises ValidationError
        Create in DB:
            raises DatabaseError
        Create in CRM:
            raises CRMClientError
from returns import result as r

class UserCreationError(Exception):
    """Base class for user creation errors."""

    def __init__(self, message: str) -> None:

class ValidationError(UserCreationError):
    """Raised when invalid user data is provided."""

    def __init__(self, message: str, *, form: forms.Form) -> None:
        self.form = form

class DatabaseError(UserCreationError):
    """Raised when some database error is encountered when creating the user."""

class CRMClientError(UserCreationError):
    """Raised when some error is encountered when creating the user on the external CRM."""

class UserManager(models.Manager):
    Create a user.

    def create_user(self, data: dict[str, str], crm_client: CRMClient) -> User:
        Create a user.

        :param data: The create user payload.
        :param crm_client: An instance of the CRM client.
        :returns: A user or user creation error.
        :raises ValidationError: If the provided user data is invalid.
        :raises DatabaseError: If a database error is encountered.
        :raises CRMClientError: If a CRM error is encountered.

        form = CreateUserForm(data=data)

        if not form.is_valid():
            raise ValidationError("Invalid create user payload.", form=form)

        first_name = form.cleaned_data["first_name"]
        last_name = form.cleaned_data["last_name"]

            user = User.objects.create(first_name=first_name, last_name=last_name)
        except Exception as ex:
            raise DatabaseError("Failed to create user record.") from ex

            crm_client.create(, first_name=first_name, last_name=last_name)
        except Exception as ex:
            raise CRMClientError("CRM is currently unavailable.") from ex

        return user

def create_user_try_except_view(request: HttpRequest) -> HttpResponse:
        CustomUser.objects.create_user(data=request.POST, crm_client=CRMClient())
        response = redirect(reverse("login-view"))
    except ValidationError as ex:
        response = render(request, "create-user-form.html", {"form": ex.form})
    except DatabaseError as ex:
        response = redirect(reverse("db-unavailable-view"))
    except CRMClientError as ex:
        response = redirect(reverse("crm-unavailable-view"))
    except Exception as ex:
        raise ex

    return response

def create_user_railway_view(request: HttpRequest) -> HttpResponse:
    safe_create_user =
    result = safe_create_user(data=request.POST, crm_client=CRMClient())

    match result:
        case r.Success(CustomUser()):
            return redirect(reverse("login-view"))
        case r.Failure(ValidationError() as error):
            return render(request, "create-user-form.html", {"form": error.form})
        case r.Failure(DatabaseError() as error):
            return redirect(reverse("db-unavailable-view"))
        case r.Failure(CRMClientError() as error):
            return redirect(reverse("crm-unavailable-view"))
        case r.Failure(Exception() as error):