Devin J. Cornell: Lessons from Rust 1: Enums for Errors and Missing Data

The Rust programming language has taken the award for “most loved programming language” in the Stack Overflow Developer Survey since 2016. The power of Rust comes in that it is nearly as fast as C++ without the potential issues that come with pointers and manual memory management. Whether or not you can convince your stakeholders to write Rust code, there are some lessons we can apply to our data pipelines in any language. In the next several blog articles I will provide some patterns that use ideas from the Rust language to improve your data science code in any language.

(current) Lessons from Rust 1: Enums for Errors and Missing Data
Lessons from Rust 2: Composition over Inheritance

The first pattern I will discuss is the use of enums to indicate missing data and errors. Rust has a robust infrastructure for handling enum types compared to languages like Python or C++, but there are two particular built-in enums that may be useful for data pipelines: Option[T] and Result[T, E]. The Option[T] enum is used to represent a value that may or may not be present (equivalent of None in Python), and the Result enum is used to represent a return value that is either a value or an error.

When `None` is a Valid Return Value

Our first example will cover the case when you need to access some variable for which None might be considered a valid value but there is also a case where the return value may be altogether invalid. This breaks typical convention where we often use None to represent invalid values. The strong typing system in rust necessitates the use of the wrapper class because None cannot be a valid integer type, but we can adapt it for our scenario even in weakly typed environments.

For example purposes we create an object that stores a single variable that can either be an integer or a None value. For a particular application we want to get that value, but we consider it to be invalid if it is not None and it is less than zero.

@dataclasses.dataclass
class MyObj:
    x: typing.Optional[int]

We can access the attribute directly, but then we would have to check the value downstream to see if it is negative - it would be better to indicate the value is invalid when we access it.

Solution using exceptions

Perhaps the most Pythonic solution to this problem is to use exceptions. We create an accessor function that raises a ValueError exception if the value is negative and passes the value otherwise.

@dataclasses.dataclass
class MyObj:
    x: typing.Optional[int]
    
    def access_x_exception(self) -> typing.Optional[int]:
        if self.x is None or self.x >= 0:
            return self.x
        else:    
            raise ValueError('x is negative so it is invalid')

As a use case, let us say we have a list of these objects and for each object we want to print the value if it is valid, and otherwise state that it is invalid. We would handle this the same way we handle any other exception.

def print_values_exception(objs: typing.List[MyObj]) -> None:
    for obj in objs:
        try:
            print(obj.access_x_exception())
        except ValueError:
            print('x is invalid')

The challenge with this approach is that the user has no way of recognizing that this function will raise an exception unless they read the implementation or details of the documentation, and the point where it should be handled is not clear.

Solution using the `Option` pattern

A better solution that emulates the Option[T] enum would be to create custom wrapper objects to indicate whether the value is valid aside from looking at the actual value. By adding type hints for this wrapper object, we can indicate to the reader exactly where the error will be handled. We can create the wrapper types using generic type hints, and create a union type hint (using typing.Union) which indicates the accessor might return either value. Using the dynamic duck typing scheme, we can check which type of object it is by accessing the is_ok attribute (there are a number of ways this could be handled) and further note that I added dummy property methods so that this design will pass strict typing checks.

T = typing.TypeVar("T")
E = typing.TypeVar("E")

@dataclasses.dataclass
class Ok(typing.Generic[T]):
    data: T
    is_ok: bool = True
    
    @property
    def error(self) -> typing.NoReturn:
        raise AttributeError(f'{self.__class__.__name__} has no attribute "error"')
    
@dataclasses.dataclass
class Err(typing.Generic[E]):
    error: typing.Optional[E] = None
    is_ok: bool = False
    
    @property
    def data(self) -> typing.NoReturn:
        raise AttributeError(f'{self.__class__.__name__} has no attribute "data"')
    
Result = typing.Union[Ok[T],  Err[E]]

We then create a new accessor which returns one of the new type hints we created.

@dataclasses.dataclass
class MyObj:
    x: typing.Optional[int]
    
    def access_x(self) -> Result[typing.Optional[int], None]:
        if self.x is None or self.x >= 0:
            return Ok(self.x)
        else:    
            return Err()

In the use case, we first check if the result is valid and then either print the value or the error information (which will always be None in this example).

def print_values(objs: typing.List[MyObj]) -> None:
    for obj in objs:
        result = obj.access_x()
        if result.is_ok:
            print(result.data)
        else:
            print(f'x is invalid: {result.error}')

Multiple Error Types

In some cases, there may be multiple situations in which the accessed values are invalid, and we want to handle them differently. As a use case, let us say we need to calculate the mean value of an attribute across a set of objects. The sum function cannot accept None values, and so we should omit those values from the mean; in the case where the value is negative, we want to replace it with a 0.

Solution using exceptions

The exception approach is simple: we could create custom exceptions, or, perhaps less optimally, use two existing exceptions to indicate the different scenarios (we will do the latter for this example, although I would encourage the former in most cases).

@dataclasses.dataclass
class MyObj:
    x: typing.Optional[int]
        
    def access_x_notnone_exception(self) -> int:
        if self.x is None:
            raise TypeError('x is None so it is invalid')
        elif self.x < 0:
            raise ValueError('x is negative so it is invalid')
        else:
            return self.x

In the use case we simply catch the exceptions and handle them as expected.

def average_values_exception(objs: typing.List[MyObj]) -> float:
    values = list()
    for obj in objs:
        try:
            values.append(obj.access_x_notnone_exception())
        except ValueError:
            values.append(0)
        except TypeError:
            pass
    return sum(values)/len(values)

Solutions using the `Result` pattern

The alternative approach would be to re-use the wrapper objects from before but provide a custom error enum to indicate which type of error was encountered. The enum module can be used to create a new enum type describing our two error types.

import enum
class MyErrorType(enum.Enum):
    IS_NONE = enum.auto()
    IS_NEGATIVE = enum.auto()

The accessor just performs the checks and returns invalid objects with the expected error type.

@dataclasses.dataclass
class MyObj:
    x: typing.Optional[int]

    def access_x_notnone(self) -> Result[int, MyErrorType]:
        if self.x is None:
            return Err(MyErrorType.IS_NONE)
        elif self.x < 0:
            return Err(MyErrorType.IS_NEGATIVE)
        else:
            return Ok(self.x)

The client can then check the error type and handle it appropriately.

def average_values(objs: typing.List[MyObj]) -> float:
    values = list()
    for obj in objs:
        v = obj.access_x_notnone()
        if v.is_ok:
            values.append(v.data)
        elif v.error is MyErrorType.IS_NEGATIVE:
            values.append(0)
    return sum(values)/len(values)

To see the true value of this approach, imagine we are propagating these types of errors up multiple levels through a call stack. For example, we could make a wrapper class that wraps another wrapper class that accesses the Result type we defined above, and it includes a method to access the original inner value. When appropriate type hints are used, the client knows when to expect a value that might possibly be invalid, and to be prepared to handle that instance when using the value. This is essentially an elaborate extension of type checking using Python’s typing.Optional[T] type hint that allows for multiple error types.

@dataclasses.dataclass
class MyObjWrapper:
    obj: MyObj
    def access_value(self) -> Result[int, MyErrorType]:
        return self.obj.access_x_notnone()
    
@dataclasses.dataclass
class MySecondWrapper:
    obj_wrapper: MyObjWrapper
    def access_value_wrapper(self) -> int:
        val = self.obj_wrapper.access_value()
        if val.is_ok:
            return val.data
        else:
            raise ValueError('SOMETHING BAD HAPPENED')

Precisely because exceptions are exceptional, they break the regular control flow in a way that may obscure important cases that you want to handle explicitly.

When None is a Valid Return Value

Solution using exceptions

Solution using the Option pattern

Multiple Error Types

Solution using exceptions

Solutions using the Result pattern

When `None` is a Valid Return Value

Solution using the `Option` pattern

Solutions using the `Result` pattern