Helper functions

We feature several helper functions to make your developer experience better.

is_successful

is_succesful is used to tell whether or not your result is a success. We treat only treat types that does not throw as a successful ones, basically: Success.

from returns.result import Success, Failure
from returns.functions import is_successful

is_successful(Success(1))
# => True

is_successful(Failure('text'))
# => False

pipeline

What is a pipeline? It is a more user-friendly syntax to work with containers.

Consider this task. We were asked to create a method that will connect together a simple pipeline of three steps:

  1. We validate passed username and email

  2. We create a new Account with this data, if it does not exists

  3. We create a new User associated with the Account

And we know that this pipeline can fail in several places:

  1. Wrong username or email might be passed, so the validation will fail

  2. Account with this username or email might already exist

  3. User creation might fail as well, since it also makes an HTTP request to another micro-service deep inside

Here’s the code to illustrate the task.

from returns.functions import pipeline
from returns.result import Result, Success, Failure


class CreateAccountAndUser(object):
    """Creates new Account-User pair."""

    # TODO: we need to create a pipeline of these methods somehow...

    # Protected methods

    def _validate_user(
        self, username: str, email: str,
    ) -> Result['UserSchema', str]:
        """Returns an UserSchema for valid input, otherwise a Failure."""

    def _create_account(
        self, user_schema: 'UserSchema',
    ) -> Result['Account', str]:
        """Creates an Account for valid UserSchema's. Or returns a Failure."""

    def _create_user(
        self, account: 'Account',
    ) -> Result['User', str]:
        """Create an User instance. If user already exists returns Failure."""

Using bind technique

We can implement this feature using a traditional bind method.

class CreateAccountAndUser(object):
    """Creates new Account-User pair."""

    def __call__(self, username: str, email: str) -> Result['User', str]:
        """Can return a Success(user) or Failure(str_reason)."""
        return self._validate_user(username, email).bind(
            self._create_account,
        ).bind(
            self._create_user,
        )

    # Protected methods
    # ...

And this will work without any problems. But, is it easy to read a code like this? No, it is not.

What alternative we can provide? @pipeline!

Using pipeline

And here’s how we can refactor previous version to be more clear.

class CreateAccountAndUser(object):
    """Creates new Account-User pair."""

    @pipeline
    def __call__(self, username: str, email: str) -> Result['User', str]:
        """Can return a Success(user) or Failure(str_reason)."""
        user_schema = self._validate_user(username, email).unwrap()
        account = self._create_account(user_schema).unwrap()
        return self._create_user(account)

    # Protected methods
    # ...

Let’s see how this new .unwrap() method works:

  • if you result is Success it will return its inner value

  • if your result is Failure it will raise a UnwrapFailedError

And that’s where @pipeline decorator becomes in handy. It will catch any UnwrapFailedError during the pipeline and then return a simple Failure result.

sequenceDiagram participant pipeline participant validation participant account creation participant user creation pipeline->>validation: runs the first step validation-->>pipeline: returns Failure(validation message) if fails validation->>account creation: passes Success(UserSchema) if valid account creation-->>pipeline: return Failure(account exists) if fails account creation->>user creation: passes Success(Account) if valid user creation-->>pipeline: returns Failure(http status) if fails user creation-->>pipeline: returns Success(user) if user is created

Pipeline execution.

See, do notation allows you to write simple yet powerful pipelines with multiple and complex steps. And at the same time the produced code is simple and readable.

And that’s it!

safe

safe is used to convert regular functions that can throw exceptions to functions that return Result type.

from returns.functions import safe

@safe
def divide(number: int) -> float:
    return number / number

divide(1)
# => Success(1.0)

divide(0)
# => Failure(ZeroDivisionError)

Limitations

There’s one limitation in typing that we are facing right now due to mypy issue:

from returns.functions import safe

@safe
def function(param: int) -> int:
    return param

reveal_type(function)
# Actual => def (*Any, **Any) -> builtins.int
# Expected => def (int) -> builtins.int

This effect can be reduced with the help of Design by Contract with these implementations:

compose

We also ship an utility function to compose two different functions together.

from returns.functions import compose

bool_after_int = compose(int, bool)
bool_after_int('1')  # => True
bool_after_int('0')  # => False

Composition is also type-safe. The only limitation is that we only support functions with one argument and one return to be composed.

API Reference

is_successful(container)[source]

Determins if a container was successful or not.

We treat container that raise UnwrapFailedError on .unwrap() not successful.

safe(function)[source]

Decorator to covert exception throwing function to ‘Result’ monad.

Show be used with care, since it only catches ‘Exception’ subclasses. It does not catch ‘BaseException’ subclasses.

pipeline(function)[source]

Decorator to enable ‘do-notation’ context.

Should be used for series of computations that rely on .unwrap method.

compose(first, second)[source]

Allows function composition.

Works as: second . first You can read it as “second after first”.

We can only compose functions with one argument and one return.