Container: the concept¶

Basics¶

The main idea behind a container is that it wraps some internal state. That’s what _inner_value is used for.

And we have several functions to create new containers based on the previous state. And we can see how this state is evolving during the execution.

Railway oriented programming¶

We use a concept of Railway oriented programming. It mean that our code can go on two tracks:

1. Successful one: where everything goes perfectly: HTTP requests work, database is always serving us data, parsing values does not failed

2. Failed one: where something went wrong

We can switch from track to track: we can fail something or we can rescue the situation.

Working with containers¶

We use two methods to create new containers from the previous one. bind and map.

The difference is simple:

• map works with functions that return regular values

• bind works with functions that return other containers

Container.bind is used to literally bind two different containers together.

from returns.result import Result, Success

def make_http_call(user_id: int) -> Result[int, str]:
    ...

result = Success(1).bind(make_http_call)
# => Will be equal to either Success[int] or Failure[str]

So, the rule is: whenever you have some impure functions, it should return a container type instead.

And we use Container.map to use containers with pure functions.

from returns.result import Success

def double(state: int) -> int:
    return state * 2

result = Success(1).map(double)
# => Will be equal to Success(2)

Note:

All containers support these methods.

from returns.result import Failure

def double(state: int) -> float:
    return state * 2.0

Failure(1).fix(double)
# => Will be equal to Success(2.0)

from returns.result import Result, Failure, Success

def fix(state: Exception) -> Result[int, Exception]:
    if isinstance(state, ZeroDivisionError):
        return Success(0)
    return Failure(state)

Failure(ZeroDivisionError).rescue(fix)
# => Will be equal to Success(0)

Not all containers support these methods.
IO cannot be fixed or rescued.

from returns.result import Failure, Success

Success(1).value_or(None)
# => 1

Success(0).unwrap()
# => 0

Failure(1).value_or(default_value=100)
# => 100

Failure(1).unwrap()
# => Traceback (most recent call last): UnwrapFailedError

Failure(1).failure()
# => 1

Success(1).failure()
# => Traceback (most recent call last): UnwrapFailedError

Not all containers support these methods.
IO cannot be unwrapped.

Immutability¶

We like to think of returns as immutable structures. You cannot mutate the inner state of the created container, because we redefine __setattr__ and __delattr__ magic methods.

You cannot also set new attributes to container instances, since we are using __slots__ for better performance and strictness.

Well, nothing is really immutable in python, but you were warned.

Type safety¶

We try to make our containers optionally type safe.

What does it mean?

1. It is still good old python, do whatever you want without mypy

2. If you are using mypy you will be notified about type violations

We also ship PEP561 compatible .pyi files together with the source code. In this case these types will be available to users when they install our application.

However, this is still good old python type system, and it has its drawbacks.

API Reference¶

class Container(inner_value)[source]¶

Bases: returns.primitives.container._BaseContainer

Represents a “context” in which calculations can be executed.

You won’t create ‘Container’ instances directly. Instead, sub-classes implement specific contexts. containers allow you to bind together a series of calculations while maintaining the context of that specific container.

This is an abstract class with the API declaration.

_inner_value¶

Wrapped internal immutable state.

abstract map(function)[source]¶

Applies ‘function’ to the contents of the functor.

And returns a new functor value. Works for containers that represent success. Is the opposite of fix().

abstract bind(function)[source]¶

Applies ‘function’ to the result of a previous calculation.

And returns a new container. Works for containers that represent success. Is the opposite of rescue().

class GenericContainerOneSlot(inner_value)[source]¶

Bases: typing.Generic, returns.primitives.container.Container

Base class for containers with one typed slot.

Use this type for generic inheritance only. Use Container as a general type for polymorphism.

class GenericContainerTwoSlots(inner_value)[source]¶

Bases: typing.Generic, returns.primitives.container.Container

Base class for containers with two typed slot.

Use this type for generic inheritance only. Use Container as a general type for polymorphism.