Maybe

The Maybe container is used when a series of computations could return None at any point.

Maybe container

Maybe consist of two types: Some and Nothing. We have a convenient method to create different Maybe types based on just a single value:

>>> from returns.maybe import Maybe

>>> assert str(Maybe.from_optional(1)) == '<Some: 1>'
>>> assert str(Maybe.from_optional(None)) == '<Nothing>'

We also have another method called .from_value that behaves a bit differently:

>>> from returns.maybe import Maybe

>>> assert str(Maybe.from_value(1)) == '<Some: 1>'
>>> assert str(Maybe.from_value(None)) == '<Some: None>'

Usage

It might be very useful for complex operations like the following one:

>>> from attr import dataclass
>>> from typing import Optional
>>> from returns.maybe import Maybe, Nothing

>>> @dataclass
... class Address(object):
...     street: Optional[str]

>>> @dataclass
... class User(object):
...     address: Optional[Address]

>>> @dataclass
... class Order(object):
...     user: Optional[User]

>>> def get_street_address(order: Order) -> Maybe[str]:
...     return Maybe.from_optional(order.user).bind_optional(
...         lambda user: user.address,
...     ).bind_optional(
...         lambda address: address.street,
...     )

>>> with_address = Order(User(Address('Some street')))
>>> empty_user = Order(None)
>>> empty_address = Order(User(None))
>>> empty_street = Order(User(Address(None)))

>>> str(get_street_address(with_address))  # all fields are not None
'<Some: Some street>'

>>> assert get_street_address(empty_user) == Nothing
>>> assert get_street_address(empty_address) == Nothing
>>> assert get_street_address(empty_street) == Nothing

Optional type

One may ask: “How is that different to the Optional[] type?” That’s a really good question!

Consider the same code to get the street name without Maybe and using raw Optional values:

order: Order  # some existing Order instance
street: Optional[str] = None
if order.user is not None:
    if order.user.address is not None:
        street = order.user.address.street

It looks way uglier and can grow even more uglier and complex when new logic will be introduced.

Pattern Matching

Maybe values can be matched using the new feature of Python 3.10, Structural Pattern Matching, see the example below:

from dataclasses import dataclass
from typing import Final

from returns.maybe import Maybe, Nothing, Some


@dataclass
class _Book(object):
    book_id: int
    name: str


_BOOK_LIST: Final = (
    _Book(book_id=1, name='Category Theory for Programmers'),
    _Book(book_id=2, name='Fluent Python'),
    _Book(book_id=3, name='Learn You Some Erlang for Great Good'),
    _Book(book_id=4, name='Learn You a Haskell for Great Good'),
)


def _find_book(book_id: int) -> Maybe[_Book]:
    for book in _BOOK_LIST:
        if book.book_id == book_id:
            return Some(book)
    return Nothing


if __name__ == '__main__':
    desired_book = _find_book(2)
    match desired_book:
        # Matches any `Some` instance that contains a book named `Fluent Python`
        case Some(_Book(name='Fluent Python')):
            print('"Fluent Python" was found')

        # Matches any `Some` instance and binds its value to the `book` variable
        case Some(book):
            print('Book found: {0}'.format(book.name))

        # Matches `Nothing` instance
        case Maybe.empty:
            print('Not found the desired book!')

Decorators

Limitations

Typing will only work correctly if our mypy plugin is used. This happens due to mypy issue.

maybe

Sometimes we have to deal with functions that dears to return Optional values!

We have to work with it the carefully and write if x is not None: everywhere. Luckily, we have your back! maybe function decorates any other function that returns Optional and converts it to return Maybe instead:

>>> from typing import Optional
>>> from returns.maybe import Maybe, Some, maybe

>>> @maybe
... def number(num: int) -> Optional[int]:
...     if num > 0:
...         return num
...     return None

>>> result: Maybe[int] = number(1)
>>> assert result == Some(1)

FAQ

How can I turn Maybe into Optional again?

When working with regular Python, you might need regular Optional[a] values.

You can easily get one from your Maybe container at any point in time:

>>> from returns.maybe import Maybe
>>> assert Maybe.from_optional(1).value_or(None) == 1
>>> assert Maybe.from_optional(None).value_or(None) == None

As you can see, revealed type of .value_or(None) is Optional[a]. Use it a fallback.

How to model absence of value vs presence of None value?

Let’s say you have this dict: values = {'a': 1, 'b': None} So, you can have two types of None here:

  • values.get('b')

  • values.get('c')

But, they are different! The first has explicit None value, the second one has no given key and None is used as a default. You might need to know exactly which case you are dealing with. For example, in validation.

So, the first thing to remember is that:

>>> assert Some(None) != Nothing

There’s a special way to work with a type like this:

>>> values = {'a': 1, 'b': None}

>>> assert Maybe.from_value(values).map(lambda d: d.get('a')) == Some(1)
>>> assert Maybe.from_value(values).map(lambda d: d.get('b')) == Some(None)

In contrast, you can ignore both None values easily:

>>> assert Maybe.from_value(values).bind_optional(
...     lambda d: d.get('a'),
... ) == Some(1)

>>> assert Maybe.from_value(values).bind_optional(
...     lambda d: d.get('b'),
... ) == Nothing

So, how to write a complete check for a value: both present and missing?

>>> from typing import Optional, Dict, TypeVar
>>> from returns.maybe import Maybe, Some, Nothing

>>> _Key = TypeVar('_Key')
>>> _Value = TypeVar('_Value')

>>> def check_key(
...    heystack: Dict[_Key, _Value],
...    needle: _Key,
... ) -> Maybe[_Value]:
...     if needle not in heystack:
...         return Nothing
...     return Maybe.from_value(heystack[needle])  # try with `.from_optional`

>>> real_values = {'a': 1}
>>> opt_values = {'a': 1, 'b': None}

>>> assert check_key(real_values, 'a') == Some(1)
>>> assert check_key(real_values, 'b') == Nothing
>>> # Type revealed: returns.maybe.Maybe[builtins.int]

>>> assert check_key(opt_values, 'a') == Some(1)
>>> assert check_key(opt_values, 'b') == Some(None)
>>> assert check_key(opt_values, 'c') == Nothing
>>> # Type revealed: returns.maybe.Maybe[Union[builtins.int, None]]

Choose wisely between .from_value and .map, and .from_optional and .bind_optional. They are similar, but do different things.

See the original issue about Some(None) for more details and the full history.

Why there’s no IOMaybe?

We do have IOResult, but we don’t have IOMaybe. Why? Because when dealing with IO there are a lot of possible errors. And Maybe represents just None and the value.

It is not useful for IO related tasks. So, use Result instead, which can represent what happened to your IO.

You can convert Maybe to Result and back again with special Converters.

Why Maybe does not have alt method?

Well, because Maybe only has a single failed value: Nothing and it cannot be altered.

But, Maybe has returns.maybe.Maybe.or_else_call() method to call a passed callback function with zero argument on failed container:

>>> from returns.maybe import Some, Nothing

>>> assert Some(1).or_else_call(lambda: 2) == 1
>>> assert Nothing.or_else_call(lambda: 2) == 2

This method is unique to Maybe container.

Further reading

API Reference

classDiagram Maybe <|-- Some SupportsKindN <|-- Maybe Maybe <|-- _Nothing MaybeBasedN <|-- Maybe BaseContainer <|-- Maybe
class Maybe(inner_value)[source]

Bases: returns.primitives.container.BaseContainer, returns.primitives.hkt.SupportsKindN[Maybe, returns.maybe._ValueType, NoReturn, NoReturn], returns.interfaces.specific.maybe.MaybeBasedN[returns.maybe._ValueType, None, NoReturn]

Represents a result of a series of computations that can return None.

An alternative to using exceptions or constant is None checks. Maybe is an abstract type and should not be instantiated directly. Instead use Some and Nothing.

empty: ClassVar[Maybe[Any]] = <Nothing>

Alias for Nothing

success_type

Success type that is used to represent the successful computation.

Parameters

inner_value (+_ValueType) –

alias of returns.maybe.Some

failure_type

Failure type that is used to represent the failed computation.

Parameters

inner_value (None) –

alias of returns.maybe._Nothing

equals(other)

Typesafe equality comparison with other Result objects.

Parameters
  • self (KindN[~_EqualType, Any, Any, Any]) –

  • other (KindN[~_EqualType, Any, Any, Any]) –

Return type

bool

map(function)[source]

Composes successful container with a pure function.

>>> from returns.maybe import Some, Nothing
>>> def mappable(string: str) -> str:
...      return string + 'b'

>>> assert Some('a').map(mappable) == Some('ab')
>>> assert Nothing.map(mappable) == Nothing
Parameters

function (Callable[[+_ValueType], ~_NewValueType]) –

Return type

Maybe[~_NewValueType]

apply(function)[source]

Calls a wrapped function in a container on this container.

>>> from returns.maybe import Some, Nothing

>>> def appliable(string: str) -> str:
...      return string + 'b'

>>> assert Some('a').apply(Some(appliable)) == Some('ab')
>>> assert Some('a').apply(Nothing) == Nothing
>>> assert Nothing.apply(Some(appliable)) == Nothing
>>> assert Nothing.apply(Nothing) == Nothing
Parameters

function (KindN[Maybe, Callable[[+_ValueType], ~_NewValueType], Any, Any]) –

Return type

Maybe[~_NewValueType]

bind(function)[source]

Composes successful container with a function that returns a container.

>>> from returns.maybe import Nothing, Maybe, Some
>>> def bindable(string: str) -> Maybe[str]:
...      return Some(string + 'b')

>>> assert Some('a').bind(bindable) == Some('ab')
>>> assert Nothing.bind(bindable) == Nothing
Parameters

function (Callable[[+_ValueType], KindN[Maybe, ~_NewValueType, Any, Any]]) –

Return type

Maybe[~_NewValueType]

bind_optional(function)[source]

Binds a function returning an optional value over a container.

>>> from returns.maybe import Some, Nothing
>>> from typing import Optional

>>> def bindable(arg: str) -> Optional[int]:
...     return len(arg) if arg else None

>>> assert Some('a').bind_optional(bindable) == Some(1)
>>> assert Some('').bind_optional(bindable) == Nothing
Parameters

function (Callable[[+_ValueType], Optional[~_NewValueType]]) –

Return type

Maybe[~_NewValueType]

lash(function)[source]

Composes failed container with a function that returns a container.

>>> from returns.maybe import Maybe, Some, Nothing

>>> def lashable(arg=None) -> Maybe[str]:
...      return Some('b')

>>> assert Some('a').lash(lashable) == Some('a')
>>> assert Nothing.lash(lashable) == Some('b')

We need this feature to make Maybe compatible with different Result like operations.

Parameters

function (Callable[[Any], KindN[Maybe, +_ValueType, Any, Any]]) –

Return type

Maybe[+_ValueType]

value_or(default_value)[source]

Get value from successful container or default value from failed one.

>>> from returns.maybe import Nothing, Some
>>> assert Some(0).value_or(1) == 0
>>> assert Nothing.value_or(1) == 1
Parameters

default_value (~_NewValueType) –

Return type

Union[+_ValueType, ~_NewValueType]

or_else_call(function)[source]

Get value from successful container or default value from failed one.

Really close to value_or() but works with lazy values. This method is unique to Maybe container, because other containers do have .alt method.

But, Maybe does not have this method. There’s nothing to alt in Nothing.

Instead, it has this method to execute some function if called on a failed container:

>>> from returns.maybe import Some, Nothing
>>> assert Some(1).or_else_call(lambda: 2) == 1
>>> assert Nothing.or_else_call(lambda: 2) == 2

It might be useful to work with exceptions as well:

>>> def fallback() -> NoReturn:
...    raise ValueError('Nothing!')

>>> Nothing.or_else_call(fallback)
Traceback (most recent call last):
  ...
ValueError: Nothing!
Parameters

function (Callable[[], ~_NewValueType]) –

Return type

Union[+_ValueType, ~_NewValueType]

unwrap()[source]

Get value from successful container or raise exception for failed one.

>>> from returns.maybe import Nothing, Some
>>> assert Some(1).unwrap() == 1

>>> Nothing.unwrap()
Traceback (most recent call last):
  ...
returns.primitives.exceptions.UnwrapFailedError
Return type

+_ValueType

failure()[source]

Get failed value from failed container or raise exception from success.

>>> from returns.maybe import Nothing, Some
>>> assert Nothing.failure() is None

>>> Some(1).failure()
Traceback (most recent call last):
  ...
returns.primitives.exceptions.UnwrapFailedError
Return type

None

classmethod from_value(inner_value)[source]

Creates new instance of Maybe container based on a value.

>>> from returns.maybe import Maybe, Some
>>> assert Maybe.from_value(1) == Some(1)
>>> assert Maybe.from_value(None) == Some(None)
Parameters

inner_value (~_NewValueType) –

Return type

Maybe[~_NewValueType]

classmethod from_optional(inner_value)[source]

Creates new instance of Maybe container based on an optional value.

>>> from returns.maybe import Maybe, Some, Nothing
>>> assert Maybe.from_optional(1) == Some(1)
>>> assert Maybe.from_optional(None) == Nothing
Parameters

inner_value (Optional[~_NewValueType]) –

Return type

Maybe[~_NewValueType]

class Some(inner_value)[source]

Bases: returns.maybe.Maybe[returns.maybe._ValueType]

Represents a calculation which has succeeded and contains the value.

Quite similar to Success type.

Parameters

inner_value (+_ValueType) –

bind(function)[source]

Binds current container to a function that returns container.

bind_optional(function)[source]

Binds a function returning an optional value over a container.

unwrap()[source]

Returns inner value for successful container.

map(function)[source]

Composes current container with a pure function.

apply(container)[source]

Calls a wrapped function in a container on this container.

lash(function)[source]

Does nothing for Some.

value_or(default_value)[source]

Returns inner value for successful container.

or_else_call(function)[source]

Returns inner value for successful container.

failure()[source]

Raises exception for successful container.

Nothing: returns.maybe.Maybe[NoReturn] = <Nothing>

Public unit value of protected _Nothing type.

maybe(function)[source]

Decorator to convert None-returning function to Maybe container.

This decorator works with sync functions only. Example:

>>> from typing import Optional
>>> from returns.maybe import Nothing, Some, maybe

>>> @maybe
... def might_be_none(arg: int) -> Optional[int]:
...     if arg == 0:
...         return None
...     return 1 / arg

>>> assert might_be_none(0) == Nothing
>>> assert might_be_none(1) == Some(1.0)

Requires our mypy plugin.

Parameters

function (Callable[…, Optional[+_ValueType]]) –

Return type

Callable[…, Maybe[+_ValueType]]

Interfaces Result