How to Use the Optional Type Hint

How should I use the Optional type hint?

Optional[...] is a shorthand notation for Union[..., None], telling the type checker that either an object of the specific type is required, or None is required. ... stands for any valid type hint, including complex compound types or a Union[] of more types. Whenever you have a keyword argument with default value None, you should use Optional. (Note: If you are targeting Python 3.10 or newer, PEP 604 introduced a better syntax, see below).

So for your two examples, you have dict and list container types, but the default value for the a keyword argument shows that None is permitted too so use Optional[...]:

from typing import Optional

def test(a: Optional[dict] = None) -> None:
    #print(a) ==> {'a': 1234}
    #or
    #print(a) ==> None

def test(a: Optional[list] = None) -> None:
    #print(a) ==> [1, 2, 3, 4, 'a', 'b']
    #or
    #print(a) ==> None

There is technically no difference between using Optional[] on a Union[], or just adding None to the Union[]. So Optional[Union[str, int]] and Union[str, int, None] are exactly the same thing.

Personally, I'd stick with always using Optional[] when setting the type for a keyword argument that uses = None to set a default value, this documents the reason why None is allowed better. Moreover, it makes it easier to move the Union[...] part into a separate type alias, or to later remove the Optional[...] part if an argument becomes mandatory.

For example, say you have

from typing import Optional, Union

def api_function(optional_argument: Optional[Union[str, int]] = None) -> None:
    """Frob the fooznar.

    If optional_argument is given, it must be an id of the fooznar subwidget
    to filter on. The id should be a string, or for backwards compatibility,
    an integer is also accepted.

    """

then documentation is improved by pulling out the Union[str, int] into a type alias:

from typing import Optional, Union

# subwidget ids used to be integers, now they are strings. Support both.
SubWidgetId = Union[str, int]

def api_function(optional_argument: Optional[SubWidgetId] = None) -> None:
    """Frob the fooznar.

    If optional_argument is given, it must be an id of the fooznar subwidget
    to filter on. The id should be a string, or for backwards compatibility,
    an integer is also accepted.

    """

The refactor to move the Union[] into an alias was made all the much easier because Optional[...] was used instead of Union[str, int, None]. The None value is not a 'subwidget id' after all, it's not part of the value, None is meant to flag the absence of a value.

Side note: Unless your code only has to support Python 3.9 or newer, you want to avoid using the standard library container types in type hinting, as you can't say anything about what types they must contain. So instead of dict and list, use typing.Dict and typing.List, respectively. And when only reading from a container type, you may just as well accept any immutable abstract container type; lists and tuples are Sequence objects, while dict is a Mapping type:

from typing import Mapping, Optional, Sequence, Union

def test(a: Optional[Mapping[str, int]] = None) -> None:
    """accepts an optional map with string keys and integer values"""
    # print(a) ==> {'a': 1234}
    # or
    # print(a) ==> None

def test(a: Optional[Sequence[Union[int, str]]] = None) -> None:
    """accepts an optional sequence of integers and strings
    # print(a) ==> [1, 2, 3, 4, 'a', 'b']
    # or
    # print(a) ==> None

In Python 3.9 and up, the standard container types have all been updated to support using them in type hints, see PEP 585. But, while you now can use dict[str, int] or list[Union[int, str]], you still may want to use the more expressive Mapping and Sequence annotations to indicate that a function won't be mutating the contents (they are treated as 'read only'), and that the functions would work with any object that works as a mapping or sequence, respectively.

Python 3.10 introduces the | union operator into type hinting, see PEP 604. Instead of Union[str, int] you can write str | int. In line with other type-hinted languages, the preferred (and more concise) way to denote an optional argument in Python 3.10 and up, is now Type | None, e.g. str | None or list | None.

Python 3.10+: Optional[Type] or Type | None

PEP 604 covers these topics in the specification section.

The existing typing.Union and | syntax should be equivalent.

int | str == typing.Union[int, str]

The order of the items in the Union should not matter for equality.

(int | str) == (str | int)
(int | str | float) == typing.Union[str, float, int]

Optional values should be equivalent to the new union syntax

None | t == typing.Optional[t]

As @jonrsharpe comments, Union and Optional are not deprecated, so the Union and | syntax are acceptable.

---

Łukasz Langa, a Python core developer, replied on a YouTube live related to the Python 3.10 release that Type | None is preferred over Optional[Type] for Python 3.10+.

How to use typing hints with an optional first parameter

Let's see how slice is hinted:

class slice(object):
    start: Any
    step: Any
    stop: Any
    @overload
    def __init__(self, stop: Any) -> None: ...
    @overload
    def __init__(self, start: Any, stop: Any, step: Any = ...) -> None: ...
    __hash__: None  # type: ignore
    def indices(self, len: SupportsIndex) -> Tuple[int, int, int]: ...

and range

class range(Sequence[int]):
    start: int
    stop: int
    step: int
    @overload
    def __init__(self, stop: SupportsIndex) -> None: ...
    @overload
    def __init__(self, start: SupportsIndex, stop: SupportsIndex, step: SupportsIndex = ...) -> None: ...
    [...]

Basically, you hint the two overloaded versions separately, one with the optional first parameter and one without. (So basically, your Attempt #2.)

from typing import overload, List

@overload
def fun(start: int, stop: int, divisors: List[int]):
    ...

@overload
def fun(stop: int, divisors: List[int]):
    ...

def fun(start, stop, divisors=None):
    if divisors is None:
        divisors = stop
        stop = start
        start = 0

    ...

fun(1, 2, [1,2,3])  # OK
fun(2, [1,2,3])  # OK

If you like, you can also make both variants accept positional arguments only:

# e.g.
def fun(start, stop, divisors=None, /):
    ...

How to type hint a function's optional return parameter?

Since Python 3.10 and PEP 604 you now can use | instead of Union.

The return type would be float | Tuple[float, float]

---

The right type hint would be:

from typing import Tuple, Union

def myfunc(x: float, return_y: bool = False) -> Union[float, Tuple[float, float]]:
    z = 1.5

    if return_y:
        y = 2.0
        return z, y

    return z

However, it is usually not a good practice to have these kinds of return. Either return something like Tuple[float, Optional[float]] or write multiple functions, it will be much easier to handle later on.

More about return statement consistency:

• PEP8 - Programming Recommendations

Be consistent in return statements. Either all return statements in a function should return an expression, or none of them should. If any return statement returns an expression, any return statements where no value is returned should explicitly state this as return None, and an explicit return statement should be present at the end of the function (if reachable).

• Why should functions return values of a consistent type?

Optional type hint in Google Style Guide

Just because it's Optional doesn't mean that it needs a default argument, or that the argument is "optional".

Optional[Text] means "it can be a Text object, or it can be None". The None value need not be a specified default though; it can be user supplied. You may, for whatever reason, want the user to pass that argument, even if it's just None.

Part of the confusion might be the use of the term "optional" here. "Optional" in this context doesn't mean that the argument is optional. It means that it's an option type.

How to type hint with an optional import?

Try sticking your import inside of an if typing.TYPE_CHECKING statement at the top of your file. This variable is always false at runtime but is treated as always true for the purposes of type hinting.

For example:

# Lets us avoid needing to use forward references everywhere
# for Python 3.7+
from __future__ import annotations
from typing import TYPE_CHECKING

if TYPE_CHECKING:
    import pandas as pd

def my_func() -> pd.DataFrame:  
    import pandas as pd                                                 
    return pd.DataFrame()

You can also do if False:, but I think that makes it a little harder for somebody to tell what's going on.

One caveat is that this does mean that while pandas will be an optional dependency at runtime, it'll still be a mandatory one for the purposes of type checking.

Another option you can explore using is mypy's --always-true and --always-false flags. This would give you finer-grained control over which parts of your code are typechecked. For example, you could do something like this:

try:
    import pandas as pd
    PANDAS_EXISTS = True
except ImportError:
    PANDAS_EXISTS = False

if PANDAS_EXISTS:
    def my_func() -> pd.DataFrame:                                                   
        return pd.DataFrame()

...then do mypy --always-true=PANDAS_EXISTS your_code.py to type check it assuming pandas is imported and mypy --always-false=PANDAS_EXISTS your_code.py to type check assuming it's missing.

This could help you catch cases where you accidentally use a function that requires pandas from a function that isn't supposed to need it -- though the caveats are that (a) this is a mypy-only solution and (b) having functions that only sometimes exist in your library might be confusing for the end-user.

Optional Union in type hint

You may think of the typing library as a specification on how to declare certain types. If something is not defined in that specification then it's always better assume it to be an undefined behavior.

However in the particular case of python and typing we have a kind-of-reference static type checker which is mypy. So in order to get an answer for your question, or just programmatically check types, we may use it and see if it shows any warnings.

Here's an example:

$ cat check_optional.py 
import typing
def fn(x: typing.Optional[int, str]):
    pass
$ mypy check_optional.py 
check_optional.py:3: error: Optional[...] must have exactly one type argument

So no, Optional[T, U] is not possible in terms of mypy even if there's no trouble declaring it within the typing library.

Besides from "functional programming" perspective both Optional and Union are two distinct but well-known and well defined monads. A combination of two monads (Union[T, U, None]) is another monad, which however behaves differently than Optional and thus should not be named so. In other words, Union[T, U, None] is isomorphic to (=same as) Optional[Union[T, U]], but not to a general Optional[X].

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