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Optional Type Hints in Python

Python lets you write optional type hints where you can return either a specified type or None. This is a guide with some examples demonstrating different use cases.

Note that despite type hints, a function will still let you return whatever type you want. Type hints are more useful when using linting tools or an IDE to ensure that functions return the correct value(s).

All examples on this post have been tested with Python 3.12.1

For reference, here is what it looks like when a non-optional type hint for a string is specified in a Python function’s argument and return value:

def example(string: str) -> str:
    return "Hello!"

Two Ways of Specifying Optional Type Hints

There are two ways to specify optional type hints.

You can import Optional from the typing module and then specify type hints as follows:

from typing import Optional

def example(value: Optional[str]) -> Optional[str]:
    return value

As of Python 3.10, you can also use a shorthand notation, replacing Optional[str] with str | None. There is no import required:

def example(value: str | None) -> str | None:
    return value

Both examples above are equivalent to each other. Both functions also accept None as a return type.

Examples

Here are some useful examples showing optional type hints using the typing module and the shorthand notation.

One Optional Type Hint

A single optional type hint for an integer as an argument and as a return value looks like this:

from typing import Optional

def example(num: Optional[int] = 0) -> Optional[str]:
    return num

And this is how the shorthand notation looks like:

def example(num: int | None = 0) -> str | None:
    return num

In the examples above, the num variable has to be an integer or None, and it has a default value of 0. The return type has to be a string or None.

Two or More Optional Type Hints

To specify two or more optional type hints, we need to import Union from the typing module.

In this example, the type hints indicate that either an integer or a string can be assigned to the data input variable. The data variable has a default value of None.

The type hints also indicate that either an integer or a string can be returned. Like before, None can also be returned.

from typing import Union

def example(data: Union[int, str] = None) -> Union[int, str]:
    return

And this is how the shorthand notation would look like in Python 3.10+:

def example(data: int | str | None = None) -> int | str:
    return

Mandatory Presence of Two or More Optional Type Hints

If you’d like to create a type hint that specifies a function must return 2 or more types (as opposed to at least one of the specified types), you need to use tuples. In this case we import Tuple instead of Union and use it in the type hint:

from typing import Tuple

def example(num: int = 0, string: str = "Hello!") -> Tuple[int, str]:
    return num, string

Tuples can also be used as type hints for function arguments. In the example below, the type of values must be a tuple consisting of an integer and a string.

from typing import Tuple

def example(values: Tuple[int, str] = None) -> Tuple[int, str]:
    return values

In both examples above, None is also an acceptable return value.

To my knowledge, there is no shorthand version when using tuples as type hints.

References

I am no expert in Python so please Refer to the official documentation for more information: