Do Union Types Actually Exist In Python?

December 27, 2023 Post a Comment

Since python is dynamically typed, of course we can do something like this: def f(x): return 2 if x else 's' But is this the way python was actually intended to be used? Or in

Solution 1:

Union typing is only needed when you have a statically typed language, as you need to declare that an object can return one of multiple types (in your case an int or str, or in the other example str or NoneType).

Python deals in objects only, so there is never a need to even consider 'union types'. Python functions return what they return, if the programmer wants to return different types for different results then that's their choice. The choice is then an architecture choice, and makes no difference to the Python interpreter (so there is nothing to 'benchmark' here).

Python 3.5 does introduce a standard for creating optional type hints, and that standard includes Union[...] and Optional[...] annotations. Type hinting adds optional static type checking outside of the runtime, the same way types in TypeScript are not part of the JavaScript runtime.

Solution 2:

the type itself does not exist because Python is just a dynamically typed language, however, in newer Python versions, Union Type is an option for Type Hinting,

from typing import Union,TypeVar

T = TypeVar('T')
def f(x: T) ->Union[str, None]:
    if x:
        return "x"

you can use that to annotate your code, thus enabling IDE/Editor level syntax checking.

Solution 3:

Here are a couple of options to deal with use-cases where you need a tagged union/sum type in Python:

Enum + Tuples

fromenum import Enum
Token = Enum('Token', ['Number', 'Operator', 'Identifier', 'Space', 'Expression'])

(Token.Number, 42)                            # int
(Token.Operator, '+')                         # str
(Token.Identifier, 'foo')                     # str
(Token.Space, )                               # void
(Token.Expression, ('lambda', 'x', 'x+x'))    # tuple[str]

A slight variation on this uses a dedicated SumType class instead of a tuple:

from dataclasses import dataclassfrom typing importAny@dataclassclassSumType:
    enum: Enum
    data: Any

SumType(Token.Number, 42)

isinstance

ifisinstance(data, int):
    ...
ifisinstance(data, str):
    ...

Or in combination with the "enum" idea from above:

token = SumType(Token.Number, 42)

if token.enum == Token.Number:
    ...

sumtypes module

These approaches all have their various drawbacks, of course.

Solution 4:

From Python 3.10 onwards, you can use | separator for union types. Taking the example from What's New In Python 3.10:

defsquare(number: int | float) -> int | float:
    return number ** 2# Instead of defsquare(number: Union[int, float]) -> Union[int, float]:
    return number ** 2

Also, if you are using Python 3.7+, you can have the feature by using the __future__ package, with some limitations, however:

from __future__ import annotations

# Works in Python 3.7+defsquare(number: int | float) -> int | float:
    return number ** 2# Works only in Python 3.10+isinstance(3.10, int | float)
numeric = int | float

For more information, see Union Types documentation and PEP 604.

Solution 5:

Adding to @MartijnPieters answer:

But is the way python was actually intended to be used?

Returning different type depending on the param is never a good practice in any language. This makes testing, maintaining and extending the code really difficult and IMHO is an anti-pattern (but of course sometimes necessary evil). The results should at least be related via having common interface.

The only reason union was introduced to C was due to performance gain. But in Python you don't have this performance gain due to dynamic nature of the language (as Martijn noticed). Actually introducing union would lower performance since the size of union is always the size of the biggest member. Thus Python will never have C-like union.

Python Manual