Python: what is the purpose of '|'

Question

So basically, when i go to the builtins.py file, i find many functions like this:

def foo(o: object, arg: int | None = ...) -> int: ...

When i tried this out myself, it actually didn't give out any errors, and still works like a normal function. I'm wondering why people use it and what the purpose of it is. When i make a function like the foo function (which is the first example given before this one):

def bar(arg: int, arg1: int | None = ...) -> int:
    try:
        return arg + arg1
    except:
        print('You cannot add a string and int together')

I don't find any difference between def bar(arg: int, arg1: int): ... and def bar(arg: int, arg1: int | None = ...) -> int: ...

This may help: https://docs.python.org/3/library/typing.html#typing.Optional — Chris, Nov 10 '21 at 01:44
Alternative syntax for `Optional[int]`, where `from typing import Optional`. — Abhijit Sarkar, Nov 10 '21 at 01:54

Donut · Accepted Answer · 2021-11-10T01:53:57.240

6

This is a type hint. In this case, int | None is a Union type indicating that the arg parameter will either be an int or None.

You might also find some valuable background info here: PEP 483.

One benefit of these hints is that IDEs and linters can use them to help analyze your code and catch certain kinds of errors. Some more high-level info is available here.

edited Nov 10 '21 at 01:53

answered Nov 10 '21 at 01:44

Donut

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Thanks, this works, but why do some functions have ```| None = ...```? I don't know what it means. Default value? – Rydex Nov 10 '21 at 01:55
`None` is an object that means "no value". See [this question](https://stackoverflow.com/questions/21095654/what-is-a-nonetype-object) for more info. – Donut Nov 10 '21 at 02:32

score 1 · Answer 2 · answered Nov 10 '21 at 01:44

1

| is the or operator. So it's basically saying the type of arg1 can be either an integer or None.

See more here: https://docs.python.org/3/library/typing.html#typing.Optional

answered Nov 10 '21 at 01:44

TYZ

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score 1 · Answer 3 · answered Nov 10 '21 at 01:45

it's a type hint or type annotation. it's described in PEP 484 and | means union so basically arg1 can be int or NoneType. and why do we need to type hint? that's because it helps us to code more safely and in handy. if you use any decent IDE, you can get help of auto-completions by IDE thanks to its type hint.

2pichar · Answer 4 · 2021-11-10T02:06:07.753

The -> int between the arguments list and the colon is the return type of that function. That allows you to specify what the function is going to return, so that if you have a larger function that spans the height of the screen, you don’t have to hunt through the function to find the returns and determine their types.

The | operator is a union, meaning that the function takes a parameter that is either one of the two types. For example, a function def func(param: int|str, param2: bool|float) would take a param of either an int or a str, and param2 would be either a bool or a float

Or, in the case of dictionaries, the | operator merges the two dictionaries together into one.

Python: what is the purpose of '|'

4 Answers4