What's the difference between Literal and Union in Python typing?

Question

What's the difference between Literal and Union in Python typing?

I can't see any difference between them. Could anyone explain this?

My IDE didn't respond to me when I use like Union[2,3].

Answer 1

Union[x, y] means "either x or y". Literal[5, 6] means "literally 5 or literally 6" (not some variable that's equal to any of those, but literally 5 or 6).

The difference is that the arguments of Union must be types. The arguments to Literal are literals. Type names (like int, float, MyClass) are not literals (but they do represent types), so they can't be used in Literal. Literals (like 5, 'Hello!' and [1,2,3]) are not types, so they cannot be used in Union.

For example, these types are not correct:

Literal[str, float]  # none of the arguments are literals
Union[5, 6] # none of the arguments are types

---

In response to your comment, None is both a literal and a type.

None is defined as a literal in Python's grammar:

atom: ('(' [yield_expr|testlist_comp] ')' |
       '[' [testlist_comp] ']' | # maybe list literal
       '{' [dictorsetmaker] '}' | # maybe dict/set literal
       NAME |  # NOT a literal
       NUMBER | # int/float literal
       STRING+ | # string literal
       '...' | # Ellipsis literal
       'None' | # None literal!
       'True' | 'False')

Here, None is a literal, thus, it can be used in Literal. However, it's not a class:

>>> import inspect
>>> inspect.isclass(None)
False
>>> inspect.isclass(5)
False
>>> inspect.isclass(str)
True

So, it can't be used in Union. BUT it actually is used in Union: Union[str, None] means "type str or nothing". From the docs on Union:

You can use Optional[X] as a shorthand for Union[X, None]