Parameterizing Literals

Literal types may contain one or more literal bools, ints, strs, bytes, andenum values. However, literal types cannot contain arbitrary expressions:types like Literal[my_string.trim()], Literal[x > 3], or Literal[3j + 4]are all illegal.

Literals containing two or more values are equivalent to the union of those values.So, Literal[-3, b"foo", MyEnum.A] is equivalent toUnion[Literal[-3], Literal[b"foo"], Literal[MyEnum.A]]. This makes writing morecomplex types involving literals a little more convenient.

Literal types may also contain None. Mypy will treat Literal[None] as beingequivalent to just None. This means that Literal[4, None],Union[Literal[4], None], and Optional[Literal[4]] are all equivalent.

Literals may also contain aliases to other literal types. For example, thefollowing program is legal:

PrimaryColors = Literal["red", "blue", "yellow"]
SecondaryColors = Literal["purple", "green", "orange"]
AllowedColors = Literal[PrimaryColors, SecondaryColors]
 
def paint(color: AllowedColors) -> None: ...
 
paint("red")        # Type checks!
paint("turquoise")  # Does not type check

Literals may not contain any other kind of type or expression. This means doingLiteral[my_instance], Literal[Any], Literal[3.14], orLiteral[{"foo": 2, "bar": 5}] are all illegal.