Literal types

Note

Literal is an officially supported feature, but is highly experimental and should be considered to be in alpha stage. It is very likely that future releases of mypy will modify the behavior of literal types, either by adding new features or by tuning or removing problematic ones.

Literal types let you indicate that an expression is equal to some specific primitive value. For example, if we annotate a variable with type Literal["foo"], mypy will understand that variable is not only of type str, but is also equal to specifically the string "foo".

This feature is primarily useful when annotating functions that behave differently based on the exact value the caller provides. For example, suppose we have a function fetch_data(...) that returns bytes if the first argument is True, and str if it’s False. We can construct a precise type signature for this function using Literal[...] and overloads:

from typing import overload, Union
from typing_extensions import Literal

# The first two overloads use Literal[...] so we can
# have precise return types:

@overload
def fetch_data(raw: Literal[True]) -> bytes: ...
@overload
def fetch_data(raw: Literal[False]) -> str: ...

# The last overload is a fallback in case the caller
# provides a regular bool:

@overload
def fetch_data(raw: bool) -> Union[bytes, str]: ...

def fetch_data(raw: bool) -> Union[bytes, str]:
    # Implementation is omitted
    ...

reveal_type(fetch_data(True))        # Revealed type is 'bytes'
reveal_type(fetch_data(False))       # Revealed type is 'str'

# Variables declared without annotations will continue to have an
# inferred type of 'bool'.

variable = True
reveal_type(fetch_data(variable))    # Revealed type is 'Union[bytes, str]'

Parameterizing Literals

Literal types may contain one or more literal bools, ints, strs, and bytes. 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", True] is equivalent to Union[Literal[-3], Literal[b"foo"], Literal[True]]. This makes writing more complex types involving literals a little more convenient.

Literal types may also contain None. Mypy will treat Literal[None] as being equivalent 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, the following 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 doing Literal[my_instance], Literal[Any], Literal[3.14], or Literal[{"foo": 2, "bar": 5}] are all illegal.

Future versions of mypy may relax some of these restrictions. For example, we plan on adding support for using enum values inside Literal[...] in an upcoming release.

Declaring literal variables

You must explicitly add an annotation to a variable to declare that it has a literal type:

a: Literal[19] = 19
reveal_type(a)          # Revealed type is 'Literal[19]'

In order to preserve backwards-compatibility, variables without this annotation are not assumed to be literals:

b = 19
reveal_type(b)          # Revealed type is 'int'

If you find repeating the value of the variable in the type hint to be tedious, you can instead change the variable to be Final:

from typing_extensions import Final, Literal

def expects_literal(x: Literal[19]) -> None: pass

c: Final = 19

reveal_type(c)          # Revealed type is 'int'
expects_literal(c)      # ...but this type checks!

If you do not provide an explicit type in the Final, the type of c becomes context-sensitive: mypy will basically try “substituting” the original assigned value whenever it’s used before performing type checking. So, mypy will type-check the above program almost as if it were written like so:

from typing_extensions import Final, Literal

def expects_literal(x: Literal[19]) -> None: pass

reveal_type(19)
expects_literal(19)

This is why expects_literal(19) type-checks despite the fact that reveal_type(c) reports int.

So while changing a variable to be Final is not quite the same thing as adding an explicit Literal[...] annotation, it often leads to the same effect in practice.

Limitations

Mypy will not understand expressions that use variables of type Literal[..] on a deep level. For example, if you have a variable a of type Literal[3] and another variable b of type Literal[5], mypy will infer that a + b has type int, not type Literal[8].

The basic rule is that literal types are treated as just regular subtypes of whatever type the parameter has. For example, Literal[3] is treated as a subtype of int and so will inherit all of int’s methods directly. This means that Literal[3].__add__ accepts the same arguments and has the same return type as int.__add__.