Ellipsis

Function and methods

The ellipsis literal ... can be used as a placeholder default value for a function parameter, in a stub file only, regardless of the type of the parameter.

def f(x: int = ...) -> None:
    reveal_type(x)  # revealed: int

def f2(x: str = ...) -> None:
    reveal_type(x)  # revealed: str

Class and module symbols

The ellipsis literal can be assigned to a class or module symbol, regardless of its declared type, in a stub file only.

y: bytes = ...
reveal_type(y)  # revealed: bytes
x = ...
reveal_type(x)  # revealed: Unknown

class Foo:
    y: int = ...

reveal_type(Foo.y)  # revealed: int

Unpacking ellipsis literal in assignment

No diagnostic is emitted if an ellipsis literal is "unpacked" in a stub file as part of an assignment statement:

x, y = ...
reveal_type(x)  # revealed: Unknown
reveal_type(y)  # revealed: Unknown

Unpacking ellipsis literal in for loops

Iterating over an ellipsis literal as part of a for loop in a stub is invalid, however, and results in a diagnostic:

# error: [not-iterable] "Object of type `ellipsis` is not iterable"
for a, b in ...:
    reveal_type(a)  # revealed: Unknown
    reveal_type(b)  # revealed: Unknown

Ellipsis usage in non stub file

In a non-stub file, there's no special treatment of ellipsis literals. An ellipsis literal can only be assigned if EllipsisType is actually assignable to the annotated type.

# error: 7 [invalid-parameter-default] "Default value of type `ellipsis` is not assignable to annotated parameter type `int`"
def f(x: int = ...) -> None: ...

# error: 1 [invalid-assignment] "Object of type `ellipsis` is not assignable to `int`"
a: int = ...
b = ...
reveal_type(b)  # revealed: ellipsis

Use of `Ellipsis` symbol

There is no special treatment of the builtin name Ellipsis in stubs, only of ... literals.

# error: 7 [invalid-parameter-default] "Default value of type `ellipsis` is not assignable to annotated parameter type `int`"
def f(x: int = Ellipsis) -> None: ...

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