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Mojo trait

SizedRaising

The SizedRaising trait describes a type that has an integer length, which might raise an error if the length can't be determined.

Any type that conforms to Sized or SizedRaising works with the built-in len() function.

The SizedRaising trait requires a type to implement the __len__() method, which can raise an error. For example:

@value
struct Foo(SizedRaising):
    var length: Int

    fn __len__(self) raises -> Int:
        if self.length < 0:
            raise Error("Length is negative")
        return self.length
@value
struct Foo(SizedRaising):
    var length: Int

    fn __len__(self) raises -> Int:
        if self.length < 0:
            raise Error("Length is negative")
        return self.length

You can pass an instance of Foo to the len() function to get its length:

fn main() raises:
    var foo = Foo(42)
    print(len(foo) == 42)
fn main() raises:
    var foo = Foo(42)
    print(len(foo) == 42)
True
True

Implemented traits​

AnyType

Methods​

__len__​

__len__(self: _Self) -> Int

Get the length of the type.

Returns:

The length of the type.

Raises:

If the length cannot be computed.