Aggregate data structure extensibility


Provide well-defined extension points for the aggregate data structure library.


RFC 1 introduces the aggregate data structure library, which allows using any shape-castable object as the shape of a field. Layouts do not consider the specific type of the shape-castable object. However, views do, and depending on whether it's a layout, a subclass of an aggregate class (Struct or Union), or any other shape-castable object, behavior differs:

>>> from amaranth import *
>>> from import *
>>> class S(Struct):
...     x: unsigned(1)
>>> layout = StructLayout({
...     "a": unsigned(1),
...     "b": ArrayLayout(unsigned(2), 4),
...     "c": S
... })
>>> View(layout).a
(slice (sig $signal) 0:1)
>>> View(layout).b
< object at 0x7f53e46934c0>
>>> View(layout).c
<__main__.S object at 0x7f53e4693a30>

At the moment this behavior is not well-defined and it special-cases the aggregate classes defined in

Guide-level explanation

Any shape-castable object can be used as the shape of a field in a layout. This includes another layout. If the object is a callable (provides a __call__ method), then when a view is accessed, the __call__ method will be called with a single argument, the slice of the underlying value, which will be returned by the view. A Layout is a callable that constructs a View from itself and the value.

Reference-level explanation

The Layout class has a method __call__. layout(value) is equivalent to View(layout, value).

The View.__getitem__ method (and by extension View.__getattr__), after extracting a field from the layout, attempts to call field.shape.__call__(value_slice), where value_slice is the slice of the underlying value corresponding to the field. If there is no such method, it iteratively calls as_shape() on field.shape or the result of the previous call to as_shape() until an object is returned that has a __call__ method, or until an instance of Shape is returned.


  • The syntax may be confusing.
    • Using __call__ to implement construction is a widespread pattern in Python. Moreover, Struct and Union are classes, whose __call__ method forwards to __new__, so implementing this behavior would remove the special case for aggregate base classes without additional code.

Rationale and alternatives

This design is, as far as the author knows, the smallest possible addition that provides the largest possible extensibility and removes all special casing of aggregate base classes. That it requires no additional functionality to be implemented on the aggregate base classes indicates that it fits well into the existing design.


  • Do not do this.

Prior art


Unresolved questions


Future possibilities