When type checking applications using lib2to3, the difference in types for lib2to3.pytree.Base.children versus Node.children makes it difficult to accept both leaves and nodes and programatically work with child nodes/leaves.  Base/Leaf both have `children` defined as an empty Tuple, while Node defines it as an empty list.  It would be more useful for Base/Leaf to also use an empty list, so that the type is the same, regardless of which type of object you are given.

Empty list takes more memory and more time for creating than an empty tuple (the latter is just a singleton). Since all Node and Leaf instances share the same children by default, making it a list is errorprone.

What is the problem of having an empty tuple instead of an empty list?

The problem I'm trying to solve is around functions that operate on a Union[Leaf, Node], and want to be able to do things like `grandchildren = node.children[0].children + node.children[1].children` (contrived example, but point being tuple+list=TypeError while list+tuple=OK).  There are similar cases around list methods not being available for tuples.  In both cases, as is, the function needs to do some amount of type checking at runtime of each child to determine what operations can be done.  Also, IMO, from a type annotation perspective, having a child class that uses a different type for an attribute from the base class is an anti-pattern, or weird at best.

re. error prone:  Since the expectation in lib2to3 is already that fixers shouldn't be modifying `node.children` directly, I don't see how this makes anything more error prone.  Base/Leaf objects lack those methods for modifications, so unless the code is already being a bad actor, the situation isn't changed.

Concatenating list and tuple (or tuple and list) you can write as:

grandchildren = [*node.children[0].children, *node.children[1].children]

The only list methods not available for tuples are mutating methods (using them with a shared empty list would be a bug), and copy() (a[:] and list(a) work with both lists and tuples).

>>> sorted(set(dir(list)) - set(dir(tuple)))
['__delitem__', '__iadd__', '__imul__', '__reversed__', '__setitem__', 'append', 'clear', 'copy', 'extend', 'insert', 'pop', 'remove', 'reverse', 'sort']

Isn't children annotated as List in typeshed?

> Isn't children annotated as List in typeshed?

Yes, lib2to3.pytree.Base.children is annotated as a list of (Node or Leafs)
https://github.com/python/typeshed/blob/ea0a9c2bd6f6a69c3e49b47870e0109d98316fc6/stdlib/2and3/lib2to3/pytree.pyi#L23

lib2to3 is deprecated as of Python 3.11, and there's an available workaround. I don't think it still makes sense to change anything here.