Deprecate creation of abstract AST nodes

[JelleZijlstra]

Feature or enhancement

Proposal:

Currently all of these are allowed:

>>> import ast
>>> ast.expr()
<ast.expr object at 0x10526cce0>
>>> ast.stmt()
<ast.stmt object at 0x1054508d0>
>>> ast.AST()
<ast.AST object at 0x105229610>

But these are meant to be abstract classes; constructing them is not meaningful. I propose to raise a DeprecationWarning when they are constructed and raise an error a few versions down.

(Originally suggested by Cornelius Krupp on discuss.python.)

Has this already been discussed elsewhere?

I have already discussed this feature proposal on Discourse

Links to previous discussion of this feature:

https://discuss.python.org/t/proposed-change-to-ast-node-constructors/46531/4

[bzoracler]

Since there are no current and planned restrictions on subclassing these abstract nodes, I have a gut feeling that deprecating this behaviour will affect code which subclass these abstract nodes and which implicitly or explicitly call the superclass constructor.

Projects which rely on AST enrichment pipelines (linters, code generators, Python-like language extensions, etc.) may rely on the current behaviour.