New submission from Batuhan Taskaya <isidenti...@gmail.com>:
Simple constructors (basically constructors with no fields) are currently
'cached' / pre-created and dispatched. What I mean by that is, when a user
parses some code with `ast.parse()` and gets a tree object, all simple
constructors with the same value would point to the same object.
>>> mod1 = ast.parse("a + b", mode="eval")
>>> mod1.body.left.ctx
<ast.Load object at 0x7f8695321d20>
>>> mod1.body.right.ctx
<ast.Load object at 0x7f8695321d20>
>>> mod1.body.left.ctx is mod1.body.right.ctx
True
>>> mod1.body.left.ctx.some_annotation_that_my_extra_process_tool_puts = 1
>>> mod1.body.right.ctx.some_annotation_that_my_extra_process_tool_puts
1
Even though I have no real evidence that, this was done on purpose, I believe
this is some sort of 'enum' replication (caching singletons but not really,
since this is only valid for the results of ast.parse)
>>> mod1.body.right.ctx is ast.Load()
False
>>> ast.parse("a + b", mode="eval").body.left.ctx is ast.parse("c + d",
>>> mode="eval").body.right.ctx
True
Obviously, we can not change these into enums like (ast.expr_ctx.Load / LOAD)
since it would break theoretically most of the code that works with ast. So
here is a tl;dr:
- Even though all ast objects are mutable (by default), we use the same objects
when converting C AST into Python AST. So that mutations on one object is
affecting the rest of the tree.
- We can either;
- Document this behavior and keep it
- Return freshly constructed objects when converting C AST into Python AST
This has a very slight (that I suspect no body uses) risk of breaking code,
but as I implied, very slight. Which would occur in such a case
import ast
from collections import defaultdict
def collect_contexts(tree):
contexts = defaultdict(list)
for node in ast.walk(tree):
if isinstance(node, ast.Name):
contexts[node.ctx].append(node.id)
return contexts
print(collect_contexts(ast.parse("a, b = c, d")))
This code can easily (and it makes it more reliable/robust) refactored into
- contexts[node.ctx].append(node.id)
- contexts[type(node.ctx)].append(node.id)
but just mentioning in case of any question appears about backwards
incompatability.
----------
components: Library (Lib)
messages: 379004
nosy: BTaskaya, lys.nikolaou, pablogsal, serhiy.storchaka
priority: normal
severity: normal
status: open
title: AST: Document / re-design? the simple constructor nodes from sums
versions: Python 3.10
_______________________________________
Python tracker <rep...@bugs.python.org>
<https://bugs.python.org/issue42086>
_______________________________________
_______________________________________________
Python-bugs-list mailing list
Unsubscribe:
https://mail.python.org/mailman/options/python-bugs-list/archive%40mail-archive.com
