bpo-32892: Support subclasses of base types in isinstance checks for AST constants. by serhiy-storchaka · Pull Request #9934 · python/cpython
I mean the change affects all the constant node types (Str, Num, Bytes, etc). I was wondering aloud if that was okay. I looked at the new ast.py code again today and tested older versions of Python. I think this change is fine. It only affects code that uses the backwards compatibility node types. The _ABC.__instancecheck__ is not enough to make perfect backwards compatibility. Old versions of Python didn't do any type checking for ast node values so you could do something like:
x = ast.Str({})
assert isinstance(x, ast.Str)
That will fail with the new ast.py module, even after this PR. Maybe the following would be better. Make Constant "remember" what kind of constant node it is when created. Then have __instancecheck__ use that as the first test.
--- a/Lib/ast.py
+++ b/Lib/ast.py
@@ -338,6 +338,9 @@ Constant.s = property(_getter, _setter)
class _ABC(type):
def __instancecheck__(cls, inst):
+ const_type = getattr(inst, '_const_type', None)
+ if const_type and const_type is cls:
+ return True
if not isinstance(inst, Constant):
return False
if cls in _const_types:
@@ -351,7 +354,9 @@ class _ABC(type):
def _new(cls, *args, **kwargs):
if cls in _const_types:
- return Constant(*args, **kwargs)
+ n = Constant(*args, **kwargs)
+ n._const_type = cls
+ return n
return Constant.__new__(cls, *args, **kwargs)
class Num(Constant, metaclass=_ABC):