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INTERNAL ERROR with allow_redefinition_new and undeclared instance variables #19996
Description
Crash Report
When using allow_redefinition_new = true using an undeclared instance variable crashes mypy
Traceback
mypy --show-traceback .\test.py
PS <project path>> mypy --show-traceback .\test.py
test.py:9: error: INTERNAL ERROR -- Please try using mypy master on GitHub:
https://mypy.readthedocs.io/en/stable/common_issues.html#using-a-development-mypy-build
Please report a bug at https://github.com/python/mypy/issues
version: 1.19.0+dev.1b8841b665efdc308828ece877cb8e5141954a7b
Traceback (most recent call last):
File "<frozen runpy>", line 198, in _run_module_as_main
File "<frozen runpy>", line 88, in _run_code
File "<project path>\.venv\Scripts\mypy.exe\__main__.py", line 10, in <module>
sys.exit(console_entry())
File "<project path>\.venv\Lib\site-packages\mypy\__main__.py", line 15, in console_entry
main()
File "<project path>\.venv\Lib\site-packages\mypy\main.py", line 127, in main
res, messages, blockers = run_build(sources, options, fscache, t0, stdout, stderr)
File "<project path>\.venv\Lib\site-packages\mypy\main.py", line 211, in run_build
res = build.build(sources, options, None, flush_errors, fscache, stdout, stderr)
File "<project path>\.venv\Lib\site-packages\mypy\build.py", line 196, in build
result = _build(
File "<project path>\.venv\Lib\site-packages\mypy\build.py", line 272, in _build
graph = dispatch(sources, manager, stdout)
File "<project path>\.venv\Lib\site-packages\mypy\build.py", line 2933, in dispatch
process_graph(graph, manager)
File "<project path>\.venv\Lib\site-packages\mypy\build.py", line 3333, in process_graph
process_stale_scc(graph, scc, manager)
File "<project path>\.venv\Lib\site-packages\mypy\build.py", line 3434, in process_stale_scc
graph[id].type_check_first_pass()
File "<project path>\.venv\Lib\site-packages\mypy\build.py", line 2312, in type_check_first_pass
self.type_checker().check_first_pass()
File "<project path>\.venv\Lib\site-packages\mypy\checker.py", line 524, in check_first_pass
self.accept(d)
File "<project path>\.venv\Lib\site-packages\mypy\checker.py", line 635, in accept
stmt.accept(self)
File "<project path>\.venv\Lib\site-packages\mypy\nodes.py", line 1445, in accept
return visitor.visit_class_def(self)
File "<project path>\.venv\Lib\site-packages\mypy\checker.py", line 2665, in visit_class_def
self.accept(defn.defs)
File "<project path>\.venv\Lib\site-packages\mypy\checker.py", line 635, in accept
stmt.accept(self)
File "<project path>\.venv\Lib\site-packages\mypy\nodes.py", line 1542, in accept
return visitor.visit_block(self)
File "<project path>\.venv\Lib\site-packages\mypy\checker.py", line 3126, in visit_block
self.accept(s)
File "<project path>\.venv\Lib\site-packages\mypy\checker.py", line 635, in accept
stmt.accept(self)
File "<project path>\.venv\Lib\site-packages\mypy\nodes.py", line 971, in accept
return visitor.visit_func_def(self)
File "<project path>\.venv\Lib\site-packages\mypy\checker.py", line 1192, in visit_func_def
self._visit_func_def(defn)
File "<project path>\.venv\Lib\site-packages\mypy\checker.py", line 1196, in _visit_func_def
self.check_func_item(defn, name=defn.name)
File "<project path>\.venv\Lib\site-packages\mypy\checker.py", line 1231, in check_func_item
self.check_func_def(defn, typ, name, allow_empty)
File "<project path>\.venv\Lib\site-packages\mypy\checker.py", line 1504, in check_func_def
self.accept(item.body)
File "<project path>\.venv\Lib\site-packages\mypy\checker.py", line 635, in accept
stmt.accept(self)
File "<project path>\.venv\Lib\site-packages\mypy\nodes.py", line 1542, in accept
return visitor.visit_block(self)
File "<project path>\.venv\Lib\site-packages\mypy\checker.py", line 3126, in visit_block
self.accept(s)
File "<project path>\.venv\Lib\site-packages\mypy\checker.py", line 635, in accept
stmt.accept(self)
File "<project path>\.venv\Lib\site-packages\mypy\nodes.py", line 1629, in accept
return visitor.visit_assignment_stmt(self)
File "<project path>\.venv\Lib\site-packages\mypy\checker.py", line 3177, in visit_assignment_stmt
self.check_assignment(s.lvalues[-1], s.rvalue, s.type is None, s.new_syntax)
File "<project path>\.venv\Lib\site-packages\mypy\checker.py", line 3385, in check_assignment
self.check_indexed_assignment(index_lvalue, rvalue, lvalue)
File "<project path>\.venv\Lib\site-packages\mypy\checker.py", line 4752, in check_indexed_assignment
basetype = get_proper_type(self.expr_checker.accept(lvalue.base))
File "<project path>\.venv\Lib\site-packages\mypy\checkexpr.py", line 6050, in accept
typ = node.accept(self)
File "<project path>\.venv\Lib\site-packages\mypy\nodes.py", line 2175, in accept
return visitor.visit_member_expr(self)
~~~~~~~~~~~~~~~~~~~~~~~~~^^^^^^
File "<project path>\.venv\Lib\site-packages\mypy\checkexpr.py", line 3318, in visit_member_expr
narrowed = self.narrow_type_from_binder(e, result)
File "<project path>\.venv\Lib\site-packages\mypy\checkexpr.py", line 6353, in narrow_type_from_binder
narrowed = narrow_declared_type(known_type, restriction)
File "<project path>\.venv\Lib\site-packages\mypy\meet.py", line 174, in narrow_declared_type
elif not is_overlapping_types(declared, narrowed, prohibit_none_typevar_overlap=True):
~~~~~~~~~~~~~~~~~~~~^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
File "<project path>\.venv\Lib\site-packages\mypy\meet.py", line 354, in is_overlapping_types
assert False, "Unexpectedly encountered partial type"
^^^^^
AssertionError: Unexpectedly encountered partial type
test.py:9: : note: use --pdb to drop into pdbTo Reproduce
import random
class Test:
# var: dict[str, str] | None # This declaration would fix the crash
def __init__(self) -> None:
if random.choice([True, False]):
self.var = None
return
self.var = {}
self.var["test"] = ""Your Environment
- Mypy version used: 1.18.2 and 1.19.0+dev.1b8841b665efdc308828ece877cb8e5141954a7b
- Mypy command-line flags:
- Mypy configuration options from
mypy.ini(and other config files):
allow_redefinition_new = true # without this it works
local_partial_types = true- Python version used: 3.13.7
- Operating system and version: Windows 11 Version 10.0.22631 Build 22631
Activity
neopexrl commentedon Oct 3, 2025
Is this still available to work on?
ilevkivskyi commentedon Oct 4, 2025
cc @JukkaL who worked on this.
@neopexrl You can of course try, but FWIW it may be not the best first issue.
bernikr commentedon Oct 4, 2025
i forgot to include the information that this bug is a regression and did not exist in 1.17
sterliakov commentedon Oct 11, 2025
This bisects to my #19400, but I don't see any obvious reason for that to be a root cause of leaking partials. I'll look deeper tomorrow.
Do not push partial types to the binder (#20202)