I can not assign a value to variable of a type built from the unpacking of another type with the new pythong 3.13 generic syntax

[GENDRAUD]

this code with the old generic syntax works perfectly well

from collections.abc import Callable
from typing import TypeAliasType, Unpack

RK_function_args = TypeAliasType("RK_function_args", tuple[float, int])

# Original function type
RK_function = TypeAliasType("RK_function", Callable[[Unpack[RK_function_args]], int])

# Attempted new function type with an additional int argument
RK_functionBIS = TypeAliasType(
    "RK_functionBIS", Callable[[Unpack[RK_function_args], int], int]
)

def ff(a: float, b: int, c: int) -> int:
    return 2

bis: RK_functionBIS = ff
res: int = bis(1.0, 2, 3)  # OK
print(res)

<
there is no problem assigning the value ff tp bis of type RK_functionBIS.
ff take a float , a int and a a int as argument. And the RK_functionBIS take as argument tuple(float,int,int) unpacked

But it fails with the new new generic syntax

from collections.abc import Callable
from typing import Unpack

type RK_function_args = tuple[float, int]
type RK_function = Callable[[Unpack[RK_function_args]], int]
type RK_functionBIS = Callable[[Unpack[RK_function_args], int], int]


def f(a: float, b: int) -> int:
    return 2

def ff(a: float, b: int, c: int) -> int:
    return 2


bis: RK_functionBIS = ff
res: int = bis(1.0, 2, 3)  # OK
print(res)

error messagee: main.py:17: error: Incompatible types in assignment (expression has type "Callable[[float, int, int], int]", variable has type "Callable[[VarArg(*tuple[*tuple[float, int], int])], int]") [assignment]

note that in both case in can run my code successfully- I am not completely sure this is a bug. Perhaps I just made a mistake but somebody in stackoverflow told me to report it as a bug:

https://stackoverflow.com/questions/79292914/why-does-unpacking-with-the-new-generic-syntax-in-python-3-13-cause-a-mypy-error?noredirect=1#comment139825314_79292914

[sterliakov]

As I commented there, this is not just a bug - mypy crashes with a traceback on given snippet. Using current master (#d33cef) I get:

q.py:16: error: Incompatible types in assignment (expression has type "Callable[[float, int, int], int]", variable has type "Callable[[VarArg(*tuple[*tuple[float, int], int])], int]")  [assignment]                                       
q.py:17: 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: 1.15.0+dev.d33cef8396c456d87db16dce3525ebf431f4b57f                                                                                                                                                                                
Traceback (most recent call last):                                                                                                                                                                                                          
  File "/tmp/.venv/bin/mypy", line 8, in <module>                                                                                                                                                                                           
    sys.exit(console_entry())                                                                                                                                                                                                               
  File "/tmp/.venv/lib/python3.13/site-packages/mypy/__main__.py", line 15, in console_entry
    main()
  File "/tmp/.venv/lib/python3.13/site-packages/mypy/main.py", line 114, in main
    res, messages, blockers = run_build(sources, options, fscache, t0, stdout, stderr)
  File "/tmp/.venv/lib/python3.13/site-packages/mypy/main.py", line 198, in run_build
    res = build.build(sources, options, None, flush_errors, fscache, stdout, stderr)
  File "/tmp/.venv/lib/python3.13/site-packages/mypy/build.py", line 194, in build
    result = _build(
  File "/tmp/.venv/lib/python3.13/site-packages/mypy/build.py", line 270, in _build
    graph = dispatch(sources, manager, stdout)
  File "/tmp/.venv/lib/python3.13/site-packages/mypy/build.py", line 2938, in dispatch
    process_graph(graph, manager)
  File "/tmp/.venv/lib/python3.13/site-packages/mypy/build.py", line 3336, in process_graph
    process_stale_scc(graph, scc, manager)
  File "/tmp/.venv/lib/python3.13/site-packages/mypy/build.py", line 3437, in process_stale_scc
    graph[id].type_check_first_pass()
  File "/tmp/.venv/lib/python3.13/site-packages/mypy/build.py", line 2310, in type_check_first_pass
    self.type_checker().check_first_pass()
  File "/tmp/.venv/lib/python3.13/site-packages/mypy/checker.py", line 483, in check_first_pass
    self.accept(d)
  File "/tmp/.venv/lib/python3.13/site-packages/mypy/checker.py", line 592, in accept
    stmt.accept(self)
  File "/tmp/.venv/lib/python3.13/site-packages/mypy/nodes.py", line 1364, in accept
    return visitor.visit_assignment_stmt(self)
  File "/tmp/.venv/lib/python3.13/site-packages/mypy/checker.py", line 3001, in visit_assignment_stmt
    self.check_assignment(s.lvalues[-1], s.rvalue, s.type is None, s.new_syntax)
  File "/tmp/.venv/lib/python3.13/site-packages/mypy/checker.py", line 3179, in check_assignment
    rvalue_type = self.check_simple_assignment(lvalue_type, rvalue, context=rvalue)
  File "/tmp/.venv/lib/python3.13/site-packages/mypy/checker.py", line 4350, in check_simple_assignment
    rvalue_type = self.expr_checker.accept(
  File "/tmp/.venv/lib/python3.13/site-packages/mypy/checkexpr.py", line 5902, in accept
    typ = node.accept(self)
  File "/tmp/.venv/lib/python3.13/site-packages/mypy/nodes.py", line 1984, in accept
    return visitor.visit_call_expr(self)
           ~~~~~~~~~~~~~~~~~~~~~~~^^^^^^
  File "/tmp/.venv/lib/python3.13/site-packages/mypy/checkexpr.py", line 484, in visit_call_expr
    return self.visit_call_expr_inner(e, allow_none_return=allow_none_return)
           ~~~~~~~~~~~~~~~~~~~~~~~~~~^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/tmp/.venv/lib/python3.13/site-packages/mypy/checkexpr.py", line 618, in visit_call_expr_inner
    ret_type = self.check_call_expr_with_callee_type(
        callee_type, e, fullname, object_type, member
    )
  File "/tmp/.venv/lib/python3.13/site-packages/mypy/checkexpr.py", line 1475, in check_call_expr_with_callee_type
    ret_type, callee_type = self.check_call(
                            ~~~~~~~~~~~~~~~^
        callee_type,
        ^^^^^^^^^^^^
    ...<6 lines>...
        object_type=object_type,
        ^^^^^^^^^^^^^^^^^^^^^^^^
    )
    ^
  File "/tmp/.venv/lib/python3.13/site-packages/mypy/checkexpr.py", line 1571, in check_call
    return self.check_callable_call(
           ~~~~~~~~~~~~~~~~~~~~~~~~^
        callee,
        ^^^^^^^
    ...<6 lines>...
        object_type,
        ^^^^^^^^^^^^
    )
    ^
  File "/tmp/.venv/lib/python3.13/site-packages/mypy/checkexpr.py", line 1798, in check_callable_call
    self.check_argument_types(
    ~~~~~~~~~~~~~~~~~~~~~~~~~^
        arg_types, arg_kinds, args, callee, formal_to_actual, context, object_type=object_type
        ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
    )
    ^
  File "/tmp/.venv/lib/python3.13/site-packages/mypy/checkexpr.py", line 2548, in check_argument_types
    assert isinstance(inner_unpacked_type, Instance)
           ~~~~~~~~~~^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
AssertionError: 
q.py:17: : note: use --pdb to drop into pdb

[GENDRAUD]

@sterliakov what do you mean when you say it is not a bug. It crashes with the new generic syntax. Although i worked with the old one

[JelleZijlstra]

He said it is not just a bug: it's a crash, which is worse.