[INTERNAL ERROR] IndexError: list index out of range

[supersergiy]

Crash Report

An internal error, possibly caused by some interaction with pytest.

To Reproduce

from __future__ import annotations

from typing import Any, Callable, Generic
from typing_extensions import ParamSpec
import pytest

P = ParamSpec("P")

class C(Generic[P]):
    def __init__(
        self,
        fn: Callable[P, Any],
        *args: P.args,
        **kwargs: P.kwargs,
    ):
        pass

def foo(x):
    return x

# No Error
c: C = C(
    fn=foo,
    x=1
)

# Internal Error
# Using a dummy decorator instead of `pytest.mark.parametrize`resolves the error.
@pytest.mark.parametrize(
    "c",
    [
        [
            C(
                fn=foo,
                x=1
            )
        ],
    ],
)
def dummy_test(
    c: C,
):
    pass

Mypy playground doesn't have pytest installed, and it's not obvious to me how to reproduce without pytest.

Traceback
version: 0.990+dev.abc9d155ffbd9ea160eec0b57c450cdf7e53ce39:

Traceback (most recent call last):
  File "/mnt/disks/sergiy-x0/envs/zetta-x3/bin/mypy", line 8, in <module>
    sys.exit(console_entry())
  File "/mnt/disks/sergiy-x0/envs/zetta-x3/lib/python3.8/site-packages/mypy/__main__.py", line 15, in console_entry
    main()
  File "/mnt/disks/sergiy-x0/envs/zetta-x3/lib/python3.8/site-packages/mypy/main.py", line 95, in main
    res, messages, blockers = run_build(sources, options, fscache, t0, stdout, stderr)
  File "/mnt/disks/sergiy-x0/envs/zetta-x3/lib/python3.8/site-packages/mypy/main.py", line 174, in run_build
    res = build.build(sources, options, None, flush_errors, fscache, stdout, stderr)
  File "/mnt/disks/sergiy-x0/envs/zetta-x3/lib/python3.8/site-packages/mypy/build.py", line 193, in build
    result = _build(
  File "/mnt/disks/sergiy-x0/envs/zetta-x3/lib/python3.8/site-packages/mypy/build.py", line 276, in _build
    graph = dispatch(sources, manager, stdout)
  File "/mnt/disks/sergiy-x0/envs/zetta-x3/lib/python3.8/site-packages/mypy/build.py", line 2903, in dispatch
    process_graph(graph, manager)
  File "/mnt/disks/sergiy-x0/envs/zetta-x3/lib/python3.8/site-packages/mypy/build.py", line 3287, in process_graph
    process_stale_scc(graph, scc, manager)
  File "/mnt/disks/sergiy-x0/envs/zetta-x3/lib/python3.8/site-packages/mypy/build.py", line 3388, in process_stale_scc
    graph[id].type_check_first_pass()
  File "/mnt/disks/sergiy-x0/envs/zetta-x3/lib/python3.8/site-packages/mypy/build.py", line 2309, in type_check_first_pass
    self.type_checker().check_first_pass()
  File "/mnt/disks/sergiy-x0/envs/zetta-x3/lib/python3.8/site-packages/mypy/checker.py", line 464, in check_first_pass
    self.accept(d)
  File "/mnt/disks/sergiy-x0/envs/zetta-x3/lib/python3.8/site-packages/mypy/checker.py", line 572, in accept
    stmt.accept(self)
  File "/mnt/disks/sergiy-x0/envs/zetta-x3/lib/python3.8/site-packages/mypy/nodes.py", line 920, in accept
    return visitor.visit_decorator(self)
  File "/mnt/disks/sergiy-x0/envs/zetta-x3/lib/python3.8/site-packages/mypy/checker.py", line 4505, in visit_decorator
    dec = self.expr_checker.accept(d)
  File "/mnt/disks/sergiy-x0/envs/zetta-x3/lib/python3.8/site-packages/mypy/checkexpr.py", line 4662, in accept
    typ = node.accept(self)
  File "/mnt/disks/sergiy-x0/envs/zetta-x3/lib/python3.8/site-packages/mypy/nodes.py", line 1838, in accept
    return visitor.visit_call_expr(self)
  File "/mnt/disks/sergiy-x0/envs/zetta-x3/lib/python3.8/site-packages/mypy/checkexpr.py", line 409, in visit_call_expr
    return self.visit_call_expr_inner(e, allow_none_return=allow_none_return)
  File "/mnt/disks/sergiy-x0/envs/zetta-x3/lib/python3.8/site-packages/mypy/checkexpr.py", line 529, in visit_call_expr_inner
    ret_type = self.check_call_expr_with_callee_type(
  File "/mnt/disks/sergiy-x0/envs/zetta-x3/lib/python3.8/site-packages/mypy/checkexpr.py", line 1180, in check_call_expr_with_callee_type
    ret_type, callee_type = self.check_call(
  File "/mnt/disks/sergiy-x0/envs/zetta-x3/lib/python3.8/site-packages/mypy/checkexpr.py", line 1299, in check_call
    result = self.check_call(
  File "/mnt/disks/sergiy-x0/envs/zetta-x3/lib/python3.8/site-packages/mypy/checkexpr.py", line 1263, in check_call
    return self.check_callable_call(
  File "/mnt/disks/sergiy-x0/envs/zetta-x3/lib/python3.8/site-packages/mypy/checkexpr.py", line 1431, in check_callable_call
    arg_types = self.infer_arg_types_in_context(callee, args, arg_kinds, formal_to_actual)
  File "/mnt/disks/sergiy-x0/envs/zetta-x3/lib/python3.8/site-packages/mypy/checkexpr.py", line 1597, in infer_arg_types_in_context
    res[ai] = self.accept(args[ai], callee.arg_types[i])
  File "/mnt/disks/sergiy-x0/envs/zetta-x3/lib/python3.8/site-packages/mypy/checkexpr.py", line 4662, in accept
    typ = node.accept(self)
  File "/mnt/disks/sergiy-x0/envs/zetta-x3/lib/python3.8/site-packages/mypy/nodes.py", line 2118, in accept
    return visitor.visit_list_expr(self)
  File "/mnt/disks/sergiy-x0/envs/zetta-x3/lib/python3.8/site-packages/mypy/checkexpr.py", line 3900, in visit_list_expr
    return self.check_lst_expr(e, "builtins.list", "<list>")
  File "/mnt/disks/sergiy-x0/envs/zetta-x3/lib/python3.8/site-packages/mypy/checkexpr.py", line 3959, in check_lst_expr
    out = self.check_call(
  File "/mnt/disks/sergiy-x0/envs/zetta-x3/lib/python3.8/site-packages/mypy/checkexpr.py", line 1263, in check_call
    return self.check_callable_call(
  File "/mnt/disks/sergiy-x0/envs/zetta-x3/lib/python3.8/site-packages/mypy/checkexpr.py", line 1403, in check_callable_call
    callee = self.infer_function_type_arguments(
  File "/mnt/disks/sergiy-x0/envs/zetta-x3/lib/python3.8/site-packages/mypy/checkexpr.py", line 1702, in infer_function_type_arguments
    arg_types = self.infer_arg_types_in_context(
  File "/mnt/disks/sergiy-x0/envs/zetta-x3/lib/python3.8/site-packages/mypy/checkexpr.py", line 1597, in infer_arg_types_in_context
    res[ai] = self.accept(args[ai], callee.arg_types[i])
  File "/mnt/disks/sergiy-x0/envs/zetta-x3/lib/python3.8/site-packages/mypy/checkexpr.py", line 4662, in accept
    typ = node.accept(self)
  File "/mnt/disks/sergiy-x0/envs/zetta-x3/lib/python3.8/site-packages/mypy/nodes.py", line 2118, in accept
    return visitor.visit_list_expr(self)
  File "/mnt/disks/sergiy-x0/envs/zetta-x3/lib/python3.8/site-packages/mypy/checkexpr.py", line 3900, in visit_list_expr
    return self.check_lst_expr(e, "builtins.list", "<list>")
  File "/mnt/disks/sergiy-x0/envs/zetta-x3/lib/python3.8/site-packages/mypy/checkexpr.py", line 3959, in check_lst_expr
    out = self.check_call(
  File "/mnt/disks/sergiy-x0/envs/zetta-x3/lib/python3.8/site-packages/mypy/checkexpr.py", line 1263, in check_call
    return self.check_callable_call(
  File "/mnt/disks/sergiy-x0/envs/zetta-x3/lib/python3.8/site-packages/mypy/checkexpr.py", line 1431, in check_callable_call
    arg_types = self.infer_arg_types_in_context(callee, args, arg_kinds, formal_to_actual)
  File "/mnt/disks/sergiy-x0/envs/zetta-x3/lib/python3.8/site-packages/mypy/checkexpr.py", line 1597, in infer_arg_types_in_context
    res[ai] = self.accept(args[ai], callee.arg_types[i])
  File "/mnt/disks/sergiy-x0/envs/zetta-x3/lib/python3.8/site-packages/mypy/checkexpr.py", line 4662, in accept
    typ = node.accept(self)
  File "/mnt/disks/sergiy-x0/envs/zetta-x3/lib/python3.8/site-packages/mypy/nodes.py", line 1838, in accept
    return visitor.visit_call_expr(self)
  File "/mnt/disks/sergiy-x0/envs/zetta-x3/lib/python3.8/site-packages/mypy/checkexpr.py", line 409, in visit_call_expr
    return self.visit_call_expr_inner(e, allow_none_return=allow_none_return)
  File "/mnt/disks/sergiy-x0/envs/zetta-x3/lib/python3.8/site-packages/mypy/checkexpr.py", line 529, in visit_call_expr_inner
    ret_type = self.check_call_expr_with_callee_type(
  File "/mnt/disks/sergiy-x0/envs/zetta-x3/lib/python3.8/site-packages/mypy/checkexpr.py", line 1180, in check_call_expr_with_callee_type
    ret_type, callee_type = self.check_call(
  File "/mnt/disks/sergiy-x0/envs/zetta-x3/lib/python3.8/site-packages/mypy/checkexpr.py", line 1263, in check_call
    return self.check_callable_call(
  File "/mnt/disks/sergiy-x0/envs/zetta-x3/lib/python3.8/site-packages/mypy/checkexpr.py", line 1403, in check_callable_call
    callee = self.infer_function_type_arguments(
  File "/mnt/disks/sergiy-x0/envs/zetta-x3/lib/python3.8/site-packages/mypy/checkexpr.py", line 1702, in infer_function_type_arguments
    arg_types = self.infer_arg_types_in_context(
  File "/mnt/disks/sergiy-x0/envs/zetta-x3/lib/python3.8/site-packages/mypy/checkexpr.py", line 1596, in infer_arg_types_in_context
    with self.allow_unions(callee.arg_types[i]):
IndexError: list index out of range

version: 0.971:

Traceback (most recent call last):
  File "mypy/checkexpr.py", line 3999, in accept
  File "mypy/nodes.py", line 1771, in accept
  File "mypy/checkexpr.py", line 306, in visit_call_expr
  File "mypy/checkexpr.py", line 384, in visit_call_expr_inner
  File "mypy/checkexpr.py", line 893, in check_call_expr_with_callee_type
  File "mypy/checkexpr.py", line 958, in check_call
  File "mypy/checkexpr.py", line 1043, in check_callable_call
  File "mypy/checkexpr.py", line 1264, in infer_function_type_arguments
  File "mypy/checkexpr.py", line 1164, in infer_arg_types_in_context
IndexError: list index out of range

Your Environment

• Mypy version used: 0.971, 0.990+dev.abc9d155ffbd9ea160eec0b57c450cdf7e53ce39
• Mypy command-line flags: --show-traceback
• Mypy configuration options from mypy.ini (and other config files): None
• Python version used: 3.8.14

[nnrepos]

i tried figuring this out but i'm not familiar enough with mypy to understand the issue.
that's what i've found so far:
in the normal definition c: C = C(...), we call check_callable_call just once, and therefore we don't convert the paramspec arguments (e.g. P`1) into the actual arguments (e.g. x: Any) in callee.arg_types.
on the other hand, in the parameterized definition parametrize(...C(...)), we call check_callable_call twice, and on the 2nd iteration it converts the paramspec arguments into actual arguments.
this is why we get:

(Pdb) p callee.arg_types
[def (*P.args, **P.kwargs) -> Any, P.args`1, P.kwargs`1]

during the normal definition (c: C = C(...)), and:

(Pdb) p callee.arg_types
[def (x: Any) -> Any, Any]

during the parameterized definition (parametrize(...C(...))).
then, we get index out of range inside infer_arg_types_in_context, because we have formal_to_actual == [[0], [], [1]], which is longer than [def (x: Any) -> Any, Any]. even though they should be the same length.

this is where i got stuck :/

[hauntsaninja]

@nnrepos Thanks for investigating! I'll take a look and see if I can provide any help.

For what it's worth, here's a self contained and somewhat minimised repro:

from typing import Callable, Generic, ParamSpec

def foo(x):
    return x

P = ParamSpec("P")

class C(Generic[P]):
    def __init__(self, fn: Callable[P, int], *args: P.args, **kwargs: P.kwargs): ...

class MarkDecorator:
    def __call__(self, *args: object, **kwargs: object) -> "MarkDecorator": ...

class MarkGenerator:
    parametrize: MarkDecorator
    
pytest_mark = MarkGenerator()

@pytest_mark.parametrize([C(fn=foo, x=1)])
def dummy_test(): ...

[hauntsaninja]

Okay, actually here's a simpler one:

from typing import Callable, Generic, ParamSpec

def foo(x): ...
def bar(x: object): ...

P = ParamSpec("P")

class C(Generic[P]):
    def __init__(self, fn: Callable[P, int], *args: P.args, **kwargs: P.kwargs): ...

bar([C(fn=foo, x=1)])