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[mypyc] AssertionError in get_sequence_type_from_type on for x in reversed(<list>) when the item type contains Any #22354
Description
Crash Report
mypyc crashes with an internal error when it compiles a for loop or a comprehension over reversed(seq), where seq is a list or tuple whose item type is Any or contains Any (for example list[Any], list[tuple[int, Any]] or tuple[Any, ...]). mypy itself accepts the code with no errors.
The cause appears to be in the for x in reversed(<list>) fast path of make_for_loop_generator (mypyc/irbuild/for_helpers.py). It checks that the argument is a sequence (is_sequence_rprimitive(builder.node_type(expr.args[0]))), but then takes the item type from the type of the call: builder.get_sequence_type(expr). Because reversed.__new__ is overloaded, mypy infers the call as plain Any when the argument's item type contains Any (several overloads match), so get_sequence_type_from_type gets AnyType and hits assert False, target_type:
$ mypy -c 'from typing import Any
def f(x: list[Any], z: list[int]) -> None:
reveal_type(reversed(x)); reveal_type(reversed(z))'
<string>:3: note: Revealed type is "Any"
<string>:3: note: Revealed type is "typing.Iterator[int]"
With list[object] or list[tuple[int, str]] the call is typed Iterator[...] and compilation works. With a plain Any argument (x: Any) the fast path is not taken and compilation also works.
A local patch that takes the item type from the argument, as the plain for x in <list> path does, fixes all the cases below, and the compiled code gives the correct results:
# Special case "for x in reversed(<list>)".
expr_reg = builder.accept(expr.args[0])
- target_type = builder.get_sequence_type(expr)
+ target_type = builder.get_sequence_type(expr.args[0])(I did not run mypyc's own test suite against that patch.)
Traceback
With the compiled mypy wheel from PyPI there is no traceback, only:
Traceback (most recent call last):
crash.py:3: AssertionError: Any
crash.py:3: note: this is an internal mypyc error; please report a bug at https://github.com/mypyc/mypyc/issues
With mypy installed from source (uv pip install --no-binary mypy mypy==2.4.1), mypyc --show-traceback crash.py gives (site-packages paths shortened):
Traceback (most recent call last):
File "build/setup.py", line 6, in <module>
ext_modules=mypycify(
File ".../site-packages/mypyc/build.py", line 975, in mypycify
groups, group_cfilenames, source_deps = mypyc_build(
File ".../site-packages/mypyc/build.py", line 727, in mypyc_build
group_cfiles, ops_text, source_deps = generate_c(
File ".../site-packages/mypyc/build.py", line 347, in generate_c
modules, ctext, mapper = emitmodule.compile_modules_to_c(
File ".../site-packages/mypyc/codegen/emitmodule.py", line 583, in compile_modules_to_c
modules = compile_modules_to_ir(result, mapper, compiler_options, errors)
File ".../site-packages/mypyc/codegen/emitmodule.py", line 357, in compile_modules_to_ir
scc_ir = compile_scc_to_ir(trees, result, mapper, compiler_options, errors)
File ".../site-packages/mypyc/codegen/emitmodule.py", line 285, in compile_scc_to_ir
modules = build_ir(scc, result.graph, result.types, mapper, compiler_options, errors)
File ".../lib/python3.14/contextlib.py", line 85, in inner
return func(*args, **kwds)
File ".../site-packages/mypyc/irbuild/main.py", line 115, in build_ir
transform_mypy_file(builder, module)
File ".../site-packages/mypyc/irbuild/main.py", line 168, in transform_mypy_file
builder.accept(node)
File ".../site-packages/mypyc/irbuild/builder.py", line 373, in accept
node.accept(self.visitor)
File ".../site-packages/mypy/nodes.py", line 1165, in accept
return visitor.visit_func_def(self)
File ".../site-packages/mypyc/irbuild/visitor.py", line 183, in visit_func_def
transform_func_def(self.builder, fdef)
File ".../site-packages/mypyc/irbuild/function.py", line 100, in transform_func_def
func_ir, func_reg = gen_func_item(builder, fdef, fdef.name, sig)
File ".../site-packages/mypyc/irbuild/function.py", line 282, in gen_func_item
func_ir, func_reg = gen_func_body(builder, sig, cdef, is_singledispatch)
File ".../site-packages/mypyc/irbuild/function.py", line 302, in gen_func_body
builder.accept(builder.fn_info.fitem.body)
File ".../site-packages/mypyc/irbuild/builder.py", line 373, in accept
node.accept(self.visitor)
File ".../site-packages/mypy/nodes.py", line 1830, in accept
return visitor.visit_block(self)
File ".../site-packages/mypyc/irbuild/visitor.py", line 192, in visit_block
transform_block(self.builder, block)
File ".../site-packages/mypyc/irbuild/statement.py", line 153, in transform_block
builder.accept(stmt)
File ".../site-packages/mypyc/irbuild/builder.py", line 373, in accept
node.accept(self.visitor)
File ".../site-packages/mypy/nodes.py", line 2011, in accept
return visitor.visit_for_stmt(self)
File ".../site-packages/mypyc/irbuild/visitor.py", line 216, in visit_for_stmt
transform_for_stmt(self.builder, stmt)
File ".../site-packages/mypyc/irbuild/statement.py", line 662, in transform_for_stmt
for_loop_helper(
File ".../site-packages/mypyc/irbuild/for_helpers.py", line 128, in for_loop_helper
for_gen = make_for_loop_generator(
File ".../site-packages/mypyc/irbuild/for_helpers.py", line 588, in make_for_loop_generator
target_type = builder.get_sequence_type(expr)
File ".../site-packages/mypyc/irbuild/builder.py", line 1123, in get_sequence_type
return self.get_sequence_type_from_type(self.types[expr])
File ".../site-packages/mypyc/irbuild/builder.py", line 1152, in get_sequence_type_from_type
assert False, target_type
crash.py:3: AssertionError: Any
crash.py:3: note: this is an internal mypyc error; please report a bug at https://github.com/mypyc/mypyc/issuesTo Reproduce
crash.py:
from typing import Any
def f(x: list[Any]) -> None:
for c in reversed(x):
passuv venv -p 3.14 .venv
uv pip install -p .venv mypy==2.4.1 setuptools
.venv/bin/mypyc crash.py
Each of these crashes the same way (each tested as its own module):
from typing import Any
def a(x: list[tuple[int, Any]]) -> None:
for i, c in reversed(x):
pass
def b(x: tuple[Any, ...]) -> None:
for c in reversed(x):
pass
def c(x: list[Any]) -> list[Any]:
return [v for v in reversed(x)]
def d(columns: list[Any]) -> dict[Any, int]:
return {col[1]: i for i, col in reversed(list(enumerate(columns)))}These compile and run correctly:
from typing import Any
def ok1(x: Any) -> None: # argument itself is Any: fast path not taken
for c in reversed(x):
pass
def ok2(x: list[tuple[int, object]]) -> None: # no Any in the item type
for i, c in reversed(x):
pass
def ok3(x: list[Any]) -> None: # slice instead of reversed()
for c in x[::-1]:
passThe original code was a dict comprehension like d above, in a larger project; we replaced reversed(...) with [::-1] as a workaround.
Your Environment
- Mypy version used: 2.4.1 (latest release, PyPI wheel and source install); also reproduced on master at ccbf32b (2.5.0+dev)
- Mypy command-line flags: none (
mypyc crash.py) - Mypy configuration options from
mypy.ini(and other config files): none - Python version used: 3.14.3 (uv-managed CPython)
- Operating system and version: macOS 26.5.2, arm64 (Apple silicon)
Possibly related (same assertion, different code path, already fixed): #20341.
Activity
darshpatel1052 commentedon Nov 3, 2026
Hi, I’d like to work on this. I’d extend the reversed-loop compiler tests with lists and tuples containing Any, trace item-type selection in the fast path, and compare compiled results with Python for loops and comprehensions. Is taking the type from the sequence argument the preferred fix, or should overload inference be addressed first?
PratyushD21 commentedon Nov 12, 2026
/take
Hi, since this was still open, I have added a PR for this. The suggested fix is essentially finding the type from the existing iterable analysis. Happy to discuss if I can add anything more to this.