Recent mojo crashes in python interop, possibly due to recursive types

mseritan · March 8, 2026, 10:33am

[BUG] Recent mojo versions hang when exporting recursive types to python

opened 10:31AM - 08 Mar 26 UTC

bug mojo Needs Triage

### Bug description ### Actual behavior Please see the minimal code in the fol…lowing repo: https://github.com/winding-lines/recursive-types Mojo hangs when running `pixi run build_python` on Mac OS Tahoe. ### Expected behavior This code works fine with a december version of mojo in this branch https://github.com/winding-lines/recursive-types/tree/mojo-dec ### Steps to reproduce - Please paste or attach the .mojo files that are needed to reproduce the issue. - If you are able to reduce the reproduction steps to a small example, this would be much appreciated! - Please quote the command line you used to compile and/or run the example. - If you see a crash or an error message, please paste it in the "actual behavior" above. - If using the Godbolt / Compiler Explorer, share the example link. - Include anything else that might help us debug the issue. Your insights and observations are welcome! Mojo hangs when running `pixi run build_python` on Mac OS Tahoe. Please see https://github.com/winding-lines/recursive-types, the code is also reproduced below. ``` from std.python import PythonObject, Python from std.python.bindings import PythonModuleBuilder from std.os import abort struct Field1(Copyable, Writable): var dtype: DataType1 var nullable: Bool fn __copyinit__(out self, copy: Field1): self.dtype = copy.dtype.copy() self.nullable = copy.nullable struct DataType1(Copyable, Writable): var code: UInt8 var fields: List[Field1] var size: Int fn __copyinit__(out self, copy: Self): self.code = copy.code self.fields = copy.fields.copy() self.size = copy.size @export fn PyInit_recursive_types() -> PythonObject: try: var m = PythonModuleBuilder("recursive_types") _ = m.add_type[DataType1]("DataType") return m.finalize() except e: abort(String("error creating Python Mojo module:", e)) ``` ### System information - Provide the system information by running `pixi info`. ``` ❯ pixi info System ------------ Pixi version: 0.59.0 Platform: osx-arm64 Virtual packages: __unix=0=0 : __osx=26.3.1=0 : __archspec=1=m2 Cache dir: /Users/mseritan/Library/Caches/rattler/cache Auth storage: /Users/mseritan/.rattler/credentials.json Config locations: No config files found Global ------------ Bin dir: /Users/mseritan/.pixi/bin Environment dir: /Users/mseritan/.pixi/envs Manifest dir: /Users/mseritan/.pixi/manifests/pixi-global.toml Workspace ------------ Name: recursive-types Version: 0.1.0 Manifest file: /Users/mseritan/dev/recursive-types/pixi.toml Last updated: 08-03-2026 12:28:03 Environments ------------ Environment: default Features: default Channels: https://conda.modular.com/max-nightly, conda-forge Dependency count: 3 Dependencies: python, pytest, mojo Target platforms: osx-arm64, linux-64 Prefix location: /Users/mseritan/dev/recursive-types/.pixi/envs/default Tasks: build_python ``` - Provide version information for MAX (includes Mojo) by pasting the output of `pixi list max`. ``` ❯ pixi list mojo Package Version Build Size Kind Source mojo 0.26.2.0.dev2026030805 release 77.5 MiB conda https://conda.modular.com/max-nightly/ mojo-compiler 0.26.2.0.dev2026030805 release 64 MiB conda https://conda.modular.com/max-nightly/ mojo-python 0.26.2.0.dev2026030805 release 23.6 KiB conda https://conda.modular.com/max-nightly/ ```

DarinSimmons · March 9, 2026, 4:30pm

Nice regression tracing and bug write-up.

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Recent mojo crashes in python interop, possibly due to recursive types

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