pocketpy/docs/features/ub.md

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icon: dot title: Unsupported Operations

order: 98

Unsupported operations

The following operations violate runtime assumptions. They may crash the interpreter or produce incorrect results instead of a Python exception.

Operation	Example or explanation
Modify or delete an attribute of a built-in type	int.__add__ = custom_add or del int.__add__. Optimized paths assume the original methods.
Call an unbound method with an invalid self	int.__add__('1', 2). Use ordinary operations on valid instances.
Return an unrelated object from T.__new__	The result must be an instance of T.
Pass an invalid class argument to __new__	Constructors expect a type object and the appropriate inheritance relationship.

These restrictions are distinct from the documented language differences, such as unsupported syntax.

At the C API boundary, respect the reference lifetimes: do not use a stale item reference, an object from another VM, or a reference after reset/finalization. Check types before low-level casts and do not access one VM concurrently from multiple threads.

Use Debug builds during binding development. They include checks for common native callback errors, such as an unbalanced stack or a missing return value.