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1683. gopy abstract

What we are porting​

Objects/abstract.c (~3000 lines). The dispatcher layer between the VM and the type slots. Every BINARY_OP, COMPARE_OP, GET_ITER, FOR_ITER opcode (and their kin) routes through here.

Five families:

  • PyObject_*: Length, GetItem, SetItem, DelItem, GetAttr, SetAttr, RichCompare, Hash, Type, IsInstance, IsSubclass, Repr, Str, Bytes, Format, Call, Iter.
  • PyNumber_*: Add, Subtract, Multiply, MatMul, TrueDivide, FloorDivide, Remainder, Divmod, Power, Negative, Positive, Absolute, Invert, Lshift, Rshift, And, Or, Xor, Index, AsSsizeT, ToBase, plus the in-place variants.
  • PySequence_*: Length, Concat, Repeat, GetItem (int index), SetItem, GetSlice, SetSlice, Tuple, List, Fast, Count, Contains, Index, InPlaceConcat, InPlaceRepeat.
  • PyMapping_*: Length, GetItem (key), SetItem, HasKey, Keys, Values, Items.
  • PyIter_*: Check, Next, Send.

Why this exists​

Each family is a thin wrapper around the type slots, with fallback rules. PyNumber_Add tries nb_add on the left operand, then the reflected slot on the right operand if the left returns NotImplemented or doesn't implement it, with the subclass-priority rule. PySequence_GetItem normalises negative indices. PyObject_Hash checks tp_hash and raises TypeError if NULL.

The fallback rules are observable through normal Python code, so the dispatcher matters for behavioural compatibility.

Go shape​

package objects

// Add is PyNumber_Add. Tries left.nb_add, falls through to
// right.nb_add (reflected) per the subclass-priority rule.
func Add(a, b Object) (Object, error)

// GetItem is PyObject_GetItem. Routes to mp_subscript first,
// falls through to sq_item with index conversion.
func GetItem(o, key Object) (Object, error)

// Iter is PyObject_GetIter. Returns o.tp_iter() if defined,
// or a sequence-fallback iter if sq_item is defined.
func Iter(o Object) (Object, error)

// IterNext is PyIter_Next. Returns (nil, ErrStopIteration) at end.
func IterNext(it Object) (Object, error)

Errors flow through Go return values; the C NULL-with-exception convention becomes a typed error.

Subclass priority​

Critical rule: if the right operand's type is a strict subclass of the left operand's type, the right's reflected slot runs first. This is how Fraction(1, 2) + 1 returns a Fraction rather than falling through to int.__add__.

File mapping​

C sourceGo target
Objects/abstract.c PyObject_*objects/abstract_object.go
Objects/abstract.c PyNumber_*objects/abstract_number.go
Objects/abstract.c PySequence_*objects/abstract_sequence.go
Objects/abstract.c PyMapping_*objects/abstract_mapping.go
Objects/abstract.c PyIter_*objects/abstract_iter.go

Checklist​

Status legend: [x] shipped, [ ] pending, [~] partial / scaffold, [n] deferred / not in scope this phase.

Files​

  • [~] objects/abstract_object.go: Length, GetItem, SetItem, DelItem, RichCompare, Hash, Repr, Str. v0.2 subset shipped.
  • [~] objects/abstract_number.go: Add, Subtract, Multiply, TrueDivide, FloorDivide, Remainder, Power, Negative, Positive, Absolute, Bool. Subclass-priority rule for binops. v0.2 subset shipped.
  • objects/abstract_number.go (full): Lshift, Rshift, And, Or, Xor, Invert, MatMul, Index, AsSsizeT, ToBase, plus all in-place variants.
  • [~] objects/abstract_sequence.go: Length, GetItem, SetItem, Tuple, List, Contains, Iter, Concat. v0.2 subset shipped.
  • objects/abstract_sequence.go (full): GetSlice, SetSlice, Count, Index, Repeat, Fast, InPlaceConcat, InPlaceRepeat.
  • objects/abstract_mapping.go: Length, GetItem, SetItem, HasKey, Keys, Values, Items.
  • [~] objects/abstract_iter.go: Iter, IterNext. v0.2 shipped.
  • objects/abstract_iter.go (full): Check, Send, GetAIter, ANext.
  • objects/abstract_test.go: subclass-priority panel, reflected-slot fallback, NotImplemented handling.

Surface guarantees​

  • Subclass priority: when type(b) is a strict subclass of type(a), a + b calls b.__radd__ first.
  • NotImplemented from both sides raises TypeError with the CPython text ("unsupported operand type(s) for +: 'X' and 'Y'").
  • PyNumber_Index follows the __index__ chain; reject bool-from-int special-cased path is preserved.
  • PyIter_Next on a non-iterator raises TypeError with the CPython text.
  • PySequence_Contains routes through __contains__ if defined, falls back to iteration.
  • PyObject_Hash raises TypeError on unhashable types with the CPython text including the type name.

Cross-references​

  • Type slots: 1672.
  • Per-type implementations: 1673-1689.

Out of scope​

  • tp_async_* async-iter slots beyond the v0.2 subset. Lands alongside generators (1687) in v0.6.