shape-inference-ops

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Guide for implementing and testing shape inference operators in src/onnx_ir/shape_inference/_ops/.

justinchuby By justinchuby schedule Updated 2/13/2026
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name: shape-inference-ops description: Guide for implementing and testing shape inference operators in src/onnx_ir/shape_inference/_ops/.

Shape Inference Op Development Skill

This skill provides guidance for implementing and testing shape inference operators in src/onnx_ir/shape_inference/_ops/.

Overview

Shape inference operators live under src/onnx_ir/shape_inference/_ops/. Each file registers one or more ops with the global registry. Tests live alongside implementation files with a _test.py suffix.

Implementing an Op

File Structure

# src/onnx_ir/shape_inference/_ops/_my_op.py
from __future__ import annotations

__all__ = ["infer_my_op"]

import onnx_ir as ir
from onnx_shape_inference import _context, _registry


@_registry.registry.register("", "MyOp", since_version=1)
def infer_my_op(ctx: _context.ShapeInferenceContext, node: ir.Node) -> None:
    (data,) = _context.check_inputs(node, "data")
    # ... inference logic ...
    ctx.set_shape_and_dtype(node.outputs[0], output_shape, data.dtype)

Import Conventions

In source files (_ops/*.py), import modules only — not names:

# GOOD
from onnx_shape_inference import _context, _registry

# BAD
from onnx_shape_inference._context import check_inputs

In test files (_ops/*_test.py), importing the ts helper directly is an approved exception:

from onnx_shape_inference._ops._testing import ts

Registration Pattern

For ops that share the same logic, stack @register decorators on a single function:

_reg = _registry.registry.register

@_reg("", "Add", since_version=7)
@_reg("", "Sub", since_version=7)
@_reg("", "Mul", since_version=7)
def infer_binary_elementwise(ctx, node):
    ...

Precondition Checks

Use check_inputs() and require_attr() for model validation. These raise OpUsageError for malformed models:

(data, shape) = _context.check_inputs(node, "data", "shape")
axis_attr = _context.require_attr(node, "axis")

Unknown Dimensions

When the exact size of a dimension is unknown, always use ctx.new_symbolic_dim() — never ir.SymbolicDim(None):

# GOOD — each unknown gets a unique name (_d0, _d1, ...)
output_dims = [ctx.new_symbolic_dim() for _ in range(rank)]

# BAD — anonymous dims are indistinguishable
output_dims = [ir.SymbolicDim(None) for _ in range(rank)]

Graceful Degradation

When input shape is unknown, set dtype only and return early:

if data.shape is None:
    ctx.set_shape_and_dtype(node.outputs[0], None, data.dtype)
    return

Semantic Errors

Use ctx.record_error() for runtime semantic issues (axis out of range, rank mismatch):

if axis >= rank:
    ctx.record_error(node, f"Axis {axis} out of range for rank {rank}")
    return

Testing an Op

The ts() Helper

All tests use ts() (TypeAndShape) for concise assertions. It creates an ir.TypeAndShape object for direct comparison with assertEqual:

from onnx_shape_inference._ops._testing import ts

FLOAT = ir.DataType.FLOAT
INT64 = ir.DataType.INT64

# Concrete shape
ts(FLOAT, [3, 4])              # Tensor(FLOAT), Shape([3, 4])

# Named symbolic dims (propagated from inputs)
ts(FLOAT, ["batch", 128])      # Tensor(FLOAT), Shape([SymbolicDim("batch"), 128])

# Auto-generated symbolic dims (from ctx.new_symbolic_dim())
ts(FLOAT, [3, "_d0"])          # Tensor(FLOAT), Shape([3, SymbolicDim("_d0")])

# Unknown rank (shape is None)
ts(FLOAT)                      # Tensor(FLOAT), shape=None

Symbolic Arithmetic in Dim Expressions

When an op's output dimension can be computed from input symbolic dims and known integer parameters (kernel size, stride, padding, blocksize, etc.), use SymbolicDim arithmetic instead of ctx.new_symbolic_dim():

# GOOD — preserves relationship between input and output dims
out_dim = (in_dim + pad_begin + pad_end - effective_kernel) // stride + 1

# BAD — loses the relationship
out_dim = ctx.new_symbolic_dim()

SymbolicDim supports +, -, *, //, % with integers and other SymbolicDims. Results are new SymbolicDim objects with SymPy expressions:

H = ir.SymbolicDim("H")
H + 2       # SymbolicDim("H + 2")
H * 3       # SymbolicDim("3*H")
H // 4      # SymbolicDim("floor(H/4)")
(H - 3) // 1 + 1  # SymbolicDim("H - 2")

Use ctx.new_symbolic_dim() only when the output size is truly data-dependent or unknowable at graph construction time:

  • Data-dependent ops: NonZero, Compress, Unique, StringSplit
  • Runtime-only values: TopK k, OneHot depth, Range length
  • Control flow: If/Loop trip counts
  • Kernel size unknown (weight shape not available)

Auto-Generated Dim Counter

The _d counter resets at the start of each run_shape_inference() call. The test helpers also name anonymous SymbolicDim(None) dims on inputs before calling the op (matching engine behavior), so _d0 may appear from that naming:

# Conv uses arithmetic expressions for computed spatial dims
actual = run_shape_inference("", "Conv", [ts(FLOAT, ["N", 3, "H", "W"]), ...], ...)
self.assertEqual(actual, [ts(FLOAT, ["N", 16, "H - 2", "W - 2"])])

# Named dims propagate through passthrough ops (unary, cast, softmax, etc.)
actual = run_shape_inference("", "Relu", [ts(FLOAT, ["N", "C"])], ...)
self.assertEqual(actual, [ts(FLOAT, ["N", "C"])])

# _d dims still appear for truly data-dependent ops
actual = run_shape_inference("", "NonZero", [ts(INT64, [3, 4])], ...)
self.assertEqual(actual, [ts(INT64, [2, "_d0"])])

When Dims Are Named vs Auto-Generated

Named dims propagate through: unary ops, Cast, Softmax, Dropout, elementwise broadcast, Where, MatMul, Gemm, Concat (non-concat axis), Squeeze, Unsqueeze, Transpose, Slice (non-sliced axis), Gather (data shape passthrough), Conv (batch dim), LSTM/GRU (seq/batch dims), Attention.

Expression-based dims (arithmetic on input symbolic dims): Conv/Pooling spatial (H - 2), ConvTranspose spatial (H + 2), Pad (N + 2), Tile (2*N), DepthToSpace/SpaceToDepth (floor(C/4), 2*H), Concat axis (a + b), Flatten (2*N), DFT (preserves input dims).

Auto-generated _d dims appear for: data-dependent outputs (NonZero, Compress, Unique), runtime-only values (TopK k, OneHot depth, Range length), control flow (If/Loop), unknown kernel sizes, Resize fallback, STFT frame/freq bins.

Test Structure

Use parameterized tests to cover multiple cases concisely:

class MyOpTest(unittest.TestCase):
    @parameterized.parameterized.expand([
        ("concrete", [3, 4], [3, 4]),
        ("symbolic", ["N", 4], ["N", 4]),
    ])
    def test_basic(self, _name, input_shape, expected_shape):
        actual = run_shape_inference("", "MyOp", [ts(FLOAT, input_shape)], opset_version=21)
        self.assertEqual(actual, [ts(FLOAT, expected_shape)])

    def test_missing_shape(self):
        actual = run_shape_inference("", "MyOp", [ts(FLOAT)], opset_version=21)
        self.assertEqual(actual, [ts(FLOAT)])

    def test_none_input_raises(self):
        with self.assertRaises(OpUsageError):
            run_shape_inference_with_values("", "MyOp", [None], opset_version=21)

Testing with Constant Inputs

Use const_value() for ops that read constant inputs:

from onnx_shape_inference._ops._testing import const_value

x = ir.Value(name="x", type=ir.TensorType(FLOAT), shape=ir.Shape([3, 4, 5]))
k = const_value([3])
actual = run_shape_inference_with_values("", "TopK", [x, k], opset_version=21, num_outputs=2)
self.assertEqual(actual[0], ts(FLOAT, [3, 4, 3]))

Assertion Style

Always prefer ts() over verbose multi-line assertions:

# GOOD — single line covers type + shape
self.assertEqual(actual, [ts(FLOAT, [3, "_d0"])])

# BAD — verbose and fragile
self.assertIsNotNone(actual[0].shape)
self.assertEqual(actual[0].shape.rank(), 2)
self.assertEqual(actual[0].shape[0], 3)
self.assertIsInstance(actual[0].shape[1], ir.SymbolicDim)
self.assertEqual(actual[0].type.dtype, FLOAT)

Running Tests and Lint

# Run all shape inference tests
python -m pytest src/onnx_ir/shape_inference/_ops/ -x -q

# Run a single test file
python -m pytest src/onnx_ir/shape_inference/_ops/_slice_test.py -x -q

# Lint and auto-fix
lintrunner -a --output oneline

Registering New Op Files

After creating a new op file, import it in src/onnx_ir/shape_inference/_ops/__init__.py to trigger registration:

from onnx_shape_inference._ops import _my_op  # noqa: F401

References

  • Design doc: docs/design/01_symbolic_shape_inference.md
  • Test infrastructure: src/onnx_ir/shape_inference/_ops/_testing.py
  • Registry: src/onnx_ir/shape_inference/_registry.py
  • Context: src/onnx_ir/shape_inference/_context.py
  • Broadcasting: src/onnx_ir/shape_inference/_broadcast.py
Install via CLI
npx skills add https://github.com/justinchuby/onnx-shape-inference --skill shape-inference-ops
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