[stefankoegl]

Supersedes #154 by @RobertCochran, which no longer applied to master. The commit credits them as co-author.

What it does

make_patch, JsonPatch.from_diff and DiffBuilder take a new keyword argument, generate_test_ops (default False). When it is set, a test operation comes before each remove and replace and checks the value about to be removed or replaced. The patch then records the previous values, and applying it to a document that differs there fails with JsonPatchTestFailed.

>>> jsonpatch.make_patch({'a': 1, 'b': 2}, {'a': 3}, generate_test_ops=True).patch
[{'op': 'test', 'path': '/b', 'value': 2}, {'op': 'remove', 'path': '/b'},
 {'op': 'test', 'path': '/a', 'value': 1}, {'op': 'replace', 'path': '/a', 'value': 3}]

• jsondiff gets a matching -t/--test-ops option.
• When the diff turns a remove into a move, that removal gets no test. With optimization=False, every remove is tested.
• A remove followed by an add at the same array index still becomes one replace, with one test in front of it.

Differences from #154

• Rewritten for the new diff engine. The diff engine has been rewritten since Add option to generate test operations in diff #154 was opened. Its code (_on_undo_*, TestOperation objects in the linked list) no longer exists, so this PR implements the feature again. Test operations use the same locations as other operations and get their array indices when the operations are replayed. Their paths therefore stay correct when earlier operations shift items.
• Existing callers are unaffected. The new argument is added at the end of each signature. Add option to generate test operations in diff #154 dropped optimization from from_diff and put the new argument before pointer_cls in make_patch, which broke callers passing those by position.
• Explicit link between a remove and its test. When a remove becomes a move, its test is found through that link. Add option to generate test operations in diff #154 instead checked whether the previous node in the list was a test.

Tests

• GenerateTestOpsTests in tests.py: checks exact outputs, including array indices, moves, optimization=False, a replace at the document root, and failing on changed values.
• CLI: a test for jsondiff --test-ops.
• Hypothesis property: with the flag, the patch equals the plain patch with a test at the same path before each remove/replace, and it still turns src into dst. It is checked with and without optimization.
• Docs: a new example in doc/tutorial.rst runs as a doctest, and doc/commandline.rst shows the new option.

python tests.py and pytest pass.

---

Generated by Claude Code

[coveralls]

coverage: 100.0%. remained the same — claude/funny-lamport-kge2fr into master

[Copilot]

🟡 Changes recommended

The generated test value aliases mutable source containers, allowing later source mutations to alter the integrity check; it needs a deep copy to preserve the original snapshot.

1 open issue

• Deep-copy mutable values when generating integrity tests · New

What changed in this PR

Adds optional test operations to generated JSON patches, allowing callers to check previous values before removing or replacing them while preserving default behavior.

Changes:

• Adds generate_test_ops to the diff APIs, with handling for moves and shifting array indices.
• Exposes the feature through jsondiff -t/--test-ops and documents its usage.
• Adds unit, CLI, and property-based coverage.

File	Description
tests.py	Tests guarded diffs, array indices, moves, and CLI options.
property_tests.py	Checks operation ordering and round trips with test generation enabled.
jsonpatch.py	Implements optional test generation in the diff engine and public APIs.
jsonpatch_cli.py	Adds and forwards the --test-ops option.
doc/​tutorial.rst	Demonstrates guarded patches and failures on changed values.
doc/​commandline.rst	Documents the new CLI option.

_{Review effort: Balanced}

[Copilot]

🟢 Approval recommended

The review identifies no active issues requiring changes to the optional JSON Patch value-check feature.

No open issues 🎉

1 resolved since last review

• Deep-copy mutable values when generating integrity tests

_{Review effort: Balanced}