Language oracle

The oracle corpus lives in the pymcu-avr repo at tests/oracle/ – it needs the AVR backend and the Avr8Sharp emulator, not just the compiler, so the 2026-06 repo split left it behind here until it was moved over. What this document describes (the probe header grammar, the divergence-citation registry, the running log of what the oracle has caught) is still the contract; the citations it names (docs/language/...) resolve in this repository.

Each probe under tests/oracle/probes/ is a small top-level-statement program that is run two ways – once directly under CPython, once compiled with PyMCU and executed in the avr8sharp ArduinoUno emulator through a small C# runner on Avr8Sharp.TestKit (UART0 captured until the program prints END) – and the two outputs are compared line by line. tests/oracle/test_oracle.py is the pytest runner; it is parametrized one test per probe file.

Each probe carries headers:

• # expect: match – CPython and the emulator must print byte-identical UART output.

• # expect: refuse <substring> – the compiler must reject the program, and the diagnostic text must contain <substring>.

• # expect: divergence <doc citation> – the compiler’s output is DELIBERATELY different from CPython’s, and the citation names where the docs say so (docs/language/type-system.md:20: bool folds to 1/0, not True/False; docs/language/roadmap.md:64: a triple-quoted string’s leading newline is stripped). The runner applies the documented transform to CPython’s own output and compares that to the emulator, so the probe passes for the documented reason and starts failing again the moment the divergence disappears (DIVERGENCE_TRANSFORMS in test_oracle.py; an unregistered citation is a hard error, not a silent pass).

• # doc: <file>:<line> – the roadmap or limitations entry the probe exercises. A probe pinned to a filed-but-not-yet-documented gap may instead cite the GitHub issue URL.

• # tracked: #<N> – optional; marks the probe as a known, filed compiler bug. The runner turns these into xfail(strict=True): the suite stays green while the issue is open, and the moment someone fixes the underlying bug the probe XPASSes, which pytest reports as a hard failure – a nudge to remove the # tracked: line and let the probe start asserting again.

• # frontend: default / # frontend: py-parser – optional; restricts the probe to one compiler front end (the C# parser, or PYMCU_PY_PARSER=1’s Python-ast one) and skips it under the other. The two front ends are meant to accept the same language subset, so this exists only for a genuine, filed disagreement between them (one refuses a construct the other accepts, sometimes with different runtime behaviour) – a shape no single # expect: can assert honestly for both engines at once. Such a gap is always covered by a pair of probes, one per front end, both citing the same issue, so the disagreement itself stays visible in the suite rather than being silently narrowed to whichever engine happens to run.

Run it from a pymcu-avr checkout (with PyMCU beside it, both venvs installed):

dotnet build tests/oracle/runner/PyMCU.OracleRunner.csproj -c Release -o build/oracle
.venv/bin/python -m pytest tests/oracle -q
PYMCU_PY_PARSER=1 .venv/bin/python -m pytest tests/oracle -q

PYMCU_BIN defaults to the pymcu-avr checkout’s .venv/bin/pymcu (editable installs resolve pymcuc through the sibling PyMCU’s src/driver/pymcuc and the backend through pymcu-avr/build/bin/pymcuc-avr); PYMCU_BACKEND_BINARY and PYMCU_ORACLE_RUNNER override the backend binary and the prebuilt runner DLL. Every probe is meant to be run, and kept green, under both front ends.

A probe is never edited to make a genuine mismatch go away. A probe IS edited when the probe itself is wrong – a stale import path, a header that cites a limitation the program does not actually hit, a # expect: refuse <substring> that does not match the compiler’s real (correct) wording – and the fix is checked by confirming the corrected probe now tests what its own header and doc citation claim. Every other mismatch is evidence about the compiler: it is filed as a GitHub issue, the probe is left exactly as it stands, and # tracked: #<N> is added so the suite reports it honestly instead of quietly skipping it.

Result of the last full run (2026-09-15, after the language-surface sweep)

178 probes (120 before this sweep, +58: probes 121-178, one per bullet of a Python-language-reference pass over builtins, integer semantics, strings, control flow, the data model, collections, exceptions, functions, modules and async – skipping only what an existing probe already covered). Two of those 58 come as # frontend:-scoped pairs (163/164, 168/169, 170/171 – six probes covering three front-end disagreements as three pairs) rather than single probes, because the two front ends give genuinely different answers.

Per front end:

Front end	match	divergence (documented)	refused (documented)	tracked (known bug)	frontend-scoped, N/A here
default (C#)	108	11	31	25 (18 distinct issues)	3
PYMCU_PY_PARSER=1	108	11	30	26 (19 distinct issues)	3

Both runs are green: every non-tracked probe matches or refuses exactly as documented, and every tracked probe’s failure is the one named issue, xfail(strict) so a fix shows up as a hard XPASS rather than a silent pass.

Compiler bugs, by cause

Cause	Issue	Probes
Unannotated integer arithmetic (loop accumulator, straight-line expression, or runtime-bounded slice sum) is sized from its first store and never widened, so it wraps mod 256	#364 (existing, open)	007, 051, 073
A field read through a with obj as name: bound name returns 0 instead of the object’s real storage	#390	029, 030
A class with no explicit __init__ cannot be constructed; CPython synthesizes a no-op default constructor	#391	037, 079, 080
self.field = bytearray(...) inside __init__ is refused, though the identical call to a local compiles	#392	049
hex()/bin()/str() folding prints the folded string’s flash address instead of its text	#393	056
List comprehensions beyond one clause: nested fors compute all zeros, a compile-time-foldable filter is refused unconditionally, one used directly as a for loop’s iterable is refused	#394	063, 064, 065
Operator dunders on a ZCA class (__add__, __lt__, and – widened by this sweep – __eq__, __le__, __sub__, __mul__) all return wrong results	#395	075, 146 (146 untracked 2026-09-25 by the #491 fix: a module-level instance is a dunder receiver now. 075 stays tracked: a CONSTRUCTOR CALL as the left operand, Num(2) + a, still reaches no method)
An explicit len(instance) call returns 0, though the same __len__ dispatches correctly for implicit truthiness	#396	076
A two-index __setitem__/__getitem__ round trip returns 0, though both calls compile	#397	078
list[T].append() on a heap-allocated list does not store the element	#398	084, 153
Indexing a one-character result out of a runtime string returns the character code, not a one-character string	#399	087
Reading an Enum member (Mode.ON, or chained .value) outside a plain assignment RHS says the enum class is not defined	#400	027
match/case sequence pattern on a real array reads phantom flattened variables instead of the array’s actual storage	#401	018
chr(n) loses its char-ness across a function return: printed as the code point, not the character, though the same chr() works at the print() call site or on a plain module-level variable	#436 (filed by this sweep)	127
+ and == on strings longer than one character fall through to the interned pool id (a small integer) instead of the text/content – the same shape as the closed #211, but past the one-character collision #211 actually fixed	#438 (filed by this sweep)	134, 135
match on a tuple pattern: the default front end correctly refuses it (tuples are not a runtime value); PYMCU_PY_PARSER=1 compiles it and silently takes the wildcard arm instead of the matching one	#439 (filed by this sweep)	168/169 (frontend-scoped pair)
match on a class pattern (case Point(x=0):): works under the default front end (undocumented, since roadmap.md:22 does not list class patterns at all), SyntaxError under PYMCU_PY_PARSER=1 – a front-end parity gap rather than a single wrong answer, so neither probe is tracked	#440 (filed by this sweep)	170/171 (frontend-scoped pair)
A type reached through a module alias (import pymcu.types as t; x: t.uint8 = ...) wraps on arithmetic instead of promoting, unlike the identical bare-imported name	#449 (filed by this sweep)	172

Probe	Feature	Doc	Expectation	Outcome	First differing line / diagnostic
001_if_elif_else.py	if elif else	docs/language/roadmap.md:13	match	match
002_while_break_continue.py	while break continue	docs/language/roadmap.md:14	match	match
003_range_runtime_bound.py	range runtime bound	docs/language/roadmap.md:15	match	match
004_range_counter_width_16.py	range counter width 16	docs/language/roadmap.md:15	match	match
005_range_signed_descending.py	range signed descending	docs/language/roadmap.md:15	match	match
006_for_fixed_array.py	for fixed array	docs/language/roadmap.md:16	match	match
007_for_list_literal.py	for list literal	docs/language/roadmap.md:16	match	tracked #364	CPython=3141, emulator=69 (mod 256)
008_for_named_tuple_long.py	for named tuple long	docs/language/roadmap.md:17	match	match
009_for_string_list.py	for string list	docs/language/roadmap.md:18	match	match
010_for_pair_list.py	for pair list	docs/language/roadmap.md:18	match	match
011_enumerate_list.py	enumerate list	docs/language/roadmap.md:19	match	match
012_enumerate_range_runtime.py	enumerate range runtime	docs/language/roadmap.md:19	match	match
013_zip_lists.py	zip lists	docs/language/roadmap.md:20	match	match
014_reversed_list.py	reversed list	docs/language/roadmap.md:21	match	match
015_reversed_range.py	reversed range	docs/language/roadmap.md:21	match	match
016_match_literal_or_wildcard.py	match literal or wildcard	docs/language/roadmap.md:22	match	match
017_match_guard.py	match guard	docs/language/roadmap.md:22	match	match
018_match_sequence.py	match sequence	docs/language/roadmap.md:22	match	tracked #401	CPython=5, emulator=0
019_match_capture.py	match capture	docs/language/roadmap.md:22	match	match
020_function_defaults_keywords.py	function defaults keywords	docs/language/roadmap.md:23	match	match
021_keyword_only_defaults.py	keyword only defaults	docs/language/roadmap.md:23	match	match
022_tuple_multireturn_inline.py	tuple multireturn inline	docs/language/roadmap.md:23	match	match
023_top_level_script.py	top level script	docs/language/roadmap.md:24	match	match
024_class_fields_methods.py	class fields methods	docs/language/roadmap.md:26	match	match
025_property_setter.py	property setter	docs/language/roadmap.md:26	match	match
026_nested_class_constants.py	nested class constants	docs/language/roadmap.md:27	match	match
027_enum_constants.py	enum constants	docs/language/roadmap.md:29	match	tracked #400	main.py:8: CompileError: name 'Mode' is not defined
028_inheritance_super_defaults.py	inheritance super defaults	docs/language/roadmap.md:28	match	match
029_with_context.py	with context	docs/language/roadmap.md:30	match	tracked #390	CPython=1, emulator=0
030_multi_with_context.py	multi with context	docs/language/roadmap.md:30	match	tracked #390	CPython=3, emulator=0
031_assert_true.py	assert true	docs/language/roadmap.md:31	match	match
032_global_access.py	global access	docs/language/roadmap.md:32	match	match
033_nonlocal_inline.py	nonlocal inline	docs/language/roadmap.md:32	match	match
034_try_except_else_finally.py	try except else finally	docs/language/roadmap.md:33	match	match
035_except_tuple.py	except tuple	docs/language/roadmap.md:33	match	match
036_except_as_args.py	except as args	docs/language/roadmap.md:33	match	match
037_raise_method_caller.py	raise method caller	docs/language/roadmap.md:33	match	tracked #391	main.py:9: CompileError: class 'Sensor' cannot be constructed: it has no __init__ method
038_unhandled_raise.py	unhandled raise	docs/language/roadmap.md:33	match	match
039_integer_promotion.py	integer promotion	docs/language/roadmap.md:34	match	match
040_int_cast_wrap.py	int cast wrap	LANGUAGE_ROADMAP.md:50	match	match
041_signed_unsigned_compare.py	signed unsigned compare	docs/language/roadmap.md:34	divergence docs/language/type-system.md:20	match
042_floor_div_mod_negative.py	floor div mod negative	docs/language/roadmap.md:35	match	match
043_true_division_float.py	true division float	docs/language/roadmap.md:35	match	match
044_divide_zero_caught.py	divide zero caught	docs/language/roadmap.md:35	match	match
045_fstring_stream_formats.py	fstring stream formats	docs/language/roadmap.md:36	match	match
046_fstring_float_stream.py	fstring float stream	docs/language/roadmap.md:36	match	match
047_print_bytearray.py	print bytearray	docs/language/roadmap.md:37	match	match
048_print_array_slice.py	print array slice	docs/language/roadmap.md:37	match	match
049_print_obj_slice.py	print obj slice	docs/language/roadmap.md:37	match	tracked #392	main.py:6: CompileError: bytearray() is a Python builtin that PyMCU does not provide
050_print_float_rounding.py	print float rounding	docs/language/roadmap.md:38	match	match
051_many_arguments.py	many arguments	docs/language/roadmap.md:39	match	tracked #364	CPython=343, emulator=87 (mod 256)
052_in_not_in.py	in not in	docs/language/roadmap.md:40	divergence docs/language/type-system.md:20	match
053_is_none.py	is none	docs/language/roadmap.md:41	divergence docs/language/type-system.md:20	match
054_divmod_builtin.py	divmod builtin	docs/language/roadmap.md:42	match	match
055_bitcast_builtin.py	bitcast builtin	docs/language/roadmap.md:43	match	match
056_hex_bin_str_pow.py	hex bin str pow	docs/language/roadmap.md:44	match	tracked #393	CPython=0xff, emulator=256 (flash address, not text)
057_sum_any_all.py	sum any all	docs/language/roadmap.md:45	divergence docs/language/type-system.md:20	match
058_bytes_literal.py	bytes literal	docs/language/roadmap.md:48	match	match
059_bytearray_mutation.py	bytearray mutation	docs/language/roadmap.md:49	match	match
060_int_from_bytes.py	int from bytes	docs/language/roadmap.md:51	match	match
061_raw_string.py	raw string	docs/language/roadmap.md:52	match	match
062_extended_unpacking.py	extended unpacking	docs/language/roadmap.md:53	match	match
063_nested_list_comprehension.py	nested list comprehension	docs/language/roadmap.md:54	match	tracked #394	CPython=13, emulator=0 (all zeros)
064_comprehension_filter.py	comprehension filter	docs/language/roadmap.md:54	match	tracked #394	main.py:4: CompileError: a list comprehension with a filter (if) is not supported
065_class_instance_comprehension.py	class instance comprehension	docs/language/roadmap.md:55	match	tracked #394	main.py:9: CompileError: for-in loop iterable must be a compile-time string constant, ...
066_class_list_parameter_instances.py	class list parameter instances	docs/language/roadmap.md:56	match	match
067_class_list_parameter_numbers.py	class list parameter numbers	docs/language/roadmap.md:57	match	match
068_class_bytearray_field.py	class bytearray field	docs/language/roadmap.md:57	match	match
069_str_join_constant.py	str join constant	docs/language/roadmap.md:58	match	match
070_str_join_runtime_buffer.py	str join runtime buffer	docs/language/roadmap.md:58	match	match
071_slice_read.py	slice read	docs/language/roadmap.md:59	match	match
072_slice_assign.py	slice assign	docs/language/roadmap.md:59	match	match
073_slice_iter_runtime_bounds.py	slice iter runtime bounds	docs/language/roadmap.md:59	match	tracked #364	CPython=297, emulator=41 (mod 256)
074_lambda_no_capture.py	lambda no capture	docs/language/roadmap.md:60	match	match
075_dunder_arithmetic_comparison.py	dunder arithmetic comparison	docs/language/roadmap.md:61	match	tracked #395	CPython=7, emulator=0
076_len_bool_truth_name.py	len bool truth name	docs/language/roadmap.md:61	match	tracked #396	CPython=2, emulator=0
077_bool_truth_field.py	bool truth field	docs/language/roadmap.md:61	match	match	fixed by PyMCU#385 (4c79b11d)
078_two_index_get_set.py	two index get set	docs/language/roadmap.md:61	match	tracked #397	CPython=39, emulator=0
079_class_attribute_instance_read.py	class attribute instance read	docs/language/roadmap.md:26	match	tracked #391	main.py:8: CompileError: class 'Device' cannot be constructed: it has no __init__ method
080_descriptor_get_set.py	descriptor get set	docs/language/roadmap.md:61	match	tracked #391	main.py:10: CompileError: class 'Slot' cannot be constructed: it has no __init__ method
081_extern_decorator_refuse_without_symbol.py	extern decorator refuse without symbol	docs/language/roadmap.md:62	refuse undefined reference	refused
082_name_guard.py	name guard	docs/language/roadmap.md:63	match	match
083_triple_quoted_string.py	triple quoted string	docs/language/roadmap.md:64	divergence docs/language/roadmap.md:64	match
084_heap_list.py	heap list	docs/language/roadmap.md:65	match	tracked #398	CPython=2, emulator=0
085_closed_dict_set_literals.py	closed dict set literals	docs/language/roadmap.md:66	divergence docs/language/type-system.md:20	match
086_fixed_dict.py	fixed dict	docs/language/roadmap.md:67	match	match
087_fstring_value.py	fstring value	docs/language/roadmap.md:68	match	tracked #399	CPython=0, emulator=48 (char code, not char)
088_generator_for.py	generator for	docs/language/roadmap.md:69	match	match
089_type_inference_unannotated.py	type inference unannotated	docs/language/roadmap.md:70	match	match
090_nested_zca_field_method.py	nested zca field method	docs/language/roadmap.md:71	match	match
091_optional_default_none.py	optional default none	docs/language/limitations.md:377	match	match
092_module_constants_reassign.py	module constants reassign	docs/language/limitations.md:78	match	match
093_import_inside_try.py	import inside try	docs/language/limitations.md:709	match	match
094_match_dotted_name.py	match dotted name	docs/language/roadmap.md:22	match	match
095_while_else.py	while else	docs/language/roadmap.md:14	match	match
096_for_else.py	for else	docs/language/roadmap.md:15	match	match
097_break_continue_unrolled.py	break continue unrolled	docs/language/roadmap.md:15	match	match
098_range_membership.py	range membership	docs/language/limitations.md:747	divergence docs/language/type-system.md:20	match
099_async_run.py	async run	LANGUAGE_ROADMAP.md:370	match	match
100_range_value_refused.py	range value refused	docs/language/limitations.md:747	refuse range	refused
101_runtime_slice_read_refused.py	runtime slice read refused	docs/language/limitations.md:643	refuse slice	refused
102_inline_fstring_expr_refused.py	inline fstring expr refused	docs/language/limitations.md:53	refuse f-string	refused
103_dict_comprehension_refused.py	dict comprehension refused	docs/language/limitations.md:500	refuse dict comprehensions	refused
104_set_comprehension_refused.py	set comprehension refused	docs/language/limitations.md:501	refuse set comprehensions	refused
105_generator_expression_refused.py	generator expression refused	docs/language/limitations.md:502	refuse Expected ')'	refused
106_yield_in_method_refused.py	yield in method refused	LANGUAGE_ROADMAP.md:375	refuse yield	refused
107_yield_expression_refused.py	yield expression refused	LANGUAGE_ROADMAP.md:375	refuse yield	refused
108_multiple_inheritance_refused.py	multiple inheritance refused	docs/language/limitations.md:323	refuse inheritance	refused
109_recursion_refused.py	recursion refused	docs/language/limitations.md:274	refuse recursive	refused
121_builtin_abs_min_max.py	builtin abs min max	docs/language/limitations.md:749	match	match
122_builtin_round_refused.py	builtin round refused	src/compiler/IR/IRGenerator/Call.cs:4511	refuse round() is a Python builtin that PyMCU does not provide	refused
123_builtin_int_float_bool_cast.py	builtin int float bool cast	LANGUAGE_ROADMAP.md:79	divergence docs/language/type-system.md:20	match
124_builtin_chr_ord_hex_bin.py	builtin chr ord hex bin	docs/language/limitations.md:760	match	match
125_builtin_oct_refused.py	builtin oct refused	LANGUAGE_ROADMAP.md:44	refuse oct() is a Python builtin that PyMCU does not provide	refused
126_len_array_and_string.py	len array and string	docs/language/limitations.md:748	match	match
127_chr_across_function_return_refused.py	chr across function return refused	docs/language/limitations.md:760	match	tracked #436	line 1: CPython=’B’, emulator=’66’
128_shift_variable_amount.py	shift variable amount	docs/language/roadmap.md:34	match	match
129_bitwise_on_signed.py	bitwise on signed	docs/language/type-system.md:156	match	match
130_comparison_chain.py	comparison chain	docs/language/type-system.md:20	divergence docs/language/type-system.md:20	match
131_augmented_assignment_all_operators.py	augmented assignment all operators	docs/language/roadmap.md:34	match	match
132_annotated_width_wraps_vs_cpython_bigint.py	annotated width wraps vs cpython bigint	docs/language/type-system.md:242	divergence docs/language/type-system.md:242	match
133_folded_overflow_refused.py	folded overflow refused	docs/language/type-system.md:189	refuse is out of range for uint8	refused
134_string_concatenation_literals.py	string concatenation literals	docs/language/limitations.md:777	match	tracked #438	line 1: CPython=’helloworld’, emulator=’257’
135_string_equality_comparison.py	string equality comparison	docs/language/limitations.md:777	match	tracked #438	line 1: CPython=’yes’, emulator=’no’
136_fstring_alignment_specs_refused.py	fstring alignment specs refused	docs/language/roadmap.md:36	refuse unsupported f-string format spec	refused
137_str_upper_lower_refused.py	str upper lower refused	docs/language/limitations.md:54	refuse undefined function	refused
138_str_strip_startswith_find_refused.py	str strip startswith find refused	docs/language/limitations.md:54	refuse undefined function	refused
139_in_on_string_refused.py	in on string refused	LANGUAGE_ROADMAP.md:40	refuse requires a list, tuple, set or dict literal	refused
140_string_slice_refused.py	string slice refused	docs/language/roadmap.md:60	refuse Slice indexing is only supported on named fixed-size arrays	refused
141_nested_loop_break_continue.py	nested loop break continue	docs/language/roadmap.md:14	match	match
142_ternary_expression.py	ternary expression	docs/language/roadmap.md:23	match	match
143_walrus_operator.py	walrus operator	docs/language/roadmap.md:23	match	match
144_pass_statement.py	pass statement	docs/language/roadmap.md:23	match	match
145_nested_function_without_inline_refused.py	nested function without inline refused	docs/language/roadmap.md:85	refuse Nested function	refused
146_dunder_eq_le_sub_mul.py	dunder eq le sub mul	docs/language/roadmap.md:61	match	tracked #395	line 1: CPython=’True’, emulator=’1’
147_dunder_str_refused.py	dunder str refused	docs/language/limitations.md:749	refuse has no runtime __str__	refused
148_dunder_contains_refused.py	dunder contains refused	LANGUAGE_ROADMAP.md:40	refuse requires a list, tuple, set or dict literal	refused
149_dunder_call.py	dunder call	docs/language/roadmap.md:61	match	match
150_dunder_iter_next_protocol_refused.py	dunder iter next protocol refused	docs/language/roadmap.md:19	refuse does not run the iterator protocol	refused
151_class_attribute_write.py	class attribute write	docs/language/roadmap.md:26	match	match
152_isinstance_refused.py	isinstance refused	docs/language/limitations.md (Built-ins summary); enhancement requested in #423/#424	refuse isinstance() is a Python builtin that PyMCU does not provide	refused
153_list_for_over_heap_list.py	list for over heap list	docs/language/roadmap.md:65	match	tracked #398	line 1: CPython=’9’, emulator=’0’
154_list_index_method_refused.py	list index method refused	docs/language/roadmap.md:65	refuse method not supported	refused
155_tuple_indexing.py	tuple indexing	docs/language/roadmap.md:17	match	match
156_dict_membership_in.py	dict membership in	docs/language/roadmap.md:67	divergence docs/language/type-system.md:20	match
157_nested_try_except.py	nested try except	docs/language/roadmap.md:33	match	match
158_bare_reraise.py	bare reraise	docs/language/roadmap.md:33	match	match
159_user_exception_class.py	user exception class	docs/language/roadmap.md:33	match	match
160_exception_crosses_inline_boundary.py	exception crosses inline boundary	docs/language/roadmap.md:33	match	match
161_args_star_compile_time.py	args star compile time	docs/language/roadmap.md:23	match	match
162_kwargs_double_star_key_access.py	kwargs double star key access	docs/language/roadmap.md:23	match	match
163_positional_only_refused_default_frontend.py	positional only refused default frontend (frontend: default)	https://github.com/PyMCU/PyMCU/issues/389	refuse Expected parameter name	refused
164_positional_only_unenforced_py_parser.py	positional only unenforced py parser (frontend: py-parser)	https://github.com/PyMCU/PyMCU/issues/389	match	match
165_default_arg_from_global.py	default arg from global	docs/language/roadmap.md:23	match	match
166_type_annotation_ignored_at_runtime.py	type annotation ignored at runtime	LANGUAGE_ROADMAP.md:79	match	match
167_inline_vs_plain_function.py	inline vs plain function	docs/language/roadmap.md:85	match	match
168_match_tuple_pattern_refused_default_frontend.py	match tuple pattern refused default frontend (frontend: default)	https://github.com/PyMCU/PyMCU/issues/439	refuse tuples are not supported as runtime values	refused
169_match_tuple_pattern_wrongcode_py_parser.py	match tuple pattern wrongcode py parser (frontend: py-parser)	https://github.com/PyMCU/PyMCU/issues/439	match	tracked #439	line 1: CPython=’5’, emulator=’0’
170_match_class_pattern_default_frontend.py	match class pattern default frontend (frontend: default)	https://github.com/PyMCU/PyMCU/issues/440	match	match
171_match_class_pattern_refused_py_parser.py	match class pattern refused py parser (frontend: py-parser)	https://github.com/PyMCU/PyMCU/issues/440	refuse match pattern MatchClass	refused
172_import_as_module_alias.py	import as module alias	docs/language/roadmap.md:34	match	tracked #449	line 1: CPython=’300’, emulator=’44’
173_import_star.py	import star	docs/language/roadmap.md:78	match	match
174_name_main_idiom.py	name main idiom	docs/language/roadmap.md:64	match	match
175_chip_conditional.py	chip conditional	docs/language/roadmap.md:88	match	match
176_sys_implementation_refused.py	sys implementation refused	docs/language/limitations.md (no interpreter to introspect)	refuse Module not found: sys	refused
177_asyncio_sleep_loop.py	asyncio sleep loop	docs/language/roadmap.md:70	match	match
178_asyncio_gather_await_refused.py	asyncio gather await refused	docs/language/roadmap.md:70	refuse “only await asyncio.sleep(n)”	refused

Probe defects fixed in this pass

Nine probes were wrong, not the compiler; each is now corrected to test what its header and doc citation actually claim:

• 018_match_sequence.py matched on a runtime tuple bound to a name first, which trips the separately-documented tuple-as-value limitation rather than exercising the sequence pattern the probe names. Rewritten to match on a list, the documented sequence-pattern subject – which then surfaced a real bug, #401 above.

• 022_tuple_multireturn_inline.py / 033_nonlocal_inline.py imported inline from pymcu instead of pymcu.types, where it actually lives; the test harness’s CPython shim was also missing pymcu.types.inline, fixed alongside the probes.

• 048_print_array_slice.py compared a uint8[4]-annotated list literal, which the oracle’s CPython shim leaves as a plain list, against PyMCU’s bytearray-style slice repr. Rewritten to build a real bytearray on both sides, the same pattern already used by 047/059/068/071.

• 050_print_float_rounding.py wrapped print(float) in a for x in [3.25, ...]: loop over a float list literal, a form the docs do not claim is an iterable. Rewritten as four direct print() calls.

• 091_optional_default_none.py wrote from typing import Optional, which the compiler refuses (typing is an explicitly-refused module); docs/language/limitations.md:377 says Optional[X] needs no import at all. The documented idiom (try: from typing import Optional / except ImportError: pass) compiles under PyMCU and imports normally under CPython, so the probe now uses that.

• 103_dict_comprehension_refused.py / 104_set_comprehension_refused.py / 105_generator_expression_refused.py named a # expect: refuse substring in the wrong case (Dict/Set/Generator) against diagnostics that actually say dict/set comprehensions in lowercase, and a generator expression that is refused at parse time (SyntaxError: Expected ')') rather than with a message naming “Generator” at all.

What the docs claim that the oracle could not exercise

• PIC / RISC-V / ARM backends: the oracle only targets atmega328p through avr8sharp; none of the other backends the roadmap documents are exercised here.

• HAL and driver modules (pymcu.hal.*, pymcu.drivers.*): out of scope for a language-feature oracle; they need real or emulated peripherals, not just UART.

• Mutable globals across separate module files: every probe is one top-level file (compile_probe() always writes a single src/main.py); a genuine cross-module probe needs multi-file project support the harness does not have yet.

• MemoryError from the bounded bump allocator: exercising it needs the arena to actually fill, which needs list[T].append() to actually store elements – currently broken (#398, probes 084/153). Revisit once that is fixed.

Regression sweep of 2026-09-21 (corpus moved to pymcu-avr)

The corpus moved out of this repository with the backend split and sat unrun while six probes went red on main, in both front ends. Bisecting each to its introducing commit:

• 009_for_string_list.py / 010_for_pair_list.py: printed the interned ids 256, 257, … where the element texts were meant. Introduced by 107fb08a (“fold const names and field ternaries in a for-in tuple”), which let the element folder run before the literal’s string branch and reduced each literal to its interned id. Fixed by binding a folded id that resolves back to an interned string as text (and keeping the character code for length-1 texts, the convention os.listdir unrolling already used).

• 070_str_join_runtime_buffer.py: print(s) after s = "".join([chr(b) for b in buf]) printed the buffer once for every string write the program made – “ABCABCABCABC” with no newline and no “END”. a38f4480 (“resolve module-scope array stores to the canonical name”, #460) gave the indexed LOAD path a ModuleScopeArrayName fallback whose main.<suffix> probe ignores function-local bindings, so the s parameter of uart_write_str read the module’s main.s and the callee never touched its argument. Fixed by the LocalScopeBinds guard the store path’s ResolveArrayVar already had.

• 134_string_concatenation_literals.py: strict XPASS. #438 was fixed the same day (a + b of two string variables folds their texts again); the # tracked: marker was dropped and the reason moved into the probe’s docstring. 135 stays tracked: == on a bound string name still does not fold.

• 139_in_on_string.py / 148_dunder_contains.py: stale refuse expectations from before the features shipped – needle in s folds on a compile-time string name (cbd581d4, roadmap.md:49) and x in <bound instance> dispatches __contains__ (334d8bef, roadmap.md:41). Both now expect the documented bool divergence (type-system.md:20: 1 where CPython prints True) and were renamed accordingly.

The corpus now has a gate: just test-oracle in pymcu-avr builds the runner and runs the suite under both front ends, and it is named among the suites a commit must keep green in AGENTS.md/CLAUDE.md.

Comparison dunders and the construction hooks, 2026-09-25 (#491)

Eleven probes, 281 to 291, added with the fix for the five silent divergences of #491. What they pin:

• 281, 282, 283 – all six comparison dunders reach their method as a condition (if, while, both operands of and / or) and in value position. Before the fix the condition path lowered the comparison as a conditional jump over the flattened instance handles, which are never written, so every if a == b: answered “equal” while the same comparison assigned to a name dispatched correctly.

• 284 – a module-level instance is a dunder receiver. print(a + b) at top level answered 0 for any operator dunder, arithmetic included, because the receiver was looked up under the synthesized module body’s scope while the binding is filed under its bare name. This is the half of #395 that closed, which is why 146 is untracked.

• 285 – a class-typed field receiver, self.lhs == self.rhs.

• 286 – two instances of a class with no comparison dunder compare by IDENTITY, as CPython does, with b = a recognised as the same object.

• 287, 288 – an ordering between two such instances, and max() / min() over instances, are refused rather than answered from the never-written handles.

• 289, 290 – __new__ and __init_subclass__ are refused where the method is written.

• 291 – a documented divergence rather than a fix: __del__ is emitted nowhere, so whatever its body prints is absent from the firmware’s output. The transform registered against docs/language/limitations.md:383 drops exactly those lines from CPython’s.