Python transpiler: fanc py command#84
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trevadelman wants to merge 6 commits intofantom-lang:masterfrom
Open
Python transpiler: fanc py command#84trevadelman wants to merge 6 commits intofantom-lang:masterfrom
trevadelman wants to merge 6 commits intofantom-lang:masterfrom
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Add Python transpiler to the fanc tool as a TranspileCmd, following the same pattern as the existing Java transpiler command. Generates Python 3.12+ code from Fantom source via 'fanc py <pod>'. The transpiler maps Fantom constructs to idiomatic Python: snake_case naming, combined getter/setter fields, Func.make_closure() for closures, ObjUtil dispatch for cross-type operations, and lazy __init__.py module loaders to avoid circular imports. See design.md for the full Fantom-to-Python mapping. New files: src/fanc/fan/py/PythonCmd.fan - Entry point (fanc py <pod>) src/fanc/fan/py/PyTypePrinter.fan - Class/type generation src/fanc/fan/py/PyExprPrinter.fan - Expression generation src/fanc/fan/py/PyStmtPrinter.fan - Statement generation src/fanc/fan/py/PyPrinter.fan - Base printer src/fanc/fan/py/PyUtil.fan - Utilities and operator maps src/fanc/fan/py/design.md - Technical reference src/fanc/fan/py/quickstart.md - Build and test guide Modified files: src/build/fan/BuildPod.fan - Add pyDirs field (mirrors jsDirs) src/compiler/fan/CompilerInput.fan - Add pyFiles field (mirrors jsFiles) src/fanc/build.fan - Add py/ to srcDirs
The combined getter/setter pattern used _val_=None as the default, making it impossible to set a nullable field to null. Now uses a module-level _UNSET = object() sentinel so field(None) correctly enters the setter path. Also fixes return type hints on field accessors to always use Optional[T] since the setter path returns None. Addresses review feedback from Matthew.
…ables
The transpiler now detects Wrap$ synthetic classes (the Fantom compiler's
signal for closure-captured mutable variables) and emits Python's nonlocal
keyword instead of ObjUtil.cvar() wrappers. This produces cleaner, more
idiomatic Python output.
PyPrinter.fan - nonlocalVars map tracks wrapper-to-original-name mappings
PyStmtPrinter.fan - detectAndRecordNonlocal() skips Wrap$.make() lines,
prescanNonlocal() pre-scans method bodies, writeClosure()
emits nonlocal declarations
PyExprPrinter.fan - isWrapValAccess() intercepts Wrap$.val field access and
outputs plain variable names instead of wrapper._val
Result: 214 generated files now use nonlocal, zero ObjUtil.cvar in output.
Addresses review feedback from Matthew.
Move generated _UNSET sentinel from per-module 'object()' to an import from sys::ObjUtil, ensuring a single global identity for cross-pod field inheritance. The corresponding _UNSET definition in ObjUtil.py lands in PR 2 (sys runtime). Field accessor setter branches now return _val_ so both code paths match the return type. Type hints reflect Fantom's actual type: non-nullable Str -> 'str', nullable Str? -> 'Optional[str]'. Addresses review feedback from Brian (Issues 1.5, 1.6).
Collapse two loops over pod.typeDefs in genPod() into one pass.
Replace ~40 printLine calls in writeLazyLoader() with a Str raw
string template using {{POD}} and {{TYPES_DICT}} placeholders.
Addresses review feedback from Matthew (Issues 1.7, 1.8).
Address all 10 points from code review:
1. mapLiteral/listLiteral: Cast to MapType/ListType directly. Removed
try/catch control flow and dynamic dispatch (->). Use ns-level types
instead of pod.resolveType().
2. DRY sysPrefix: Extracted sysPrefix() helper, replacing 15 inline
occurrences of the curPod != sys check.
3. checkInCtor/enterCtor/exitCtor: Scoped to sys::Func parent type
check instead of bare name matching.
4. call() broken up: From ~120 lines to 15-line dispatcher delegating
to callSafe, callDynamic, callFunc, callPrivate, callNormal.
5. isObjUtilMethod: m.parent.isObj routes ALL Obj methods through
ObjUtil (no name checks). Matches ES compiler pattern.
6. isPrimitiveType: Err/Func confirmed NOT primitives (matches ES
compiler pmap). Num routed via ObjUtil.
7. primitiveMap: Single Str:Str hashmap replaces parallel if-chains
in isPrimitiveType and primitiveClassName.
8. usesMethodStyleSetters: Removed dead code.
9. TODO defaults: throw UnsupportedErr instead of outputting comments
or None for unimplemented paths.
10. CType methods: Replaced string comparisons (targetSig == sys::Str)
with CType methods (isStr, isObj, isFloat, isNum, isDecimal, isRange).
Additional DRY improvements:
- writeTypeRef(): Extracted pod-qualified type reference (4 duplicates)
- writeArgs(): Extracted comma-separated arg writing (6+ duplicates)
- compoundAssign(): Extracted from doShortcutBinaryOp
- objUtilOp(): Extracted from divOp/modOp
- incDec(): Merged identical increment/decrement methods
- Eliminated dead cmp() method (redundant with comparison())
PyTypePrinter: fieldInit now writes type defaults only, matching ES
compiler pattern. Actual initialization handled by instance$init.
Net reduction: ~325 lines removed (15% smaller).
Addresses review feedback from Brian.
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Summary
Add Python transpiler to the fanc tool -- generates Python 3.12+ code from Fantom source via
fanc py <pod>.Context
This is PR 1 of 6 for adding Python transpilation support to Fantom. Subsequent PRs add the hand-written runtime (sys, concurrent, util), test framework, and additional pod support (inet, crypto).
Changes
New files (8):
src/fanc/fan/py/PythonCmd.fan-- Entry point (fanc py <pod>)src/fanc/fan/py/PyTypePrinter.fan-- Class/type generationsrc/fanc/fan/py/PyExprPrinter.fan-- Expression generationsrc/fanc/fan/py/PyStmtPrinter.fan-- Statement generationsrc/fanc/fan/py/PyPrinter.fan-- Base printer with indentation and output managementsrc/fanc/fan/py/PyUtil.fan-- Utilities, operator maps, naming conventionssrc/fanc/fan/py/design.md-- Technical reference for the Fantom-to-Python mappingsrc/fanc/fan/py/quickstart.md-- Build and test guideModified files (3):
src/build/fan/BuildPod.fan-- AddpyDirsfield (mirrorsjsDirs),pod.native.pymeta,ci.pyFiles = pyDirssrc/compiler/fan/CompilerInput.fan-- AddpyFilesfield (mirrorsjsFiles)src/fanc/build.fan-- Addfan/py/tosrcDirsDependencies
Testing
The transpiler generates code but cannot run it yet -- the hand-written sys runtime (PR 2) and test framework (PR 4) are needed for execution.
Design Decisions
PythonCmdextendsTranspileCmd, same asJavaCmd. UsesPyTypePrinter/PyExprPrinter/PyStmtPrinterhierarchy mirroring the Java transpiler.camelCasemethods become Python'ssnake_case. A compile-time map handles the conversion.__init__.pyloaders: Avoids circular imports by deferring module loads until first access.Func.make_closure(): Closures are wrapped to support Fantom's captured variable semantics.ObjUtildispatch: Cross-type operations (compare, equals, hash) route through a utility module matching the JS transpiler's approach.pyDirsin BuildPod: MirrorsjsDirs-- pods with apy/directory contain hand-written Python natives that override transpiled output.See
design.mdfor the complete Fantom-to-Python mapping reference.