THRIFT-6115: Escape Python-keyword names in service extends and cross-module type references

[Jens-G]

Summary

Depends on #3659 (THRIFT-6114) -- this branch is based on THRIFT-6114, not master, so the diff below includes THRIFT-6114's commit until that one merges. Please review/merge #3659 first; this will show a clean 1-commit diff once it's rebased onto master after that lands. See "Why the 6114 dependency" below for why.

Fourth in the THRIFT-5927 follow-up chain (THRIFT-6113: regression test wasn't running in CI; THRIFT-6114: service module filename and -remote script weren't escaped).

t_py_generator::type_name() is the shared helper that renders any struct/enum/exception/service reference as a Python expression (class instantiation, type hints, extends declarations, deserialization, except clauses, etc.). None of its three return paths escaped the identifier:

string t_py_generator::type_name(t_type* ttype) {
  ...
  if (ttype->is_service()) {
    return get_real_py_module(...) + "." + ttype->get_name();        // service extends / references
  }
  if (program != nullptr && program != program_) {
    return get_real_py_module(...) + ".ttypes." + ttype->get_name(); // cross-module (include) type references
  }
  return ttype->get_name();                                          // same-module type references
}

This meant:

• A service extendsing another service whose name is a Python keyword generated a broken import module.<keyword> statement (t_py_generator.cc:1298-1301, a direct call that doesn't go through type_name()) and a broken class Client(module.<keyword>.Client): (via type_name()).
• A struct/enum/exception referenced across an include boundary whose name is a keyword generated a broken module.ttypes.<keyword> reference wherever it's used -- including the except <Type> as <name>: clauses generated for service exceptions.

Same class of regression as THRIFT-6114 (5927 escaped identifiers in some code paths, not others), just reached via extends/include instead of the service module and -remote script.

One added wrinkle: for exception types specifically, True/False/None are Python keyword literals, not statement keywords -- except True as e: parses without a SyntaxError (True is a valid expression atom), so py_compile-based testing alone doesn't catch that sub-case; it silently binds the wrong object and would raise a runtime TypeError if ever hit. Same fix either way (escape to True_), but the test needed a small addition to see it.

Fix

• Escape the identifier at type_name()'s three return points.
• Escape the parent service name in the direct import module.<service> extends statement (t_py_generator.cc:1298-1301).
• Extend test_keyword_escape.py with an AST-based check: fail if any generated file has an except clause whose type expression is a bare True/False/None constant (which py_compile alone can't see).

Why the 6114 dependency

The service-extends-a-keyword-named-service scenario needs both fixes to work end-to-end: 6114 fixes the parent service's own module filename (continue.py -> continue_.py); this fix corrects the child's reference to it. Neither alone is sufficient for that one scenario. The struct/enum/exception cross-module and same-file cases in this PR are independent of 6114 (those types live in ttypes.py, already correctly escaped by the original THRIFT-5927) -- only the service-extends-service path is coupled.

Test fixtures

• Same-file: added service AlsoDerived extends continue to the existing Thrift5927.thrift, exercising extends of a keyword-named parent in the same file.
• Cross-module: added lib/py/test/test_compiler/thrift5927include.thrift, modeled on tutorial/shared.thrift + tutorial/tutorial.thrift's include/extends pattern, providing a keyword-named struct (except) and service (class). Thrift5927.thrift now includes it, adds struct UsesIncluded { 1: thrift5927include.except item }, and service Derived extends thrift5927include.class.

Out of scope

Found while investigating, same missed-escaping category but each a different specific variable bolted onto an otherwise-correct type_name() call site (not part of type_name()'s own output, so not covered by this fix) -- flagging for a possible future consolidated pass:

• t_py_generator.cc:647, render_const_value(): the enum value name in EnumClass.VALUE constant rendering is unescaped, inconsistent with that same value being escaped everywhere else (e.g. as a class attribute).
• t_py_generator.cc:925: a trailing field-name use in a __setattr__ override's __members__.get(...) call is unescaped, inconsistent with two escaped sibling uses of the same identifier two lines earlier in the same statement.
• t_py_generator.cc:2180, generate_service_client(): the exception field name (xname) is unescaped in one except <Type> as xname: binding and its two subsequent uses, inconsistent with the same pattern at :2246 and :2322 which do escape it.

Test plan

• Before the fix: extending the fixture (extends + include) reproduced three distinct SyntaxErrors via py_compile -- import thrift5927.continue, import thrift5927include.class, and thrift5927include.ttypes.except().
• After the fix: test_keyword_escape.py passes (OK: All 15 generated Python files compile successfully), including the new AST check.
• Real import/cross-reference test (not just syntax): AlsoDerived.Client/Derived.Client correctly inherit from the escaped parent Client classes; UsesIncluded's field type correctly resolves to the escaped except_ class from the included module.
• Regression check: an ordinary (non-keyword) two-file extends+include fixture generates byte-identical output to before.
• Wider regression: make -C lib/py check (full suite, not just this test) and make -C test/py check's code-generation step both still pass -- 189 generated files across ThriftTest.thrift/DebugProtoTest.thrift/DoubleConstantsTest.thrift/Recursive.thrift in all 8 generation flavors (default/slots/oldstyle/no_utf8strings/dynamic/dynamicslots/enum/type_hints) still compile cleanly, since type_name() is used file-wide.

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This PR includes AI-assisted changes (Claude Code); see commit trailer.