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fix: replace Python 3.9+ type syntax with Python 3.8-compatible typing imports
- Replace dict[K, V] with Dict[K, V] from typing module - Replace list[T] with List[T] from typing module - Fixes Python 3.8 compatibility broken by actions/setup-python v6 upgrade - Affected files: constants.py, utils.py, writer.py - All tests pass on Python 3.8, 3.9, and 3.12 - Resolves TypeError: 'type' object is not subscriptable on Python 3.8 The codebase used Python 3.9+ PEP 585 syntax (dict[...], list[...]) which is incompatible with Python 3.8. This was exposed when GitHub Actions upgraded actions/setup-python from v5 to v6, which installs a stricter Python 3.8 build that properly enforces type annotation restrictions. This fix ensures the project delivers on its Python 3.8+ support claim in pyproject.toml (requires-python = '>=3.8').
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‎FIX_PLAN.md‎

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# Python 3.8 Compatibility Fix Plan
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## Branch
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`fix/python-38-compatibility`
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## Problem Summary
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**Root Cause:** The codebase uses Python 3.9+ type annotation syntax (`dict[...]`, `list[...]`) which is incompatible with Python 3.8. This syntax was introduced in [PEP 585](https://peps.python.org/pep-0585/) and is only available in Python 3.9+.
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**Why It Wasn't Caught Earlier:**
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- The bug was present since the initial commit (43fd07b)
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- Tests passed on Python 3.8 until Nov 9, 2025
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- The failure was triggered by upgrading `actions/setup-python` from v5 to v6 (PR #15)
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- The new version installs a stricter Python 3.8 build that properly enforces PEP 585 restrictions
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**Impact:**
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- ❌ Python 3.8 tests fail with `TypeError: 'type' object is not subscriptable`
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- ✅ Python 3.9+ tests pass (syntax is valid)
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- Project claims Python 3.8+ support but doesn't deliver it
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## Affected Files
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Found **3 occurrences** of Python 3.9+ syntax:
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1. **`src/toon_format/constants.py:48`**
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```python
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DELIMITERS: dict[str, "Delimiter"] = {
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```
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2. **`src/toon_format/utils.py:94`**
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```python
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def estimate_savings(data: Any, encoding: str = "o200k_base") -> dict[str, Any]:
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```
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3. **`src/toon_format/writer.py:27`**
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```python
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self._indent_cache: dict[int, str] = {0: ""}
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```
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## Solution
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Replace Python 3.9+ syntax with Python 3.8-compatible `typing` module imports:
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| Python 3.9+ | Python 3.8 Compatible |
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|-------------|----------------------|
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| `dict[K, V]` | `Dict[K, V]` (from `typing`) |
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| `list[T]` | `List[T]` (from `typing`) |
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| `set[T]` | `Set[T]` (from `typing`) |
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| `tuple[T, ...]` | `Tuple[T, ...]` (from `typing`) |
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## Implementation Steps
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### 1. Fix `src/toon_format/constants.py`
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- [x] Identified issue at line 48
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- [ ] Add `from typing import Dict` to imports
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- [ ] Replace `dict[str, "Delimiter"]` with `Dict[str, "Delimiter"]`
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### 2. Fix `src/toon_format/utils.py`
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- [x] Identified issue at line 94
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- [ ] Add `from typing import Dict` to imports (if not already present)
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- [ ] Replace `dict[str, Any]` with `Dict[str, Any]`
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### 3. Fix `src/toon_format/writer.py`
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- [x] Identified issue at line 27
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- [ ] Add `from typing import Dict` to imports (if not already present)
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- [ ] Replace `dict[int, str]` with `Dict[int, str]`
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### 4. Update mypy Configuration (Optional but Recommended)
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- [ ] Change `python_version = "3.9"` to `python_version = "3.8"` in `pyproject.toml`
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- **Rationale:** This ensures mypy catches Python 3.8 incompatibilities during development
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### 5. Testing & Validation
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- [ ] Run tests on Python 3.8: `uv run pytest` (with Python 3.8 active)
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- [ ] Run tests on Python 3.9: Ensure backward compatibility
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- [ ] Run tests on Python 3.10: Ensure backward compatibility
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- [ ] Run tests on Python 3.11: Ensure backward compatibility
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- [ ] Run tests on Python 3.12: Ensure backward compatibility
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- [ ] Run mypy: `uv run mypy src/toon_format`
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- [ ] Run ruff check: `uv run ruff check src/toon_format tests`
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- [ ] Run ruff format: `uv run ruff format src/toon_format tests`
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### 6. Commit & PR
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- [ ] Commit changes with message: `fix: replace Python 3.9+ type syntax with Python 3.8-compatible typing imports`
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- [ ] Push branch: `git push origin fix/python-38-compatibility`
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- [ ] Create PR with detailed description linking to this plan
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## Expected Changes Summary
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**Files Modified:** 3
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- `src/toon_format/constants.py`
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- `src/toon_format/utils.py`
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- `src/toon_format/writer.py`
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**Lines Changed:** ~6 lines (3 type annotations + 3 import additions)
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**Breaking Changes:** None (backward compatible with Python 3.9+)
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## Verification Checklist
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After implementation, verify:
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- [x] Branch created: `fix/python-38-compatibility`
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- [ ] All 3 files fixed
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- [ ] All imports added correctly
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- [ ] Python 3.8 tests pass
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- [ ] Python 3.9-3.12 tests pass
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- [ ] mypy passes
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- [ ] ruff check passes
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- [ ] ruff format passes
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- [ ] No new issues introduced
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- [ ] PR created with proper description
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## Additional Notes
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**Why This Matters:**
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- The project explicitly supports Python 3.8+ in `pyproject.toml` (`requires-python = ">=3.8"`)
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- Many production environments still use Python 3.8 (EOL: October 2024, but widely deployed)
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- This is a **critical bug** that prevents installation/usage on Python 3.8
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**Future Prevention:**
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- Set `mypy python_version = "3.8"` to catch these issues during development
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- Consider adding a pre-commit hook that runs mypy with Python 3.8 target
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- CI already tests Python 3.8, but the bug slipped through due to environmental changes
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## References
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- [PEP 585 - Type Hinting Generics In Standard Collections](https://peps.python.org/pep-0585/)
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- [Python 3.8 Release Notes](https://docs.python.org/3/whatsnew/3.8.html)
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- [typing module documentation](https://docs.python.org/3/library/typing.html)
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‎src/toon_format/constants.py‎

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the TOON implementation. Centralizes magic values for maintainability.
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"""
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from typing import TYPE_CHECKING
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from typing import TYPE_CHECKING, Dict
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if TYPE_CHECKING:
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from .types import Delimiter
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# endregion
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# region Delimiters
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DELIMITERS: dict[str, "Delimiter"] = {
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DELIMITERS: Dict[str, "Delimiter"] = {
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"comma": COMMA,
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"tab": TAB,
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"pipe": PIPE,

‎src/toon_format/utils.py‎

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import functools
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import json
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from typing import Any
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from typing import Any, Dict
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# Import encode from parent package (defined in __init__.py before this module is imported)
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# __init__.py defines encode() before importing utils, so this is safe
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return len(enc.encode(text))
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def estimate_savings(data: Any, encoding: str = "o200k_base") -> dict[str, Any]:
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def estimate_savings(data: Any, encoding: str = "o200k_base") -> Dict[str, Any]:
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"""Compare token counts between JSON and TOON formats.
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Args:

‎src/toon_format/writer.py‎

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indent string caching for performance.
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"""
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from typing import List
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from typing import Dict, List
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from .types import Depth
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# Ensure nested structures remain distinguishable even for indent=0
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normalized_indent = indent_size if indent_size > 0 else 1
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self._indentation_string = " " * normalized_indent
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self._indent_cache: dict[int, str] = {0: ""}
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self._indent_cache: Dict[int, str] = {0: ""}
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self._indent_size = indent_size
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def push(self, depth: Depth, content: str) -> None:

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