extract subtitle writer creation: resolve_subtitle_format() + make_subtitle_writer()

- infrastructure/subtitle_writer.py: add resolve_subtitle_format() that
  maps format strings (e.g. 'ass_centered_narrow') to (extension, alignment),
  and make_subtitle_writer() convenience that resolves + creates writer or None
- conversion_runner.py: replace _create_subtitle_writer() with make_subtitle_writer()
- pyqt/conversion.py: replace _subtitle_alignment_from_format() and 3 manual
  create_subtitle_writer() call sites with resolve_subtitle_format()/make_subtitle_writer()

abogen/infrastructure/subtitle_writer.py

@@ -290,6 +290,71 @@ def create_subtitle_writer(
         raise ValueError(f"Unsupported subtitle format: {format}")
 
 
+def resolve_subtitle_format(
+    subtitle_format: str | None,
+    subtitle_mode: str,
+) -> tuple[str, str]:
+    """Resolve a subtitle_format setting string to (file_extension, alignment).
+
+    Handles the PyQt convention where format strings encode alignment
+    (e.g. ``"ass_centered_narrow"`` → extension ``"ass"``, alignment
+    ``"center_narrow"``).
+
+    Also enforces that ``"Sentence + Highlighting"`` mode requires ASS.
+
+    Returns:
+        Tuple of (file_extension, alignment) suitable for
+        :func:`create_subtitle_writer`.
+    """
+    fmt = (subtitle_format or "srt").lower()
+
+    if subtitle_mode == "Sentence + Highlighting" and fmt == "srt":
+        fmt = "ass"
+
+    if "ass" in fmt:
+        extension = "ass"
+        if "centered_narrow" in fmt:
+            alignment = "center_narrow"
+        elif "centered" in fmt:
+            alignment = "center"
+        elif "narrow" in fmt:
+            alignment = "narrow"
+        else:
+            alignment = "left"
+    else:
+        extension = fmt if fmt in ("srt", "vtt") else "srt"
+        alignment = "left"
+
+    return extension, alignment
+
+
+def make_subtitle_writer(
+    audio_path: Path,
+    subtitle_format: str | None,
+    subtitle_mode: str,
+    max_words: int = 50,
+) -> SubtitleWriter | None:
+    """Convenience: resolve format and create a writer, or return None if disabled.
+
+    Returns ``None`` when ``subtitle_mode`` is ``"Disabled"`` or the
+    format is unsupported.
+    """
+    if subtitle_mode == "Disabled":
+        return None
+
+    extension, alignment = resolve_subtitle_format(subtitle_format, subtitle_mode)
+    try:
+        return create_subtitle_writer(
+            audio_path.with_suffix(f".{extension}"),
+            extension,
+            subtitle_mode,
+            alignment=alignment,
+            max_words=max_words,
+        )
+    except (ValueError, KeyError):
+        return None
+
+
 __all__ = [
     "SubtitleFormat",
     "SubtitleMode",
@@ -300,4 +365,6 @@ __all__ = [
     "VttWriter",
     "AssWriter",
     "create_subtitle_writer",
+    "resolve_subtitle_format",
+    "make_subtitle_writer",
 ]

abogen/pyqt/conversion.py

@@ -21,7 +21,7 @@ from abogen.constants import (
     SUPPORTED_SOUND_FORMATS,
     SUPPORTED_SUBTITLE_FORMATS,
 )
-from abogen.infrastructure.subtitle_writer import create_subtitle_writer
+from abogen.infrastructure.subtitle_writer import make_subtitle_writer, resolve_subtitle_format
 from abogen.domain.split_pattern import get_split_pattern
 from abogen.domain.subtitle_processor import (
     parse_subtitle_file,
@@ -65,17 +65,6 @@ import threading  # for efficient waiting
 import subprocess
 
 
-def _subtitle_alignment_from_format(subtitle_format: str) -> str:
-    """Map PyQt subtitle format string to SubtitleAlignment value."""
-    if subtitle_format in ("ass_centered_wide", "ass_centered_narrow"):
-        if subtitle_format == "ass_centered_narrow":
-            return "center_narrow"
-        return "center"
-    if subtitle_format in ("ass_narrow",):
-        return "narrow"
-    return "left"
-
-
 
 # Configuration constants
 _USER_RESPONSE_TIMEOUT = (
@@ -743,18 +732,17 @@ class ConversionThread(QThread):
                 merged_subtitle_path = None
                 if self.subtitle_mode != "Disabled":
                     subtitle_format = getattr(self, "subtitle_format", "srt")
-                    file_extension = "ass" if "ass" in subtitle_format else "srt"
+                    extension, _ = resolve_subtitle_format(subtitle_format, self.subtitle_mode)
                     merged_subtitle_path = (
-                        os.path.splitext(merged_out_path)[0] + f".{file_extension}"
+                        os.path.splitext(merged_out_path)[0] + f".{extension}"
                     )
-                    alignment = _subtitle_alignment_from_format(subtitle_format)
+                    merged_subtitle_writer = make_subtitle_writer(
-                    merged_subtitle_writer = create_subtitle_writer(
                         Path(merged_subtitle_path),
-                        file_extension,
+                        subtitle_format,
                         self.subtitle_mode,
-                        alignment=alignment,
                         max_words=self.max_subtitle_words,
                     )
+                    if merged_subtitle_writer:
                         merged_subtitle_writer.open()
                 else:
                     merged_subtitle_path = None
@@ -817,18 +805,17 @@ class ConversionThread(QThread):
                         continue
                     if self.subtitle_mode != "Disabled":
                         subtitle_format = getattr(self, "subtitle_format", "srt")
-                        file_extension = "ass" if "ass" in subtitle_format else "srt"
+                        extension, _ = resolve_subtitle_format(subtitle_format, self.subtitle_mode)
                         chapter_subtitle_path = os.path.join(
-                            chapters_out_dir, f"{chapter_filename}.{file_extension}"
+                            chapters_out_dir, f"{chapter_filename}.{extension}"
                         )
-                        alignment = _subtitle_alignment_from_format(subtitle_format)
+                        chapter_subtitle_writer = make_subtitle_writer(
-                        chapter_subtitle_writer = create_subtitle_writer(
                             Path(chapter_subtitle_path),
-                            file_extension,
+                            subtitle_format,
                             self.subtitle_mode,
-                            alignment=alignment,
                             max_words=self.max_subtitle_words,
                         )
+                        if chapter_subtitle_writer:
                             chapter_subtitle_writer.open()
                     else:
                         chapter_subtitle_writer = None
@@ -1189,16 +1176,15 @@ class ConversionThread(QThread):
             subtitle_writer = None
             subtitle_path = None
             subtitle_format = getattr(self, "subtitle_format", "srt")
-            file_extension = "ass" if "ass" in subtitle_format else "srt"
+            extension, _ = resolve_subtitle_format(subtitle_format, self.subtitle_mode)
-            subtitle_path = f"{base_filepath_no_ext}.{file_extension}"
+            subtitle_path = f"{base_filepath_no_ext}.{extension}"
-            alignment_str = _subtitle_alignment_from_format(subtitle_format)
+            subtitle_writer = make_subtitle_writer(
-            subtitle_writer = create_subtitle_writer(
                 Path(subtitle_path),
-                file_extension,
+                subtitle_format,
                 self.subtitle_mode,
-                alignment=alignment_str,
                 max_words=self.max_subtitle_words,
             )
+            if subtitle_writer:
                 subtitle_writer.open()
 
             # Load voice

abogen/webui/conversion_runner.py

@@ -33,7 +33,7 @@ from abogen.utils import (
 )
 from abogen.voice_profiles import load_profiles, normalize_profile_entry
 from abogen.llm_client import LLMClientError
-from abogen.infrastructure.subtitle_writer import create_subtitle_writer
+from abogen.infrastructure.subtitle_writer import make_subtitle_writer
 from abogen.domain.chapter_titles import (
     simplify_heading_text as _simplify_heading_text,
     headings_equivalent as _headings_equivalent,
@@ -338,7 +338,18 @@ def run_conversion_job(job: Job) -> None:
                     cancel_check=lambda: job.cancel_requested,
                 )
             )
-            subtitle_writer = _create_subtitle_writer(job, audio_path)
+            subtitle_writer = make_subtitle_writer(
+                audio_path,
+                job.subtitle_format,
+                job.subtitle_mode or "Line",
+                max_words=job.max_subtitle_words,
+            )
+            if subtitle_writer is None and job.subtitle_mode != "Disabled":
+                fmt = (job.subtitle_format or "srt").lower()
+                if job.subtitle_mode == "Sentence + Highlighting" and fmt == "srt":
+                    job.add_log("Highlighting requires ASS subtitles. Switching format.", level="warning")
+                else:
+                    job.add_log(f"Unsupported subtitle format '{job.subtitle_format}'. Skipping.", level="warning")
             job.result.audio_path = audio_path
             if subtitle_writer:
                 job.result.subtitle_paths.append(subtitle_writer.path)
@@ -1063,25 +1074,6 @@ def _resolve_voice(pipeline, voice_spec: str, use_gpu: bool):
     return voice_spec
 
 
-def _create_subtitle_writer(job: Job, audio_path: Path):
-    if job.subtitle_mode == "Disabled":
-        return None
-
-    fmt = (job.subtitle_format or "srt").lower()
-    if job.subtitle_mode == "Sentence + Highlighting" and fmt == "srt":
-        job.add_log("Highlighting requires ASS subtitles. Switching format.", level="warning")
-        fmt = "ass"
-
-    try:
-        return create_subtitle_writer(
-            audio_path.with_suffix(f".{fmt}"),
-            fmt,
-            job.subtitle_mode or "Line",
-        )
-    except (ValueError, KeyError):
-        job.add_log(f"Unsupported subtitle format '{job.subtitle_format}'. Skipping.", level="warning")
-        return None
-
 
 def _make_canceller(job: Job) -> Callable[[], None]:
     def _cancel() -> None: