[DOCS] Add ffmpeg path docs (#487)

src/ytdl_sub/config/config_validator.py

@@ -6,6 +6,7 @@
 
 from ytdl_sub.prebuilt_presets import PREBUILT_PRESETS
 from ytdl_sub.utils.system import IS_WINDOWS
+from ytdl_sub.validators.file_path_validators import ExistingFileValidator
 from ytdl_sub.validators.strict_dict_validator import StrictDictValidator
 from ytdl_sub.validators.validators import LiteralDictValidator
 from ytdl_sub.validators.validators import StringValidator
@@ -39,12 +40,11 @@ def __init__(self, name: str, value: Any):
         self._lock_directory = self._validate_key(
             key="lock_directory", validator=StringValidator, default=_DEFAULT_LOCK_DIRECTORY
         )
-        # TODO: Validate these exist
         self._ffmpeg_path = self._validate_key(
-            key="ffmpeg_path", validator=StringValidator, default=_DEFAULT_FFMPEG_PATH
+            key="ffmpeg_path", validator=ExistingFileValidator, default=_DEFAULT_FFMPEG_PATH
         )
         self._ffprobe_path = self._validate_key(
-            key="ffprobe_path", validator=StringValidator, default=_DEFAULT_FFPROBE_PATH
+            key="ffprobe_path", validator=ExistingFileValidator, default=_DEFAULT_FFPROBE_PATH
         )
 
     @property
@@ -102,14 +102,16 @@ def lock_directory(self) -> str:
     @property
     def ffmpeg_path(self) -> str:
         """
-        TODO: Fill out!
+        Optional. Path to ffmpeg executable. Defaults to ``/usr/bin/ffmpeg`` for Linux, and
+        ``ffmpeg.exe`` for Windows (in the same directory as ytdl-sub).
         """
         return self._ffmpeg_path.value
 
     @property
     def ffprobe_path(self) -> str:
         """
-        TODO: Fill out!
+        Optional. Path to ffprobe executable. Defaults to ``/usr/bin/ffprobe`` for Linux, and
+        ``ffprobe.exe`` for Windows (in the same directory as ytdl-sub).
         """
         return self._ffprobe_path.value
 

src/ytdl_sub/validators/file_path_validators.py

@@ -1,9 +1,27 @@
 import os
 from pathlib import Path
+from typing import Any
 from typing import Dict
 
 from ytdl_sub.validators.string_formatter_validators import OverridesStringFormatterValidator
 from ytdl_sub.validators.string_formatter_validators import StringFormatterValidator
+from ytdl_sub.validators.validators import StringValidator
+
+
+class ExistingFileValidator(StringValidator):
+    _expected_value_type_name = "file"
+
+    def __init__(self, name: str, value: Any):
+        super().__init__(name, value)
+        if not os.path.isfile(self._value):
+            raise self._validation_exception(
+                f"Expects an existing file, but '{self.value}' is not a file"
+            )
+
+    @property
+    def value(self) -> str:
+        """Turn into a Path, then a string, to get correct directory separators"""
+        return str(Path(self._value))
 
 
 class StringFormatterFilePathValidator(StringFormatterValidator):