Optional parameter to check if the encoder exists

If `must_exists` is `False` when intializing `EncoderFFmpeg()`, skip
skip checking whether the FFmpeg binary exists.

Fixes #722.

spotdl/encode/encode_base.py

@@ -37,12 +37,12 @@ class EncoderBase(ABC):
     """
 
     @abstractmethod
-    def __init__(self, encoder_path, loglevel, additional_arguments=[]):
+    def __init__(self, encoder_path, must_exist, loglevel, additional_arguments=[]):
         """
         This method must make sure whether specified encoder
         is available under PATH.
         """
-        if shutil.which(encoder_path) is None:
+        if must_exist and shutil.which(encoder_path) is None:
             raise EncoderNotFoundError(
                 "{} executable does not exist or was not found in PATH.".format(
                     encoder_path

spotdl/encode/encoders/ffmpeg.py

@@ -27,11 +27,11 @@ RULES = {
 
 
 class EncoderFFmpeg(EncoderBase):
-    def __init__(self, encoder_path="ffmpeg"):
+    def __init__(self, encoder_path="ffmpeg", must_exist=True):
         _loglevel = "-hide_banner -nostats -v panic"
         _additional_arguments = ["-b:a", "192k", "-vn"]
         try:
-            super().__init__(encoder_path, _loglevel, _additional_arguments)
+            super().__init__(encoder_path, must_exist, _loglevel, _additional_arguments)
         except EncoderNotFoundError as e:
             raise FFmpegNotFoundError(e.args[0])
         self._rules = RULES

spotdl/track.py

@@ -44,9 +44,9 @@ class Track:
         return progress_bar
 
     def download_while_re_encoding(self, stream, target_path, target_encoding=None,
-                                   encoder=EncoderFFmpeg, show_progress=True):
+                                   encoder=EncoderFFmpeg(must_exist=False), show_progress=True):
         total_chunks = self.calculate_total_chunks(stream["filesize"])
-        process = encoder().re_encode_from_stdin(
+        process = encoder.re_encode_from_stdin(
             stream["encoding"],
             target_path,
             target_encoding=target_encoding
@@ -80,10 +80,10 @@ class Track:
                 writer(response, progress_bar, file_io)
 
     def re_encode(self, input_path, target_path, target_encoding=None,
-                  encoder=EncoderFFmpeg, show_progress=True):
+                  encoder=EncoderFFmpeg(must_exist=False), show_progress=True):
         stream = self.metadata["streams"].getbest()
         total_chunks = self.calculate_total_chunks(stream["filesize"])
-        process = encoder().re_encode_from_stdin(
+        process = encoder.re_encode_from_stdin(
             stream["encoding"],
             target_path,
             target_encoding=target_encoding