More helpful error message on unrecognized input file format

See peterjc/thapbi-pict#627

src/cutadapt/files.py

@@ -297,7 +297,7 @@ def has_qualities(self) -> bool:
 
 
 # TODO copied and adjusted from dnaio; upstream this
-def detect_file_format(file: BinaryIO) -> Optional[FileFormat]:
+def detect_file_format(file: BinaryIO) -> FileFormat:
     if file.seekable():
         original_position = file.tell()
         magic = file.read(4)
@@ -313,4 +313,7 @@ def detect_file_format(file: BinaryIO) -> Optional[FileFormat]:
         return FileFormat.FASTA
     elif magic == b"BAM\1":
         return FileFormat.BAM
-    return None
+    raise dnaio.exceptions.UnknownFileFormat(
+        f"Input file format not recognized. The file starts with {magic!r}, "
+        "but files in supported formats start with '>' (FASTA), '@' (FASTQ) or 'BAM'"
+    )

src/cutadapt/runners.py

@@ -101,9 +101,8 @@ def run(self):
                     self._file_format_connection.send((e, traceback.format_exc()))
                     raise
                 self._file_format_connection.send(file_format)
-                if file_format is not None:
-                    for index, chunks in enumerate(self._read_chunks(*files)):
-                        self.send_to_worker(index, *chunks)
+                for index, chunks in enumerate(self._read_chunks(*files)):
+                    self.send_to_worker(index, *chunks)
             self.shutdown()
         except Exception as e:
             # TODO better send this to a common "something went wrong" Queue
@@ -321,12 +320,7 @@ def __init__(
         )
         self._reader_process.daemon = True
         self._reader_process.start()
-        file_format: Optional[FileFormat] = self._try_receive(file_format_connection_r)
-        if file_format is None:
-            raise dnaio.exceptions.UnknownFileFormat(
-                f"Format of input file '{self._inpaths.paths[0]}' not recognized."
-            )
-        self._input_file_format = file_format
+        self._input_file_format = self._try_receive(file_format_connection_r)
 
     def _start_workers(
         self, pipeline, proxy_files
@@ -434,12 +428,7 @@ def close(self):
         self._infiles.close()
 
     def input_file_format(self) -> FileFormat:
-        detected = detect_file_format(self._infiles._files[0])
-        if detected is None:
-            raise dnaio.exceptions.UnknownFileFormat(
-                f"Format of input file '{self._infiles._files[0].name}' not recognized."
-            )
-        return detected
+        return detect_file_format(self._infiles._files[0])
 
 
 def make_runner(