Defined file transferer abstract base class

This will allow for different file transfer classes uploading to different
services.

blender_asset_tracer/pack/transfer.py

@@ -0,0 +1,44 @@
+import abc
+import pathlib
+import typing
+
+
+class FileTransferError(IOError):
+    """Raised when one or more files could not be transferred."""
+
+    def __init__(self, message, files_remaining: typing.List[pathlib.Path]) -> None:
+        super().__init__(message)
+        self.files_remaining = files_remaining
+
+
+class FileTransferer(metaclass=abc.ABCMeta):
+    """Interface for file transfer classes."""
+
+    @abc.abstractmethod
+    def start(self):
+        """Starts the file transfer thread/process.
+
+        This could spin up a separate thread to perform the actual file
+        transfer. After start() is called, implementations should still accept
+        calls to the queue_xxx() methods. In other words, this is not to be
+        used as queue-and-then-start, but as start-and-then-queue.
+        """
+
+    @abc.abstractmethod
+    def queue_copy(self, src: pathlib.Path, dst: pathlib.Path) -> None:
+        """Queue a copy action from 'src' to 'dst'."""
+
+    @abc.abstractmethod
+    def queue_move(self, src: pathlib.Path, dst: pathlib.Path) -> None:
+        """Queue a move action from 'src' to 'dst'."""
+
+    @abc.abstractmethod
+    def done_and_join(self) -> None:
+        """Indicate all files have been queued, and wait until done.
+
+        After this function has been called, the queue_xxx() methods should not
+        be called any more.
+
+        :raises FileTransferError: if there was an error transferring one or
+            more files.
+        """