Defined file transferer abstract base class

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

blender_asset_tracer/cli/pack.py

@@ -4,6 +4,7 @@ import pathlib
 import sys
 import typing
 
+import blender_asset_tracer.pack.transfer
 from blender_asset_tracer import pack
 
 log = logging.getLogger(__name__)
@@ -34,7 +35,7 @@ def cli_pack(args):
 
     try:
         packer.execute()
-    except pack.queued_copy.FileCopyError as ex:
+    except blender_asset_tracer.pack.transfer.FileTransferError as ex:
         log.error("%d files couldn't be copied, starting with %s",
                   len(ex.files_remaining), ex.files_remaining[0])
         raise SystemExit(1)

blender_asset_tracer/pack/queued_copy.py

@@ -6,23 +6,17 @@ import queue
 import shutil
 import typing
 
+from . import transfer
+
 log = logging.getLogger(__name__)
 
 
-class FileCopyError(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 Action(enum.Enum):
     COPY = 1
     MOVE = 2
 
 
-class FileCopier(threading.Thread):
+class FileCopier(threading.Thread, transfer.FileTransferer):
     """Copies or moves files in source directory order."""
 
     def __init__(self, *args, **kwargs) -> None:
@@ -62,8 +56,9 @@ class FileCopier(threading.Thread):
                 src, dst = self.queue.get_nowait()
                 files_remaining.append(src)
             assert files_remaining
-            raise FileCopyError("%d files couldn't be transferred" % len(files_remaining),
-                                files_remaining)
+            raise transfer.FileTransferError(
+                "%d files couldn't be transferred" % len(files_remaining),
+                files_remaining)
 
     def run(self):
         files_transferred = 0
@@ -98,8 +93,8 @@ class FileCopier(threading.Thread):
                 dst.parent.mkdir(parents=True, exist_ok=True)
 
                 # TODO(Sybren): when we target Py 3.6+, remove the str() calls.
-                transfer = transfer_funcs[act]
-                transfer(str(src), str(dst))
+                tfunc = transfer_funcs[act]
+                tfunc(str(src), str(dst))
 
                 files_transferred += 1
             except Exception:

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.
+        """