fix the write in double bug

Example.ipynb

@@ -14,7 +14,6 @@
    "outputs": [],
    "source": [
     "import dropboxdrivefs as dbx\n",
-    "from fklab.io.cloud import dropboxAPI\n",
     "import logging"
    ]
   },
@@ -24,8 +23,7 @@
    "metadata": {},
    "outputs": [],
    "source": [
-    "db = dropboxAPI()\n",
-    "fs = dbx.DropboxDriveFileSystem(token=db.load_token())"
+    "fs = dbx.DropboxDriveFileSystem(token=\"****************\")"
    ]
   },
   {
@@ -129,6 +127,23 @@
     "    f.write('hello')"
    ]
   },
+  {
+   "cell_type": "markdown",
+   "metadata": {},
+   "source": [
+    "Be careful, the mode \"append\" does not append the existing file. It creates a new file with the text wrotten by adding (2) in the path name. If this new file is identical with the existing file, nothing is uploaded. "
+   ]
+  },
+  {
+   "cell_type": "code",
+   "execution_count": null,
+   "metadata": {},
+   "outputs": [],
+   "source": [
+    "with fs.open(\"/test_dropbox/test.txt\", mode='a') as f:\n",
+    "    f.write('hello')"
+   ]
+  },
   {
    "cell_type": "markdown",
    "metadata": {},

dropboxdrivefs/core.py

@@ -155,7 +155,7 @@ def __init__(
         path : str
             file path to inspect in dropbox
         mode: str
-            Normal file modes.'rb' or 'wb'
+            Normal file modes.'rb', 'wb' or 'ab'
         block_size: int or None
             The amount of read-ahead to do, in bytes. Default is 5MB, or the value
             configured for the FileSystem creating this file
@@ -171,21 +171,35 @@ def __init__(
     # return self.httpfile.read(length=length)
 
     def _upload_chunk(self, final=False):
-        self.cursor.offset += self.buffer.seek(0, 2)
         if final:
+
             self.dbx.files_upload_session_finish(
                 self.buffer.getvalue(), self.cursor, self.commit
             )
         else:
-            self.dbx.files_upload_session_append(
+
+            self.dbx.files_upload_session_append_v2(
                 self.buffer.getvalue(), self.cursor.session_id, self.cursor.offset
             )
 
+        self.cursor.offset += self.buffer.seek(0, 2)
+
     def _initiate_upload(self):
         """ Initiate the upload session
         """
-        session = self.dbx.files_upload_session_start(self.buffer.getvalue())
-        self.commit = dropbox.files.CommitInfo(path=self.path)
+
+        session = self.dbx.files_upload_session_start(b"")
+
+        if "w" in self.mode:
+            self.commit = dropbox.files.CommitInfo(
+                path=self.path, mode=dropbox.files.WriteMode("overwrite", None)
+            )
+        elif "a" in self.mode:
+            self.commit = dropbox.files.CommitInfo(
+                path=self.path, mode=dropbox.files.WriteMode("add")
+            )
+
         self.cursor = dropbox.files.UploadSessionCursor(
             session_id=session.session_id, offset=self.offset
         )
+        print(self.offset)