Core: Support append mode for write API (#63)

tosfs/core.py

@@ -1775,6 +1775,17 @@ def __init__(
         self.append_block = False
         self.buffer: Optional[io.BytesIO] = io.BytesIO()
 
+        if "a" in mode and fs.exists(path):
+            head = self.fs.tos_client.head_object(bucket, key)
+            loc = head.content_length
+
+            if loc < 5 * 2**20:
+                # existing file too small for multi-upload: download
+                self.write(self.fs.cat(self.path))
+            else:
+                self.append_block = True
+            self.loc = loc
+
     def _initiate_upload(self) -> None:
         """Create remote file/upload."""
         if self.autocommit and not self.append_block and self.tell() < self.blocksize:

tosfs/tests/test_tosfs.py

@@ -712,6 +712,22 @@ def test_file_write_mpu(
     )
 
 
+def test_file_write_append(
+    tosfs: TosFileSystem, bucket: str, temporary_workspace: str
+) -> None:
+    file_name = random_str()
+    content = "hello world"
+    with tosfs.open(f"{bucket}/{temporary_workspace}/{file_name}", "w") as f:
+        f.write(content)
+    with tosfs.open(f"{bucket}/{temporary_workspace}/{file_name}", "a") as f:
+        f.write(content)
+    assert tosfs.info(f"{bucket}/{temporary_workspace}/{file_name}")["size"] == 2 * len(
+        content
+    )
+    with tosfs.open(f"{bucket}/{temporary_workspace}/{file_name}", "r") as f:
+        assert f.read() == content + content
+
+
 def test_file_read(tosfs: TosFileSystem, bucket: str, temporary_workspace: str) -> None:
     file_name = random_str()
     content = "hello world"