Raise OSError on attempt to append to dataset stored in a file

Resolves #1027

CHANGES.txt

@@ -8,6 +8,8 @@ All issue numbers are relative to https://github.com/Toblerity/Fiona/issues.
 
 Changes:
 
+- OSError is raised on attempts to open a dataset in a Python file object in
+  "a" mode (see #1027).
 - Upgrade attrs, cython, etc to open up Python 3.10 support (#1049).
 
 Bug fixes:

fiona/__init__.py

@@ -199,8 +199,8 @@ def open(fp, mode='r', driver=None, schema=None, crs=None, encoding=None,
     Returns
     -------
     Collection
-    """
 
+    """
     if mode == 'r' and hasattr(fp, 'read'):
 
         @contextmanager
@@ -243,6 +243,11 @@ def fp_writer(fp):
 
         return fp_writer(fp)
 
+    elif mode == "a" and hasattr(fp, "write"):
+        raise OSError(
+            "Append mode is not supported for datasets in a Python file object."
+        )
+
     else:
         # If a pathlib.Path instance is given, convert it to a string path.
         if isinstance(fp, Path):

tests/test_memoryfile.py

@@ -160,6 +160,12 @@ def test_write_bytesio(profile_first_coutwildrnp_shp):
             assert len(col) == 1
 
 
+def test_append_bytesio_exception(profile_first_coutwildrnp_shp):
+    """Append is not supported, see #1027."""
+    with pytest.raises(OSError):
+        fiona.open(BytesIO(data_coutwildrnp_json), "a")
+
+
 def test_mapinfo_raises():
     """Reported to be a crasher in #937"""
     driver = 'MapInfo File'