From 55d039e3429c2be31c432127b57eda61ac0a8f43 Mon Sep 17 00:00:00 2001
From: Sean Gillies <sean.gillies@gmail.com>
Date: Tue, 1 Feb 2022 09:47:05 -0700
Subject: [PATCH] Raise OSError on attempt to append to dataset stored in a
 file

Resolves #1027
---
 CHANGES.txt              | 2 ++
 fiona/__init__.py        | 7 ++++++-
 tests/test_memoryfile.py | 6 ++++++
 3 files changed, 14 insertions(+), 1 deletion(-)

diff --git a/CHANGES.txt b/CHANGES.txt
index d18a31321..796f8be89 100644
--- a/CHANGES.txt
+++ b/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:
diff --git a/fiona/__init__.py b/fiona/__init__.py
index 56b6db5c8..41648c90a 100644
--- a/fiona/__init__.py
+++ b/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):
diff --git a/tests/test_memoryfile.py b/tests/test_memoryfile.py
index fbdbbf83a..ca6490e05 100644
--- a/tests/test_memoryfile.py
+++ b/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'