Create missing ZIP archives in append mode

docs/source/changelog.rst

@@ -10,6 +10,8 @@ Fixes
   not exist, as ``rm_file`` and the other filesystems do, so deleting a missing
   key from a memory-backed mapper raises ``KeyError``
 
+- Create missing ZIP archives in append mode without truncating existing archives
+
 - Allow filesystem implementations to assign ``protocol`` per instance
 
 - Avoid mutating live ``BlockCache`` and ``BackgroundBlockCache`` instances

fsspec/implementations/tests/test_zip.py

@@ -124,6 +124,25 @@ def test_zip_glob_star(m):
     assert len(outfiles) == 1
 
 
+@pytest.mark.parametrize("backend", ["memory", "local"])
+def test_append_creates_archive(m, tmp_path, backend):
+    path = "memory://new.zip" if backend == "memory" else tmp_path / "new.zip"
+    for name, content in [("first", b"original"), ("second", b"appended")]:
+        fs = ZipFileSystem(fo=path, mode="a")
+        try:
+            fs.pipe_file(name, content)
+        finally:
+            fs.close()
+
+    fs = ZipFileSystem(fo=path)
+    try:
+        assert fs.cat("first") == b"original"
+        assert fs.cat("second") == b"appended"
+        assert fs.find("") == ["first", "second"]
+    finally:
+        fs.close()
+
+
 def test_append(m, tmpdir):
     fs = fsspec.filesystem("zip", fo="memory://out.zip", mode="w")
     with fs.open("afile", "wb") as f:

fsspec/implementations/zip.py

@@ -32,7 +32,8 @@ def __init__(
         Parameters
         ----------
         fo: str or file-like
-            Contains ZIP, and must exist. If a str, will fetch file using
+            Contains ZIP. In append mode, a missing archive is created.
+            If a str, will fetch file using
             :meth:`~fsspec.open_files`, which must return one file exactly.
         mode: str
             Accept: "r", "w", "a"
@@ -59,7 +60,13 @@ def __init__(
             )
         self.force_zip_64 = allowZip64
         self.of = fo
-        self.fo = fo.__enter__()  # the whole instance is a context
+        try:
+            self.fo = fo.__enter__()  # the whole instance is a context
+        except FileNotFoundError:
+            if mode != "a" or not isinstance(fo, fsspec.core.OpenFile):
+                raise
+            fo.mode = "w+b"
+            self.fo = fo.__enter__()
         self.zip = zipfile.ZipFile(
             self.fo,
             mode=mode,