Add seek() method to ZstdCompressionReader

c-ext/compressionreader.c

@@ -83,6 +83,11 @@ static PyObject *compressionreader_seekable(ZstdCompressionReader *self) {
     Py_RETURN_FALSE;
 }
 
+static PyObject *compressionreader_seek(PyObject *self, PyObject *args) {
+    PyErr_SetString(PyExc_OSError, "stream is not seekable");
+    return NULL;
+}
+
 static PyObject *compressionreader_readline(PyObject *self, PyObject *args) {
     set_io_unsupported_operation();
     return NULL;
@@ -760,6 +765,8 @@ static PyMethodDef compressionreader_methods[] = {
      PyDoc_STR("Not implemented")},
     {"readlines", (PyCFunction)compressionreader_readlines, METH_VARARGS,
      PyDoc_STR("Not implemented")},
+    {"seek", (PyCFunction)compressionreader_seek, METH_VARARGS,
+     PyDoc_STR("Raises OSError")},
     {"seekable", (PyCFunction)compressionreader_seekable, METH_NOARGS,
      PyDoc_STR("Returns False")},
     {"tell", (PyCFunction)compressionreader_tell, METH_NOARGS,

rust-ext/src/compression_reader.rs

@@ -169,6 +169,10 @@ impl ZstdCompressionReader {
         false
     }
 
+    fn seek(&self, _data: &Bound<'_, PyAny>) -> PyResult<()> {
+        Err(PyOSError::new_err("stream is not seekable"))
+    }
+
     fn readline(&self, py: Python) -> PyResult<()> {
         let io = py.import("io")?;
         let exc = io.getattr("UnsupportedOperation")?;

tests/test_compressor_stream_reader.py

@@ -54,6 +54,20 @@ def test_not_implemented(self):
             with self.assertRaises(OSError):
                 reader.write(b"foo")
 
+    def test_seek(self):
+        cctx = zstd.ZstdCompressor()
+
+        with cctx.stream_reader(b"foo" * 60) as reader:
+            with self.assertRaises(OSError):
+                reader.seek(0)
+
+            reader.read(1)
+            with self.assertRaises(OSError):
+                reader.seek(0)
+
+        with self.assertRaises(OSError):
+            reader.seek(0)
+
     def test_constant_methods(self):
         cctx = zstd.ZstdCompressor()
 

zstandard/__init__.pyi

@@ -216,6 +216,7 @@ class ZstdCompressionReader(BinaryIO):
     def readable(self) -> bool: ...
     def writable(self) -> bool: ...
     def seekable(self) -> bool: ...
+    def seek(self, pos: int, whence: int = ...) -> int: ...
     def readline(self, limit: int = ...) -> bytes: ...
     def readlines(self, hint: int = ...) -> List[bytes]: ...
     def write(self, data: ByteString): ...

zstandard/backend_cffi.py

@@ -1427,6 +1427,9 @@ def writable(self):
     def seekable(self):
         return False
 
+    def seek(self, offset, whence=None):
+        raise io.UnsupportedOperation()
+
     def readline(self):
         raise io.UnsupportedOperation()