Remove warning about resetting file pointer to zero

File pointer is reset to zero after reading the magic byte number anyway, so should be ok not to warn about this.

Author: weiji14

xarray/core/utils.py

@@ -655,12 +655,6 @@ def read_magic_number_from_file(filename_or_obj, count=8) -> bytes:
     elif isinstance(filename_or_obj, io.IOBase):
         if filename_or_obj.tell() != 0:
             filename_or_obj.seek(0)
-            warnings.warn(
-                "cannot guess the engine, "
-                "file-like object read/write pointer not at the start of the file, "
-                "so resetting file pointer to zero. If this does not work, "
-                "please close and reopen, or use a context manager"
-            )
         magic_number = filename_or_obj.read(count)
         filename_or_obj.seek(0)
     else:

xarray/tests/test_backends.py

@@ -3159,13 +3159,12 @@ def test_open_badbytes(self) -> None:
     def test_open_twice(self) -> None:
         expected = create_test_data()
         expected.attrs["foo"] = "bar"
-        with pytest.warns(match=r"read/write pointer not at the start"):
-            with create_tmp_file() as tmp_file:
-                expected.to_netcdf(tmp_file, engine="h5netcdf")
-                with open(tmp_file, "rb") as f:
+        with create_tmp_file() as tmp_file:
+            expected.to_netcdf(tmp_file, engine="h5netcdf")
+            with open(tmp_file, "rb") as f:
+                with open_dataset(f, engine="h5netcdf"):
                     with open_dataset(f, engine="h5netcdf"):
-                        with open_dataset(f, engine="h5netcdf"):
-                            pass
+                        pass
 
     @requires_scipy
     def test_open_fileobj(self) -> None: