# Buggy code
def open(
    cls,
    filename,
    mode="r",
    format=None,
    group=None,
    lock=None,
    autoclose=False,
    invalid_netcdf=None,
    phony_dims=None,
):
    import h5netcdf

    if format not in [None, "NETCDF4"]:
        raise ValueError("invalid format for h5netcdf backend")

    kwargs = {"invalid_netcdf": invalid_netcdf}
    if phony_dims is not None:
        if LooseVersion(h5netcdf.__version__) >= LooseVersion("0.8.0"):
            kwargs["phony_dims"] = phony_dims
        else:
            raise ValueError(
                "h5netcdf backend keyword argument 'phony_dims' needs "
                "h5netcdf >= 0.8.0."
            )

    if lock is None:
        if mode == "r":
            lock = HDF5_LOCK
        else:
            lock = combine_locks([HDF5_LOCK, get_write_lock(filename)])

    manager = CachingFileManager(h5netcdf.File, filename, mode=mode, kwargs=kwargs)
    return cls(manager, group=group, mode=mode, lock=lock, autoclose=autoclose)


# Traceback (TypeError)
xr.open_dataset('netCDF4.nc', engine='scipy')
---------------------------------------------------------------------------
KeyError                                  Traceback (most recent call last)
[...]
TypeError: Error: netCDF4.nc is not a valid NetCDF 3 file
If this is a NetCDF4 file, you may need to install the
netcdf4 library, e.g.,

$ pip install netcdf4

fp = open('netCDF4.nc')
xr.open_dataset(fp, engine='scipy')
<xarray.Dataset>
Dimensions:       (line: 41587, pixel: 17751)
Coordinates:
* line          (line) int64 0 1 2 3 4 5 ... 41582 41583 41584 41585 41586
* pixel         (pixel) int64 0 1 2 3 4 5 ... 17746 17747 17748 17749 17750
Data variables:
phase         (line, pixel) float32 ...
Attributes:
Conventions:             CF-1.7
[...]

# Fixed code
def open(
    cls,
    filename,
    mode="r",
    format=None,
    group=None,
    lock=None,
    autoclose=False,
    invalid_netcdf=None,
    phony_dims=None,
):
    import h5netcdf

    if isinstance(filename, bytes):
        raise ValueError(
            "can't open netCDF4/HDF5 as bytes try passing a path or file-like object"
        )
    elif hasattr(filename, "tell"):
        if filename.tell() != 0:
            raise ValueError(
                "file-like object read/write pointer not at zero "
                "please close and reopen, or use a context manager"
            )
        else:
            magic_number = filename.read(8)
            filename.seek(0)
            if not magic_number.startswith(b"\211HDF\r\n\032\n"):
                raise ValueError(
                    f"{magic_number} is not the signature of a valid netCDF file"
                )

    if format not in [None, "NETCDF4"]:
        raise ValueError("invalid format for h5netcdf backend")

    kwargs = {"invalid_netcdf": invalid_netcdf}
    if phony_dims is not None:
        if LooseVersion(h5netcdf.__version__) >= LooseVersion("0.8.0"):
            kwargs["phony_dims"] = phony_dims
        else:
            raise ValueError(
                "h5netcdf backend keyword argument 'phony_dims' needs "
                "h5netcdf >= 0.8.0."
            )

    if lock is None:
        if mode == "r":
            lock = HDF5_LOCK
        else:
            lock = combine_locks([HDF5_LOCK, get_write_lock(filename)])

    manager = CachingFileManager(h5netcdf.File, filename, mode=mode, kwargs=kwargs)
    return cls(manager, group=group, mode=mode, lock=lock, autoclose=autoclose)