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Solve the following problem using Python, implementing the functions described below, one line at a time
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Description:
 def shift(self, shifts=None, fill_value=dtypes.NA, **shifts_kwargs):
 """Shift this dataset by an offset along one or more dimensions. Only data variables are moved; coordinates stay in place. This is consistent with the behavior of ``shift`` in pandas. Parameters shifts : Mapping with the form of {dim: offset} Integer offset to shift along each of the given dimensions. Positive offsets shift to the right; negative offsets shift to the left. fill_value: scalar, optional Value to use for newly missing values **shifts_kwargs: The keyword arguments form of ``shifts``. One of shifts or shifts_kwarg must be provided. Returns ------- shifted : Dataset Dataset with the same coordinates and attributes but shifted data variables. See also -------- roll Examples -------- <xarray.Dataset> Dimensions: (x: 5) Coordinates: * x (x) int64 0 1 2 3 4 Data variables: foo (x) object nan nan 'a' 'b' 'c' """

        shifts = either_dict_or_kwargs(shifts, shifts_kwargs, 'shift')
        invalid = [k for k in shifts if k not in self.dims]
        if invalid:
            raise ValueError("dimensions %r do not exist" % invalid)

        variables = OrderedDict()
        for name, var in self.variables.items():
            if name in self.data_vars:
                var_shifts = {k: v for k, v in shifts.items()
                              if k in var.dims}
                variables[name] = var.shift(
                    fill_value=fill_value, shifts=var_shifts)
            else:
                variables[name] = var

        return self._replace_vars_and_dims(variables)