def METHOD_NAME(self, expression, data, key, dims):
    """Parse the expression and return the result
    Parameters:
    expression -- the string expression to parse
    data -- the xarray or netcdf dataset to pull data from
    key -- the key passed along from the __getitem__ call, a tuple of
           integers and/or slices
    dims -- the dimensions that correspond to the key, a list of strings
    Returns a numpy array of data.
    """
    self.data = data
    self.result = np.array(np.nan)  # populated by p_statement_expr()
    self.key = key
    self.dims = dims
    self.expression = expression
    self.parser.METHOD_NAME(expression)
    self.data = None
    self.key = None
    self.dims = None
    return self.result