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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 fetch_turbine_data(self, fetch_curve, data_source):
 r""" Fetches data of the requested wind turbine. Method fetches nominal power as well as power coefficient curve or power curve from a data set provided in the OpenEnergy Database (oedb). You can also import your own power (coefficient) curves from a file. For that the wind speeds in m/s have to be in the first row and the corresponding power coefficient curve values or power curve values in W in a row where the first column contains the turbine name. See `example_power_curves.csv' and `example_power_coefficient_curves.csv` in example/data for the required form of a csv file (more columns can be added). See :py:func:`~.get_turbine_data_from_file` for an example reading data from a csv file. Parameters fetch_curve : string Parameter to specify whether a power or power coefficient curve should be retrieved from the provided turbine data. Valid options are 'power_curve' and 'power_coefficient_curve'. Default: None. data_source : string Specifies whether turbine data (f.e. nominal power, power curve, power coefficient curve) is loaded from the OpenEnergy Database ('oedb') or from a csv file ('<path including file name>'). Default: 'oedb'. Returns ------- self Examples -------- 0.44 4200000.0 """

        if data_source == 'oedb':
            curve_df, nominal_power = get_turbine_data_from_oedb(
                turbine_type=self.name, fetch_curve=fetch_curve)
        else:
            curve_df, nominal_power = get_turbine_data_from_file(
                turbine_type=self.name, file_=data_source)
        if fetch_curve == 'power_curve':
            self.power_curve = curve_df
        elif fetch_curve == 'power_coefficient_curve':
            self.power_coefficient_curve = curve_df
        else:
            raise ValueError("'{0}' is an invalid value. ".format(
                             fetch_curve) + "`fetch_curve` must be " +
                             "'power_curve' or 'power_coefficient_curve'.")
        if self.nominal_power is None:
            self.nominal_power = nominal_power
        return self