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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 homogenize_types(df, groupby_cols):
 """Converting all GROUPBY columns to strings When grouping by a numeric (say FLOAT) column, pydruid returns strings in the dataframe. This creates issues downstream related to having mixed types in the dataframe Here we replace None with <NULL> and make the whole series a str instead of an object. """

        for col in groupby_cols:
            df[col] = df[col].fillna('<NULL>').astype('unicode')
        return df