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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 convert_types_slow(df):
    '''This is a slow operation.'''
    dtypes = get_types(df)
    for k, v in dtypes.items():
        t = df[df['key']==k]
        t['value'] = t['value'].astype(v)
    df = df.apply(convert_row, axis=1)
    return df