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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 pop_density(data: CityInfo) -> str: """Calculate the population density from the data entry"""

    if not isinstance(data, CityInfo):
        raise AttributeError("Argument to pop_density() must be an instance of CityInfo")
    return no_dec(data.get_population() / data.get_area())