Practice: intro to classes - Python initiation: core language, modern workflows, and scientific applications

Let us suppose we have a set of weather stations that do measurements of wind speed and temperature in a weather station (for example in Paris). We need to store these measurements in a clear way so that computing statistics (maximum wind, average temperature...) is easy and not error prone. This chapter shows different ways of storing the data, its advantages and disadvantages.

The basic way: only with lists¶

Exercise 1 (A list of lists)

A basic representation of our Paris weather station will be a unique list of lists. Write one list with two elements containing the wind values and temperature values. Find the values of the maximum temperature and wind.

wind_values = [10, 0, 20, 30, 20, 4]
temperature_values = [1, 5, 1, -1, -1, 3]

Solution to Exercise 1

paris = [wind_values, temperature_values]

# get wind when temperature is maximal
idx_max_temp = paris[1].index(max(paris[1]))
print(f"max temp is {paris[1][idx_max_temp]}°C at index {idx_max_temp} ")
print(f"wind speed at max temp = {paris[0][idx_max_temp]} km/h")

max temp is 5°C at index 1 
wind speed at max temp = 0 km/h

Comments on this solution¶

Many problems:

if the number of measurements increases (e.g. having rainfall, humidity, ...) the previous indexing will not be valid (what will paris[5] represent? wind, temperature, ..., ?)
Code analysis is not (that) straightforward

More complex structures: mixing dicts and lists¶

Solution to Exercise 2

paris = {"wind": wind_values, "temperature": temperature_values}

# get wind when temperature is minimal
paris_temp = paris["temperature"]
idx_max_temp = paris_temp.index(max(paris_temp))

print(f"max temp is {paris_temp[idx_max_temp]}°C at index {idx_max_temp}")
print(f"wind speed at max temp = {paris['wind'][idx_max_temp]} km/h")

max temp is 5°C at index 1
wind speed at max temp = 0 km/h

Comments¶

Pro
- More readable code (reading paris["temperature"] is clearer than paris[1]).
- Less error prone code (i.e. using words as keys allow to not use index numbers that are easily mistaken and lead to code that is hard to read and debug)
Con
- The code to compute the final result is not very readable

Improve database: add functions¶

Solution to Exercise 3

paris = {"wind": wind_values, "temperature": temperature_values}


def max_temp(station):
    """returns the maximum temperature available in the station"""
    return max(station["temperature"])


def arg_max_temp(station):
    """returns the index of maximum temperature available in the station"""
    max_temperature = max_temp(station)
    return station["temperature"].index(max_temperature)


idx_max_temp = arg_max_temp(paris)

print(f"max temp is {max_temp(paris)}°C at index {arg_max_temp(paris)}")
print(f"wind speed at max temp = {paris['wind'][idx_max_temp]} km/h")

max temp is 5°C at index 1
wind speed at max temp = 0 km/h

Comments¶

Pro:
- Adding functions leads to a code that is easier to read, hence easier to debug.
- Testing functions can be done separately from the rest of the code.
- The computation done on the second part depends upon the functions (i.e it depends on the function definitions not their implementations).
- Adding function allows to reuse code: computing the max temperature is something one could want to do in other places.
Con
- We rely on the fact that the dictionaries have been built correctly (for example wind and temperature arrays have the same length).

Continue improving: build a init function¶

Solution to Exercise 4

def build_station(wind, temp):
    """Build a station given wind and temp
    :param wind: (list) floats of winds
    :param temp: (list) float of temperatures
    """
    if len(wind) != len(temp):
        raise ValueError("wind and temperature should have the same size")
    return {"wind": list(wind), "temperature": list(temp)}


def max_temp(station):
    """returns the maximum temperature available in the station"""
    return max(station["temperature"])


def arg_max_temp(station):
    """returns the index of maximum temperature available in the station"""
    max_temperature = max_temp(station)
    return station["temperature"].index(max_temperature)


paris = build_station(wind_values, temperature_values)
idx_max_temp = arg_max_temp(paris)

print(f"max temp is {max_temp(paris)}°C at index {arg_max_temp(paris)}")
print(f"wind speed at max temp = {paris['wind'][idx_max_temp]} km/h")

max temp is 5°C at index 1
wind speed at max temp = 0 km/h

Comments¶

If the dedicated function build_station is used, the returned dictionary is well structured.
If one changes build_station, only max_temp and arg_max_temp have to be changed accordingly
We use a list comprehension to be able to have parameters wind and temp provided by any ordered iterable (e.g. see test_build_station_with_iterable wtih range)
BUT if we have a new kind of station, i.e. that holds only wind and humidity, we want to avoid to be able to use max_temp with it.

Last improvement: using a class¶

We would like to “embed” the max_temp and the arg_max_temp in the “dictionary station” in order to address the last point.

And here comes object-oriented programming !

A class defines a template used for building object. In our example, the class (named WeatherStation) defines the specifications of what is a weather station (i.e, a weather station should contain an array for wind speeds, named “wind”, and an array for temperatures, named “temp”). paris should now be an object that answers to these specifications. Is is called an instance of the class WeatherStation.

When defining the class, we need to define how to initialize the object (special “function” __init__).

Exercise 5 (Build the class)

We provide a skeleton for the class. Take the code defined in the previous exercises and fill the holes

class WeatherStation:
    """A weather station that holds wind and temperature

    :param wind: any ordered iterable
    :param temperature: any ordered iterable

    wind and temperature must have the same length.

    """

    def __init__(self, wind, temperature):
        """initialize the weather station.
        Precondition: wind and temperature must have the same length.
                      ValueError is raised if this is not the case
        :param wind: any ordered iterable
        :param temperature: any ordered iterable"""
        pass

    def max_temp(self):
        """returns the maximum temperature recorded in the station"""
        pass

    def arg_max_temp(self):
        """returns the index of (one of the) maximum temperature recorded in the station"""
        pass

Solution to Exercise 5

class WeatherStation:
    """A weather station that holds wind and temperature

    :param wind: any ordered iterable
    :param temperature: any ordered iterable

    wind and temperature must have the same length.

    """

    def __init__(self, wind, temperature):
        """initialize the weather station.
        Precondition: wind and temperature must have the same length.
                      ValueError is raised if this is not the case
        :param wind: any ordered iterable
        :param temperature: any ordered iterable"""
        self.wind = list(wind)
        self.temp = list(temperature)
        if len(self.wind) != len(self.temp):
            raise ValueError(
                "wind and temperature should have the same size"
                f" got len(wind)={len(self.wind)} vs "
                f" len(temp)={len(self.temp)}"
            )

    def max_temp(self):
        """returns the maximum temperature recorded in the station"""
        return max(self.temp)

    def arg_max_temp(self):
        """returns the index of (one of the) maximum temperature recorded in the station"""
        return self.temp.index(self.max_temp())


paris = WeatherStation(wind_values, temperature_values)
idx_max_temp = paris.arg_max_temp()

print(f"max temp is {paris.max_temp()}°C at index {paris.arg_max_temp()}")
print(f"wind speed at max temp = {paris.wind[idx_max_temp]} km/h")

max temp is 5°C at index 1
wind speed at max temp = 0 km/h

Comments¶

The max_temp and the arg_max_temp are now part of the class WeatherStation. Functions attached to classes are named methods. Similarly, wind and temp lists are also now part this class. Variables attached to classes are named members or attributes.
if max_temp method is called in many places, we can improve it by caching the result. This will not affect code the uses the class.
arg_max_temp method should be rewritten as we implicitly check equality of floats.

An object (here paris) thus contains both attributes (holding data for example) and methods to access and/or process the data.

Solution to Exercise 6

class WeatherStation(object):
    """A weather station that holds wind and temperature"""

    def __init__(self, wind, temperature):
        """initialize the weather station.
        Precondition: wind and temperature must have the same length
                      ValueError is raised if this is not the case
        :param wind: any ordered iterable
        :param temperature: any ordered iterable"""
        self.wind = [x for x in wind]
        self.temp = [x for x in temperature]
        if len(self.wind) != len(self.temp):
            raise ValueError(
                "wind and temperature should have the same size"
                f" got len(wind)={len(self.wind)} vs "
                f" len(temp)={len(self.temp)}"
            )

    def perceived_temp(self, index):
        """computes the perceived temp according to
        https://en.wikipedia.org/wiki/Wind_chill
        i.e. The standard Wind Chill formula for Environment Canada is:
        apparent = 13.12 + 0.6215*air_temp - 11.37*wind_speed^0.16 + 0.3965*air_temp*wind_speed^0.16

        :param index: the index for which the computation must be made
        :return: the perceived temperature"""
        air_temp = self.temp[index]
        wind_speed = self.wind[index]
        # Perceived temperature does not have a sense without wind...
        if wind_speed == 0:
            apparent_temp = air_temp
        else:
            apparent_temp = (
                13.12
                + 0.6215 * air_temp
                - 11.37 * wind_speed**0.16
                + 0.3965 * air_temp * wind_speed**0.16
            )
        # Let's round to avoid trailing decimals...
        return round(apparent_temp, 2)

    def perceived_temperatures(self):
        """Returns an array of perceived temp computed from the temperatures and wind speed data"""
        apparent_temps = []
        for index in range(len(self.wind)):
            # Reusing the method perceived_temp defined above
            apparent_temperature = self.perceived_temp(index)
            apparent_temps.append(apparent_temperature)
        return apparent_temps

    def max_temp(self, perceived):
        """returns the maximum temperature record in the station"""
        if perceived:
            apparent_temp = self.perceived_temperatures()
            return max(apparent_temp)
        else:
            return max(self.temp)

    def arg_max_temp(self, perceived):
        """returns the index of (one of the) maximum temperature record in the station"""
        if perceived:
            temp_array_to_search = self.perceived_temperatures()
        else:
            temp_array_to_search = self.temp
        return temp_array_to_search.index(self.max_temp(perceived))

Comments¶

The wind array was changed to have different maximum temperatures for the air and perceived temperatures: for air temperatures, the max is 5°C (with a wind speed 50 km/h). For perceived temperatures, the max is 3°C (as the wind speed is 0).
It was a choice to set the apparent/perceived temperature to the air temperature if the wind speed is 0 so the tests were written with this in mind. Testing such choices allows to have clear inputs/outputs.
isinstance allows to test the type of an object (in this case, we test if apparent_temps is a list)
When testing boolean in if structures: use if perceived: rather than if perceived == True:. It is equivalent but clearer and shorter !

Coming next: inheritance¶

What if we now have a weather station that also measure humidity ?

Do we need to rewrite everything ?

What if we rewrite everything and we find a bug ?

Here comes inheritance