Getting attributes from a list containing classes

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Getting attributes from a list containing classes - midarq - Dec-02-2019

Hi!

I have a small problem:

I defined a class Tone which inherits from the class Recorder. (i.e. a recorder is an instrument which has many notes). Each Tone has certain attributes like frequency, fingering, etc. Currently, I am doing the following:

class Recorder(object):
    def __init__(self):
        self.alltones=[Tone(ii) for ii in range(10)]
class Tone(Recorder):
    def __init__(self,ii):
        self.frequency=2**(ii/12)

This however doesn't satisfy me as I would like to be able to easily create a list of all frequencies from an instance of Recorder. Something along these lines

a=Recorder()
a.alltones.frequency

In particular, I am looking for a clean way of creating lists of the following type

[Tone(ii).attribute for ii in range(10)]

Thank you in advance for your help :)

RE: Getting attributes from a list containing classes - Clunk_Head - Dec-02-2019

You have a fundamental misunderstanding of inheritance.
Inheritance is base on the is-a relationship. What you have is the has-a relationship.

The is-a relationship is easily demonstrated using vehicles:
Car is-a Ground_Vehicle is-a Vehicle
Truck is-a Ground_Vehicle is-a Vehicle
Hang_Glider is-a Flying_Vehicle is-a Vehicle
Biplane is-a Flying_Vehicle is-a Vehicle

These relationships can be expressed as a tree
[Image: Screenshot-154.png]

So we would create:

class Vehicle

class Ground_Vehicle(Vehicle)
class Car(Ground_Vehicle)
class Truck(Ground_Vehicle)

class Flying_Vehicle(Vehicle)
class Hang_Glider(Flying_Vehicle)
class Biplane(Flying_Vehicle)

This allows us to express the is-a relationship and inherit the properties of the parent class(es) as we go.

You have a has-a relationship. A recorder has-a tone, or in this case a list of tones. To handle this relationship you need to make class Tone and class Recorder and in the init of recorder give it a list of Tones:

class Recorder:
    def __init__(self, tones):
        self.alltones = tones

bass_recorder = Recorder([Tone('Gs'), Tone(Af), Tone(An), Tone(As), . . .])

#then you can access all of the tones in any given recorder as a property of that recorder
print(bass_recorder.alltones)

Since a Tone is not a recorder you just create class Tone with its own properties, such as note name and frequency.

RE: Getting attributes from a list containing classes - midarq - Dec-03-2019

Fantastic explanation, thank you!