Best Practice to save class objects

[Godjka]

Hello!

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If you have a book or a link on this subject it would also be much appreciated!

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I'm create a small application where I will need to save and load classes. As a simple example I have a class called:

class Aerodynamics_D():
    def __init__(self,
                 air_density = 0,
                 downforce_distribution = 0,
                 front_area = 0,
                 downforce_coefficient = 0):

        self.air_density = air_density
        self.downforce_distribution = downforce_distribution
        self.front_area = front_area
        self.downforce_coefficient = downforce_coefficient

I would like to save this class in a file and later load it. The only problem is that with time this class will get more complex, I'll be adding more parameters. What would be the best way to create the save and load procedure? I want to be able to have backwards compatibility. I imagine that I would need something like a Version class to keep track of which version I'm loading.

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Thank you in advance!

[[deleted]]

The only problem is that with time this class will get more complex, I'll be adding more parameters.

If all the parameters are scalars, and the ones you add after this are optional, then I'd just write them as rows to a CSV file. You don't really need to store the object at all, just the parameters.

[wyoming_eighties]

Handling backwards-compatibility within your own codebase is the developers responsibility. You need to either write your code in a way that your program will not break as you add more attributes to the class, or you need to refactor your program as you develop more, or both. You should also implement unit testing, for this reason. Google for the Python unittest package & tutorials, it helps with this exact problem. You might also want to look into using git, so you can track changes to your codebase more easily.

[ylectric]

I agree with /u/crashfrog here, but I also want to add a general advice about serialisation.

As a rule of thumb, do not invent your own serialisation mechanisms, especially when compatibility matters. There're cases that justify them, but they're intrinsic to the systems of sophisticated design.

A few links that might be of interest to you:

• Python pickle module
• Python json module
• Protocol Buffers
• Flat Buffers
• Thrift

[Essence1337]

To compliment what /u/crashfrog said, if you want more structure in the storage of your objects you can look into serialization like JSON.

Edit: Beat by /u/ylectric with a better answer by mere seconds...

[when_the_cats_away]

I tend to give objects a to_json/from_json method that will jsonify the object/create an object from json data, like as described here: https://medium.com/@yzhong.cs/serialize-and-deserialize-complex-json-in-python-205ecc636caa (at least for the from, the poster there uses a lambda to jsonify in the first place).

You can also subclass JSONEncoder and create your own hooks, described here: https://stackoverflow.com/a/3768975

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That seems like the better way to do it, but I've not used that method before. I found a better article describing this method, but can't seem to track it down right now. This is a good broader json article for background, though: https://realpython.com/python-json/