How to run some code in a class at the time that it is defined?

Question

In Python, I can have the equivalent of C# static members:

class MyClass:
  i = 0  # This is like a C# static member

print(MyClass.i)

gives output

But maybe my static member needs to be calculated somehow. I can do:

class MyClass:
  i = 0
  i += 10

print(MyClass.i)

gives output

In practice, I'm writing a config class which needs to read a json file from disk, validate it, and then populate some static members. The closest thing to what I want to do in Python would look like:

class GlobalConfig:
  with open('config.json', 'r') as f:
    config_dict = json.read(f)
  # Maybe do some validation here.
  i = config_dict["i"]
  a = config_dict["hello_world"]

Truth be told, I wouldn't really do this in Python, but I'm borrowing from C# in that everything needs to go in classes.

In practice in my C# code, I would have a GlobalConfig class in a Config.cs file and all my other files would have access to this.

But it seems I can't do anything other than declare/define members in the class body. How can I do the work of loading up and parsing my config file and have the result of that be accessible as static members to the rest of my program?

PS: I don't really want this to influence the answers I get in unless it has to, but FYI I am working with Unity.

you can have a static function that anyone can use. And even a static constructor, that does some global initialization so that your `Cnonfig`-class is allways fully initialized. — MakePeaceGreatAgain, Oct 20 '22 at 13:54
Oh a static constructor. Cool. I supposed this is it then https://learn.microsoft.com/en-us/dotnet/csharp/programming-guide/classes-and-structs/static-constructors — Alexander Soare, Oct 20 '22 at 13:55
However I'd rather not go that path as it's pretty undeterministic when the static constructor is called - only that it *will* be called. You may have a private load-function that is called within all the others of not already done. — MakePeaceGreatAgain, Oct 20 '22 at 13:57
@MakePeaceGreatAgain are you saying that with a static constructor it's possible I try to access the static members before the constructor has run? And if so, wouldn't C# just try to call the static constructor first anyway? — Alexander Soare, Oct 20 '22 at 14:02
see https://stackoverflow.com/questions/1437352/when-is-a-static-constructor-called-in-c for when it is called. It's a bit over oiut of scope for your question. — MakePeaceGreatAgain, Oct 20 '22 at 14:04
@MakePeaceGreatAgain Then a static constructor sounds perfect because it's called "before any static members are referenced" — Alexander Soare, Oct 20 '22 at 14:05
Also see https://stackoverflow.com/questions/10465961/potential-pitfalls-with-static-constructors-in-c-sharp for potential pitfalls when using static constructor. — MakePeaceGreatAgain, Oct 20 '22 at 14:07
@MakePeaceGreatAgain thanks you've been super helpful. For the time being I'll accept Dmitry's answer as it's more direct, but you've provided a lot of value! — Alexander Soare, Oct 20 '22 at 14:10

score 2 · Accepted Answer · answered Oct 20 '22 at 13:56

Are you looking for static constructor?

public class MyClass { 
  private static int i = 0;

  private static int j;

  private static int loadJ(int value) {...}

  // Static constructor will be called before the first MyClass call
  static MyClass() {
    // If you want to compute anything with static you can do it here
    i += 10; 
    // you can call static methods here, address static fields and properties
    j = loadJ(i);
  }
}

MakePeaceGreatAgain · Answer 2 · 2022-10-20T14:11:41.283

1

An alternative to static constrcutors is some lazy-loading approach. In essence you just initialize your class when it is used in any way calling the loading-function internally:

public class MyClass 
{ 
    static int i;
    private static int loadJ() 
    {
        i = ReadValueFromJson();
    }

    public static DoSomething()
    {
        // load if not yet done
        if(!loaded) 
            loadj();
        // do the actual work
    }
}

The advantage here is that you get more fine-grained controll about if or if not initialization should happen. Imagine some constant within your config-class that allways has the exact same value regardless of any initialization. The downsie is that you need to add this boilerplate to all the members that need loading.

edited Oct 20 '22 at 14:11

answered Oct 20 '22 at 13:59

MakePeaceGreatAgain

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Not sure if this would have any influence on your answer, but I don't want `i` and `j` to be private. I want theme to be accessed from anywhere else in the program **after** the setup has been done. – Alexander Soare Oct 20 '22 at 14:01
1

@AlexanderSoare You can make it a public property. – MakePeaceGreatAgain Oct 20 '22 at 14:03
So is the idea then that I must call `DoSomething` in any place I want to use this? Just in case it hasn't been called already by some other thing? – Alexander Soare Oct 20 '22 at 14:03
@AlexanderSoare well, just within all the functions in your config-class. But I agree, that may become brittle when your config-class has dozenzs of members that all assume the class to be initialized in front. – MakePeaceGreatAgain Oct 20 '22 at 14:05
Hm yeah I think I'm not seeing any issues with the static constructor so it seems like the more suitable approach. – Alexander Soare Oct 20 '22 at 14:06
1

@AlexanderSoare Just wait until your static constructor throws an exception... Fun times when that happens. My advice: NEVER allow an exception to be thrown from a static constructor. – Matthew Watson Oct 20 '22 at 14:44

How to run some code in a class at the time that it is defined?

2 Answers2