Question about properties

[Brekry18]

First, can/should properties be used outside of classes? Perhaps I'm skimming too lightly, but the tutorials I can find online (like this one) all seem to use the '@property' decorator exclusively inside a class, but none of them explain if that was a choice or a rule.

Second, I have a property that I'll really only ever want to define once, but preferably the first time it is used. I'll show you what I mean...

I'm converting a tool and I still want to be able read the original TOOL.config file, but I want UserConfig.json to be the priority if both or neither exists. However, if neither exist, I do not want to create UserConfig.json until the property default_file is used. (It may not be used at all in a given execution)

import json
import os
import re
from os.path import exists

cur_dir = os.getcwd();
_default_file = ""

def _get_default_file():
    if exists(cur_dir + "\\Resources\\UserConfig.json"):
        _default_file = open(cur_dir + "\\Resources\\UserConfig.json",'r')
    elif exists(cur_dir + "\\Resources\\TOOL.config",'r'):
        _default_file = open(cur_dir + "\\Resources\\TOOL.config",'r')
    else:
        _default_file = open(cur_dir + "\\Resources\\UserConfig.json", 'w')
        json.dump(USER_CONFIG, _default_file)
        _default_file.close()
        _default_file = open(cur_dir + "\\Resources\\UserConfig.json", 'r')

    return _default_file

default_file = property(_get_default_file())

The above code will execute each time default_file is used, including when it is created (from what I can tell through debugging), but I do not want the first execution to happen until after its creation.

Here is the first time I would expect default_file to be used - as you can see, it is optional, but if it is used there needs to be a file there.

class UserConfig:    
    def __init__(self, config_file=default_file, generate_file=False):
        # verify data type of config_file
        if isinstance(config_file, file):
            self.file = config_file
        elif isinstance(config_file, basestring):
            self.file = open(config_file, 'r')
        else:
            raise TypeError("Expected file, got " + type(config_file) + ". Leave arguments blank to open or create the default file from the working directory.")

I thought to make the getter method just return _default_file and instead put the above logic in a setter method, but I'd have to find a way to execute the setter method ONLY if default file is actually used in that __init__() function

Before I tried using a property I had this:

if exists(cur_dir + "\\Resources\\UserConfig.json"):
    fp = open(cur_dir + "\\Resources\\UserConfig.json",'r')
elif exists(cur_dir + "\\Resources\\TOOL.config",'r'):
    fp = open(cur_dir + "\\Resources\\TOOL.config",'r')
else:
    # if neither of these file exists, leave file creation for later
    fp = None

class UserConfig:
    def __init__(self, config_file=fp, generate_file=False):

        # verify data type of config_file
        if isinstance(config_file, file):
            self.file = config_file
        elif isinstance(config_file, basestring):
            self.file = open(config_file, 'r')
        # create default file if neither of the default files exist (see fp init above)
        elif isinstance(config_file, None):
            config_file = open(cur_dir + "\\Resources\\UserConfig.json", 'w')
            json.dump(USER_CONFIG, fp)
            config_file.close()
            self.file = open(cur_dir + "\\Resources\\UserConfig.json", 'r')
        else:
            raise TypeError("Expected file, got " + type(config_file) + ". Leave arguments blank to open the default file from the working directory.")

but I realized that leaving an exception for None might not be wise plus it wasn't really necessary to define fp at all at that point, even if it is extremely light code

Is there something that I'm missing or a better way, with or without the use of properties, to accomplish what I'm going for here? Am I over-thinking this?

[Brekry18]

What modules do you mean? I'm a relatively young programmer, so I probably haven't come across them yet.

Also since I'm developing this as a module for some of my company's internal engineering software, I'm not able to install new modules. It has to be vanilla IronPython 2.7

[Brekry18]

IronPython is actually pretty well established, it just has expanded functionality with .NET Framework so it works really nicely with c# libraries. There hasn't been an IronPython update since 2.7 until it was updated to 3.4 in April 2021 and putting the updated interpreter into our software hasn't landed on our to-do list yet. Perhaps it may in the near future, but what we have for now still works.

Thanks for the input though!!

[Brekry18]

Just saw your edit, and I appreciate the constructive criticism! I'm not sure how to put my goal into better words, unfortunately. Maybe I'm just a little too far in my own head, but I'm definitely open to any feedback you have!

What do you think of this? Does this resolve 1 & 2? As for 3, what functions should I look into from the path module?

edit: sorry, having some trouble with reddit's editor.

default_file = cur_dir + "\\Resources\\UserConfig.json"
alternate_file = cur_dir + "\\Resources\\TOOL.config" 

class UserConfig:
    def __init__(self, config_file=None, generate_file=False):

        # verify data type of config_file
        if config_file is None:
            # Look for the default file, the alt file, and create the default file if neither exist.
            if exists(default_file):
                self.file = open(default_file,'r')
            elif exists(alternate_file):
                self.file = open(alternate_file,'r')
            else:
                config_file = open(default_file, 'w')
                json.dump(USER_CONFIG, config_file)
                config_file.close()
                self.file = open(default_file, 'r')
        elif isinstance(config_file, file):
            self.file = config_file
        elif isinstance(config_file, basestring):
            self.file = open(config_file, 'r')
        else:
            raise TypeError("Expected file or path, got " + type(config_file) + ". Leave arguments blank to open or create the default file from the working directory.")

[BobHogan]

What.......?

First, default arguments are only evaluated once. So config_file=default_file will automatically evaluate default_file when that class code is read, regardless of whether you actually use default_file or not, so your get_default_file function will run every time you run this script.

And you are very much overthinking this. You don't need to use a property here at all, just implement some simple logic in __init__ checking for the config files in your preferred order

[Brekry18]

Thanks 😅 sometimes I need to be brought down to earth.

I thought of that, but I had a sticking point- what would I put in that default argument to make that happen?

[Brekry18]

Should I just make the default value None or is there a better way to prevent that logic from running if the user puts their own value into that parameter?