Support for default values for optional config

[mavwolverine]

Something along the lines of

config.postgres_port or 5432
config.postgres_host or 'localhost'

[mavwolverine]

I guess I could still do config.values.get('pg_port', 5432), but a simpler syntax would be great.

[RobertoPrevato]

Thanks for your feedback! I have some mixed feelings about this, because supporting something like:

config.postgres_port or 5432

presents this issue:

• if a user forgets to set at all a certain setting, (and doesn't use the or ... part), the code returns None, which might fail later (more details below on what is the original philosophy of this library)

As a side note, even Łukasz Langa recently commented on a similar subject, stating that it's more Pythonic to throw exception if an attribute is missing, rather than returning T | None (https://twitter.com/llanga/status/1522922855335436289).
Of course there are exceptions even in the standard library: Regex match returns a Match object or None - so I take this with a grain of salt.

---

The original philosophy of this library is to enable composition (something I like in .NET Core and wanted to have also for my Python projects), like:

1. first read default values from example-settings.yaml / json
2. then read values from example-test-settings.yaml / json (for the test environment - using an env variable to define the environment)
3. then - for example - read values from environmental variables
4. then read values from an Azure Key Vault / AWS Key Management Service

In other words, if you wish to have defaults, the idea of this library is that you would configure them either in memory:

    builder = ConfigurationBuilder()
    
    builder.add_map({
        "postgres_port ": 5432,
        "postgres_host": "localhost"
    })
    
    builder.add_source(YAMLFile("./yaml_example_01.yaml"))

    config = builder.build()

or, more likely, in a base app settings file:

{
    "postgres_port": 5432,
    "postgres_host": "localhost"
}

I know by practice that this has the good quality that applications settings are more understandable: it's sufficient to open this base settings file to get an overview of what settings are used by the app, and what are their defaults.

In conclusion, as I am thinking while writing these sentences, I don't think that returning default None would be the best option in this case.

[RobertoPrevato]

But I like your recommendation, we could add a method similar to what dict offers:

configuration.get("key", default_value)

[RobertoPrevato]

What about something like this? 368e9a4

[mavwolverine]

Yes, that works. Thanks