Support loading a specific nested key from YAML in YamlConfigSettingsSource

[Seunghan-Jung]

Description

It would be great if YamlConfigSettingsSource could support loading settings from a specific nested key within a YAML file.

Use Case

Often, YAML files contain configuration for multiple environments or sections, such as:

A_service:
  key: "A_key"
 
B_service:
  key: "B_key"

In this case, it would be useful to load settings from a specific key, for example, development, rather than requiring separate YAML files for each environment.

Suggested Feature

Allow an optional argument (e.g., nested_key) to be passed to YamlConfigSettingsSource like this:

YamlConfigSettingsSource(settings_cls, yaml_file="config.yaml", yaml_nested_key="A_service")

This would tell the source to load settings from config["development"] instead of the root.

Benefits
• Reduces the need for multiple YAML files
• Simplifies environment-specific configuration management
• Aligns with how many existing tools and frameworks organize config

[hramezani]

Thanks @Seunghan-Jung for the PR.

I left some comments. please update

[hramezani]

Thanks @Seunghan-Jung

[Seunghan-Jung]

Thanks @hramezani