### Install OmegaConf

Source: https://omegaconf.readthedocs.io/en/2.3_branch/_sources/usage.rst.txt

Use pip to install the library.

```bash
pip install omegaconf
```

--------------------------------

### Config interpolation example

Source: https://omegaconf.readthedocs.io/en/2.3_branch/_sources/usage.rst.txt

Demonstrates primitive, string, and relative interpolation using a loaded configuration.

```python
>>> conf = OmegaConf.load('source/config_interpolation.yaml')
>>> def show(x):
...     print(f"type: {type(x).__name__}, value: {repr(x)}")
>>> # Primitive interpolation types are inherited from the reference
>>> show(conf.client.server_port)
type: int, value: 80
>>> # String interpolations concatenate fragments into a string
>>> show(conf.client.url)
type: str, value: 'http://localhost:80/'
>>> # Relative interpolation example
>>> show(conf.client.description)
type: str, value: 'Client of http://localhost:80/'
```

--------------------------------

### Basic custom resolver usage

Source: https://omegaconf.readthedocs.io/en/2.3_branch/custom_resolvers.html

A simple example of a resolver that adds 10 to a provided value.

```python
>>> OmegaConf.register_new_resolver("plus_10", lambda x: x + 10)
>>> c = OmegaConf.create({'key': '${plus_10:990}'})
>>> c.key
1000
```

--------------------------------

### Define configuration with interpolation in YAML

Source: https://omegaconf.readthedocs.io/en/2.3_branch/usage.html

Example YAML structure demonstrating absolute and relative variable interpolation.

```yaml
server:
  host: localhost
  port: 80

client:
  url: http://${server.host}:${server.port}/
  server_port: ${server.port}
  # relative interpolation
  description: Client of ${.url}
```

--------------------------------

### Example of Clearing a Built-in Resolver

Source: https://omegaconf.readthedocs.io/en/2.3_branch/_sources/custom_resolvers.rst.txt

Shows how to remove a default resolver like oc.env using clear_resolver.

```python
>>> OmegaConf.has_resolver("oc.env")
    True
    >>> # This will remove the default resolver: oc.env
    >>> OmegaConf.clear_resolver("oc.env")
    True
    >>> OmegaConf.has_resolver("oc.env")
    False
```

--------------------------------

### Providing default values

Source: https://omegaconf.readthedocs.io/en/2.3_branch/_sources/usage.rst.txt

Uses the get method to provide a fallback value if a key is missing.

```python
>>> conf.get('missing_key', 'a default value')
'a default value'
```

--------------------------------

### Handle Configuration Cache

Source: https://omegaconf.readthedocs.io/en/2.3_branch/_modules/omegaconf/omegaconf.html

Utility methods to get, set, clear, or copy the resolver cache for a given configuration container.

```python
@staticmethod
def get_cache(conf: BaseContainer) -> Dict[str, Any]:
    return conf._metadata.resolver_cache
```

```python
@staticmethod
def set_cache(conf: BaseContainer, cache: Dict[str, Any]) -> None:
    conf._metadata.resolver_cache = copy.deepcopy(cache)
```

```python
@staticmethod
def clear_cache(conf: BaseContainer) -> None:
    OmegaConf.set_cache(conf, defaultdict(dict, {}))
```

```python
@staticmethod
def copy_cache(from_config: BaseContainer, to_config: BaseContainer) -> None:
    OmegaConf.set_cache(to_config, OmegaConf.get_cache(from_config))
```

--------------------------------

### Define example2.yaml configuration file

Source: https://omegaconf.readthedocs.io/en/2.3_branch/usage.html

A sample YAML configuration file defining server port and user list.

```yaml
server:
  port: 80
users:
  - user1
  - user2
```

--------------------------------

### Define example3.yaml configuration file

Source: https://omegaconf.readthedocs.io/en/2.3_branch/usage.html

A sample YAML configuration file defining log settings.

```yaml
log:
  file: log.txt
```

--------------------------------

### Create from List

Source: https://omegaconf.readthedocs.io/en/2.3_branch/_sources/usage.rst.txt

Initialize a configuration from a Python list.

```python
>>> conf = OmegaConf.create([1, {"a":10, "b": {"a":10, 123: "int_key"}}])
>>> print(OmegaConf.to_yaml(conf))
- 1
- a: 10
  b:
    a: 10
    123: int_key
<BLANKLINE>
```

--------------------------------

### Get object type

Source: https://omegaconf.readthedocs.io/en/2.3_branch/_modules/omegaconf/omegaconf.html

Retrieves the type of the object or a specific child node within the configuration.

```python
@staticmethod
def get_type(obj: Any, key: Optional[str] = None) -> Optional[Type[Any]]:
    if key is not None:
        c = obj._get_child(key)
    else:
        c = obj
    return OmegaConf._get_obj_type(c)
```

--------------------------------

### Create from YAML String

Source: https://omegaconf.readthedocs.io/en/2.3_branch/_sources/usage.rst.txt

Initialize a configuration from a raw YAML string.

```python
>>> s = """
... a: b
... b: c
... list:
... - item1
... - item2
... 123: 456
... """
>>> conf = OmegaConf.create(s)
>>> print(OmegaConf.to_yaml(conf))
a: b
b: c
list:
- item1
- item2
123: 456
<BLANKLINE>
```

--------------------------------

### Create from Dictionary

Source: https://omegaconf.readthedocs.io/en/2.3_branch/_sources/usage.rst.txt

Initialize a configuration from a standard Python dictionary.

```python
>>> conf = OmegaConf.create({"k" : "v", "list" : [1, {"a": "1", "b": "2", 3: "c"}]})
>>> print(OmegaConf.to_yaml(conf))
k: v
list:
- 1
- a: '1'
  b: '2'
  3: c
<BLANKLINE>
```

--------------------------------

### Create from Dot-List

Source: https://omegaconf.readthedocs.io/en/2.3_branch/_sources/usage.rst.txt

Initialize a configuration from a list of dot-notation strings.

```python
>>> dot_list = ["a.aa.aaa=1", "a.aa.bbb=2", "a.bb.aaa=3", "a.bb.bbb=4"]
>>> conf = OmegaConf.from_dotlist(dot_list)
>>> print(OmegaConf.to_yaml(conf))
a:
  aa:
    aaa: 1
    bbb: 2
  bb:
    aaa: 3
    bbb: 4
<BLANKLINE>
```

--------------------------------

### Initialize and customize structured configs

Source: https://omegaconf.readthedocs.io/en/2.3_branch/_sources/structured_config.rst.txt

Demonstrates creating a config from a class or instance, and overriding default values during initialization.

```python
conf1 = OmegaConf.structured(SimpleTypes)
conf2 = OmegaConf.structured(SimpleTypes())
# The two configs are identical in this case
assert conf1 == conf2
# But the second form allow for easy customization of the values:
conf3 = OmegaConf.structured(
  SimpleTypes(num=20,
  height=Height.TALL))
print(OmegaConf.to_yaml(conf3))
```

--------------------------------

### Merge configurations from files and CLI

Source: https://omegaconf.readthedocs.io/en/2.3_branch/usage.html

Demonstrates loading configurations from files and command line arguments, then merging them into a single object.

```python
>>> from omegaconf import OmegaConf
>>> import sys
>>>
>>> # Simulate command line arguments
>>> sys.argv = ['program.py', 'server.port=82']
>>>
>>> base_conf = OmegaConf.load('source/example2.yaml')
>>> second_conf = OmegaConf.load('source/example3.yaml')
>>> cli_conf = OmegaConf.from_cli()
>>>
>>> # merge them all
>>> conf = OmegaConf.merge(base_conf, second_conf, cli_conf)
>>> print(OmegaConf.to_yaml(conf))
server:
  port: 82
users:
- user1
- user2
log:
  file: log.txt
```

--------------------------------

### Accessing configuration elements

Source: https://omegaconf.readthedocs.io/en/2.3_branch/_sources/usage.rst.txt

Demonstrates object-style, dictionary-style, and list-index access patterns for configuration nodes.

```python
>>> # object style access of dictionary elements
>>> conf.server.port
80

>>> # dictionary style access
>>> conf['log']['rotation']
3600

>>> # items in list
>>> conf.users[0]
'user1'
```

--------------------------------

### Create from Structured Config

Source: https://omegaconf.readthedocs.io/en/2.3_branch/_sources/usage.rst.txt

Initialize configurations using dataclasses for type safety.

```python
>>> from dataclasses import dataclass
>>> @dataclass
... class MyConfig:
...     port: int = 80
...     host: str = "localhost"
>>> # For strict typing purposes, prefer OmegaConf.structured() when creating structured configs
>>> conf = OmegaConf.structured(MyConfig)
>>> print(OmegaConf.to_yaml(conf))
port: 80
host: localhost
<BLANKLINE>
```

```python
>>> conf = OmegaConf.structured(MyConfig(port=443))
>>> print(OmegaConf.to_yaml(conf))
port: 443
host: localhost
<BLANKLINE>
```

--------------------------------

### Manipulating configuration

Source: https://omegaconf.readthedocs.io/en/2.3_branch/_sources/usage.rst.txt

Shows how to update existing keys, add new keys, and insert new dictionary structures.

```python
>>> # Changing existing keys
>>> conf.server.port = 81

>>> # Adding new keys
>>> conf.server.hostname = "localhost"

>>> # Adding a new dictionary
>>> conf.database = {'hostname': 'database01', 'port': 3306}
```

--------------------------------

### Create Empty Config

Source: https://omegaconf.readthedocs.io/en/2.3_branch/usage.html

Initialize an empty configuration object.

```python
>>> from omegaconf import OmegaConf
>>> conf = OmegaConf.create()
>>> print(OmegaConf.to_yaml(conf))
{}
```

--------------------------------

### Create Empty OmegaConf Object

Source: https://omegaconf.readthedocs.io/en/2.3_branch/_sources/usage.rst.txt

Initialize an empty configuration object.

```python
>>> from omegaconf import OmegaConf
>>> conf = OmegaConf.create()
>>> print(OmegaConf.to_yaml(conf))
{}
<BLANKLINE>
```

--------------------------------

### OmegaConf.from_cli

Source: https://omegaconf.readthedocs.io/en/2.3_branch/_modules/omegaconf/omegaconf.html

Creates a configuration object from command line arguments.

```APIDOC
## OmegaConf.from_cli(args_list=None)

### Description
Creates a DictConfig from the provided list of arguments, or from sys.argv if no list is provided.

### Parameters
- **args_list** (Optional[List[str]]) - Optional - A list of dotlist-style strings. If None, defaults to sys.argv[1:].
```

--------------------------------

### Create from CLI Arguments

Source: https://omegaconf.readthedocs.io/en/2.3_branch/_sources/usage.rst.txt

Parse command-line arguments into a configuration object.

```python
>>> # Simulating command line arguments
>>> sys.argv = ['your-program.py', 'server.port=82', 'log.file=log2.txt']
>>> conf = OmegaConf.from_cli()
>>> print(OmegaConf.to_yaml(conf))
server:
  port: 82
log:
  file: log2.txt
<BLANKLINE>
```

--------------------------------

### Using oc.select for defaults and special keys

Source: https://omegaconf.readthedocs.io/en/2.3_branch/custom_resolvers.html

Demonstrates using oc.select to provide default values, access keys containing colons, and handle missing values.

```python
>>> cfg = OmegaConf.create({
...  "a": "Saving output to ${oc.select:output,/tmp}"
... })
>>> print(cfg.a)
Saving output to /tmp
>>> cfg.output = "/etc/config"
>>> print(cfg.a)
Saving output to /etc/config
```

```python
>>> cfg = OmegaConf.create({
...    # yes, there is a : in this key
...    "a:b": 10,
...    "bad": "${a:b}",
...    "good": "${oc.select:'a:b'}",
... })
>>> print(cfg.bad)
Traceback (most recent call last):
...
UnsupportedInterpolationType: Unsupported interpolation type a
>>> print(cfg.good)
10
```

```python
>>> cfg = OmegaConf.create({
...         "missing": "???",
...         "interpolation": "${missing}",
...         "select": "${oc.select:missing}",
...         "with_default": "${oc.select:missing,default value}",
...     }
... )
...
>>> print(cfg.interpolation)
Traceback (most recent call last):
...
InterpolationToMissingValueError: MissingMandatoryValue while ...
>>> print(cfg.select)
None
>>> print(cfg.with_default)
default value
```

--------------------------------

### Using lists in structured configurations

Source: https://omegaconf.readthedocs.io/en/2.3_branch/_sources/structured_config.rst.txt

Demonstrates defining and validating lists of types within structured configurations.

```python
>>> from dataclasses import dataclass, field
>>> from typing import List, Tuple
>>> @dataclass
... class User:
...     name: str = MISSING

>>> @dataclass
... class ListsExample:
...     # Typed list can hold Any, int, float, bool, str,
...     # bytes, pathlib.Path and Enums as well as arbitrary Structured configs.
...     ints: List[int] = field(default_factory=lambda: [10, 20, 30])
...     bools: Tuple[bool, bool] = field(default_factory=lambda: (True, False))
...     users: List[User] = field(default_factory=lambda: [User(name="omry")])
```

```python
>>> conf: ListsExample = OmegaConf.structured(ListsExample)

>>> # Okay, 10 is an int
>>> conf.ints.append(10)
>>> # Okay, "20" can be converted to an int
>>> conf.ints.append("20")

>>> conf.bools.append(True)
>>> conf.users.append(User(name="Joe"))
>>> # Not okay, 10 cannot be converted to a User
>>> with raises(ValidationError):
...     conf.users.append(10)
```

--------------------------------

### Select configuration nodes with OmegaConf.select

Source: https://omegaconf.readthedocs.io/en/2.3_branch/usage.html

Use dot-notation or brackets to retrieve specific keys from a configuration object. Supports default values and error handling for missing nodes.

```python
>>> cfg = OmegaConf.create({
...     "foo" : {
...         "missing" : "???",
...         "bar": {
...             "zonk" : 10,
...         }
...     }
... })
>>> assert OmegaConf.select(cfg, "foo") == {
...     "missing" : "???",
...     "bar":  {
...         "zonk" : 10,
...     }
... }
>>> assert OmegaConf.select(cfg, "foo.bar") == {
...     "zonk" : 10,
... }
>>> assert OmegaConf.select(cfg, "foo.bar.zonk") == 10    # dots
>>> assert OmegaConf.select(cfg, "foo[bar][zonk]") == 10  # brackets
>>> assert OmegaConf.select(cfg, "no_such_key", default=99) == 99
>>> assert OmegaConf.select(cfg, "foo.missing") is None
>>> assert OmegaConf.select(cfg, "foo.missing", default=99) == 99
>>> OmegaConf.select(cfg,
...     "foo.missing",
...     throw_on_missing=True
... )
Traceback (most recent call last):
...
omegaconf.errors.MissingMandatoryValue: missing node selected
    full_key: foo.missing
```

--------------------------------

### Load from YAML File

Source: https://omegaconf.readthedocs.io/en/2.3_branch/_sources/usage.rst.txt

Load configuration data directly from a YAML file.

```python
>>> conf = OmegaConf.load('source/example.yaml')
>>> # Output is identical to the YAML file
>>> print(OmegaConf.to_yaml(conf))
server:
  port: 80
log:
  file: ???
  rotation: 3600
users:
- user1
- user2
<BLANKLINE>
```

--------------------------------

### Define and instantiate nested structured configs

Source: https://omegaconf.readthedocs.io/en/2.3_branch/structured_config.html

Demonstrates nesting dataclasses within a parent class and initializing them using OmegaConf.structured.

```python
>>> @dataclass
... class User:
...     # A simple user class with two missing fields
...     name: str = MISSING
...     height: Height = MISSING
>>>
>>> @dataclass
... class DuperUser(User):
...     duper: bool = True
...
>>> # Group class contains two instances of User.
>>> @dataclass
... class Group:
...     name: str = MISSING
...     # data classes can be nested
...     admin: User = field(default_factory=User)
...
...     # You can also specify different defaults for nested classes
...     manager: User = field(default_factory=lambda: User(name="manager", height=Height.TALL))

>>> conf: Group = OmegaConf.structured(Group)
>>> print(OmegaConf.to_yaml(conf))
name: ???
admin:
  name: ???
  height: ???
manager:
  name: manager
  height: TALL
```

--------------------------------

### Register and Manage Custom Resolvers

Source: https://omegaconf.readthedocs.io/en/2.3_branch/_sources/custom_resolvers.rst.txt

Demonstrates registering a new resolver, checking its existence, and clearing all custom resolvers.

```python
>>> # register a new resolver: str.lower
    >>> OmegaConf.register_new_resolver(
    ...     name='str.lower',
    ...     resolver=lambda x: str(x).lower(),
    ... )
    >>> # check if resolver exists (after adding, before removal)
    >>> OmegaConf.has_resolver("str.lower")
    True
    >>> # clear all custom-resolvers
    >>> OmegaConf.clear_resolvers()
    >>> # check if resolver exists (after removal)
    >>> OmegaConf.has_resolver("str.lower")
    False
    >>> # default resolvers are not affected
    >>> OmegaConf.has_resolver("oc.env")
    True
```

--------------------------------

### OmegaConf.load

Source: https://omegaconf.readthedocs.io/en/2.3_branch/_modules/omegaconf/omegaconf.html

Loads a configuration from a file path or file-like object.

```APIDOC
## OmegaConf.load

### Description
Loads a configuration from a YAML file or a file-like object.

### Parameters
- **file_** (Union[str, pathlib.Path, IO[Any]]) - Required - The file path or file-like object to load from.

### Returns
- **Union[DictConfig, ListConfig]** - The loaded configuration object.
```

--------------------------------

### Nesting structured configurations

Source: https://omegaconf.readthedocs.io/en/2.3_branch/_sources/structured_config.rst.txt

Illustrates how to nest dataclasses and handle type validation for nested assignments.

```python
>>> @dataclass
... class User:
...     # A simple user class with two missing fields
...     name: str = MISSING
...     height: Height = MISSING
>>>
>>> @dataclass
... class DuperUser(User):
...     duper: bool = True
...
>>> # Group class contains two instances of User.
>>> @dataclass
... class Group:
...     name: str = MISSING
...     # data classes can be nested
...     admin: User = field(default_factory=User)
...
...     # You can also specify different defaults for nested classes
...     manager: User = field(default_factory=lambda: User(name="manager", height=Height.TALL))

>>> conf: Group = OmegaConf.structured(Group)
>>> print(OmegaConf.to_yaml(conf))
name: ???
admin:
  name: ???
  height: ???
manager:
  name: manager
  height: TALL
<BLANKLINE>
```

```python
>>> with raises(ValidationError):
...     conf.manager = 10
```

```python
>>> conf.manager = DuperUser()
>>> assert conf.manager.duper == True
```

--------------------------------

### OmegaConf.from_dotlist

Source: https://omegaconf.readthedocs.io/en/2.3_branch/api_reference.html

Creates a configuration object from a list of dot-notation strings.

```APIDOC
## OmegaConf.from_dotlist(dotlist: List[str]) -> DictConfig

### Description
Creates a configuration object from a list of dotlist-style strings (e.g., ['foo.bar=1']).

### Parameters
- **dotlist** (List[str]) - Required - A list of strings in dotlist format.

### Returns
- **DictConfig** - The created configuration object.
```

--------------------------------

### Create configuration from CLI arguments

Source: https://omegaconf.readthedocs.io/en/2.3_branch/_modules/omegaconf/omegaconf.html

Generates a DictConfig from command line arguments or a provided list of strings.

```python
@staticmethod
def from_cli(args_list: Optional[List[str]] = None) -> DictConfig:
    if args_list is None:
        # Skip program name
        args_list = sys.argv[1:]
    return OmegaConf.from_dotlist(args_list)
```

--------------------------------

### OmegaConf.create

Source: https://omegaconf.readthedocs.io/en/2.3_branch/_modules/omegaconf/omegaconf.html

Creates a DictConfig or ListConfig from a dictionary, list, or string.

```APIDOC
## OmegaConf.create

### Description
Creates a configuration object from the provided input. The input can be a dictionary, list, or a string representation.

### Parameters
- **obj** (Any) - Optional - The input object to convert into a configuration (dict, list, or string).
- **parent** (BaseContainer) - Optional - The parent container for the new configuration.
- **flags** (Dict[str, bool]) - Optional - Configuration flags to apply.

### Returns
- **Union[DictConfig, ListConfig]** - The resulting configuration object.
```

--------------------------------

### OmegaConf.select

Source: https://omegaconf.readthedocs.io/en/2.3_branch/_sources/usage.rst.txt

Selects a config node or value using dot-notation or bracket-notation. Supports a default value if the key is missing.

```APIDOC
## OmegaConf.select(cfg, key, default=None, throw_on_missing=False)

### Description
Allows you to select a config node or value, using either a dot-notation or brackets to denote sub-keys.

### Parameters
- **cfg** (Any) - The configuration object to search.
- **key** (str) - The key path to select.
- **default** (Any, optional) - The value to return if the key is missing.
- **throw_on_missing** (bool, optional) - If True, raises a MissingMandatoryValue error if the key is missing.
```

--------------------------------

### Dynamically generate nodes with oc.create

Source: https://omegaconf.readthedocs.io/en/2.3_branch/custom_resolvers.html

Use oc.create to convert Python objects or YAML strings into configuration nodes.

```python
>>> OmegaConf.register_new_resolver("make_dict", lambda: {"a": 10})
>>> cfg = OmegaConf.create(
...     {
...         "plain_dict": "${make_dict:}",
...         "dict_config": "${oc.create:${make_dict:}}",
...         "dict_config_env": "${oc.create:${oc.env:YAML_ENV}}",
...     }
... )
>>> os.environ["YAML_ENV"] = "A: 10\nb: 20\nC: ${.A}"
>>> show(cfg.plain_dict)  # `make_dict` returns a Python dict
type: dict, value: {'a': 10}
>>> show(cfg.dict_config)  # `oc.create` converts it to DictConfig
type: DictConfig, value: {'a': 10}
>>> show(cfg.dict_config_env)  # YAML string to DictConfig
type: DictConfig, value: {'A': 10, 'b': 20, 'C': '${.A}'}
>>> cfg.dict_config_env.C  # interpolations work in a DictConfig
10
```

--------------------------------

### Internal configuration creation implementation

Source: https://omegaconf.readthedocs.io/en/2.3_branch/_modules/omegaconf/omegaconf.html

Low-level implementation for creating configuration objects from various input types.

```python
    @staticmethod
    def _create_impl(  # noqa F811
        obj: Any = _DEFAULT_MARKER_,
        parent: Optional[BaseContainer] = None,
        flags: Optional[Dict[str, bool]] = None,
    ) -> Union[DictConfig, ListConfig]:
        try:
            from ._utils import get_yaml_loader
            from .dictconfig import DictConfig
            from .listconfig import ListConfig

            if obj is _DEFAULT_MARKER_:
                obj = {}
            if isinstance(obj, str):
                obj = yaml.load(obj, Loader=get_yaml_loader())
                if obj is None:
                    return OmegaConf.create({}, parent=parent, flags=flags)
                elif isinstance(obj, str):
                    return OmegaConf.create({obj: None}, parent=parent, flags=flags)
                else:
                    assert isinstance(obj, (list, dict))
                    return OmegaConf.create(obj, parent=parent, flags=flags)
```

--------------------------------

### Define structured configs with lists and tuples

Source: https://omegaconf.readthedocs.io/en/2.3_branch/structured_config.html

Shows how to use typing.List and typing.Tuple within dataclasses to enforce types for collection fields.

```python
>>> from dataclasses import dataclass, field
>>> from typing import List, Tuple
>>> @dataclass
... class User:
...     name: str = MISSING

>>> @dataclass
... class ListsExample:
...     # Typed list can hold Any, int, float, bool, str,
...     # bytes, pathlib.Path and Enums as well as arbitrary Structured configs.
...     ints: List[int] = field(default_factory=lambda: [10, 20, 30])
...     bools: Tuple[bool, bool] = field(default_factory=lambda: (True, False))
...     users: List[User] = field(default_factory=lambda: [User(name="omry")])
```

--------------------------------

### oc.create

Source: https://omegaconf.readthedocs.io/en/2.3_branch/custom_resolvers.html

Dynamically generate configuration nodes from Python objects or YAML strings.

```APIDOC
## oc.create

### Description
Used for dynamic generation of config nodes from Python dictionaries, lists, or YAML strings.

### Syntax
`${oc.create:value}`

### Parameters
- **value** (string/object) - Required - The input data to be converted into a DictConfig or ListConfig.
```

--------------------------------

### Define YAML configuration file

Source: https://omegaconf.readthedocs.io/en/2.3_branch/structured_config.html

A sample YAML configuration file used for merging with structured schemas.

```yaml
server:
  port: 80
log:
  file: ???
  rotation: 3600
users:
  - user1
  - user2
```

--------------------------------

### Handling variadic arguments and escaping

Source: https://omegaconf.readthedocs.io/en/2.3_branch/custom_resolvers.html

Demonstrates how to pass multiple arguments and use escaping or quotes for special characters.

```python
>>> OmegaConf.register_new_resolver("concat", lambda x, y: x+y)
>>> c = OmegaConf.create({
...     'key1': '${concat:Hello,World}',
...     'key_trimmed': '${concat:Hello , World}',
...     'escape_whitespace': '${concat:Hello,\ World}',
...     'quoted': '${concat:"Hello,", " World"}',
... })
>>> c.key1
'HelloWorld'
>>> c.key_trimmed
'HelloWorld'
>>> c.escape_whitespace
'Hello World'
>>> c.quoted
'Hello, World'
```

--------------------------------

### Static Type Checking with Structured Configs

Source: https://omegaconf.readthedocs.io/en/2.3_branch/structured_config.html

Demonstrates how structured configurations enable static type checking and validation for attribute assignments.

```python
>>> conf: SimpleTypes = OmegaConf.structured(SimpleTypes)
>>> # Passes static type checking
>>> conf.description = "text"
>>> # Fails static type checking (but will also raise a Validation error)
>>> with raises(ValidationError):
...     conf.num = "foo"
```

--------------------------------

### Saving and loading pickle

Source: https://omegaconf.readthedocs.io/en/2.3_branch/_sources/usage.rst.txt

Uses pickle to persist configuration while retaining type information.

```python
>>> conf = OmegaConf.create({"foo": 10, "bar": 20, 123: 456})
>>> with tempfile.TemporaryFile() as fp:
...     pickle.dump(conf, fp)
...     fp.flush()
...     assert fp.seek(0) == 0
...     loaded = pickle.load(fp)
...     assert conf == loaded
```

--------------------------------

### omegaconf.read_write

Source: https://omegaconf.readthedocs.io/en/2.3_branch/api_reference.html

Context manager to toggle read/write access for a configuration node.

```APIDOC
## omegaconf.read_write()

### Description
Provides a generator to manage the read/write state of a configuration node.
```

--------------------------------

### Merging configurations

Source: https://omegaconf.readthedocs.io/en/2.3_branch/_sources/usage.rst.txt

Combines multiple configuration objects into a single instance.

```python
conf = OmegaConf.merge(base_cfg, model_cfg, optimizer_cfg, dataset_cfg)
```

```python
conf = OmegaConf.merge(server_cfg, plugin1_cfg, site1_cfg, site2_cfg)
```

```python
>>> from omegaconf import OmegaConf
>>> import sys
>>>
>>> # Simulate command line arguments
>>> sys.argv = ['program.py', 'server.port=82']
>>>
>>> base_conf = OmegaConf.load('source/example2.yaml')
>>> second_conf = OmegaConf.load('source/example3.yaml')
>>> cli_conf = OmegaConf.from_cli()
>>>
>>> # merge them all
>>> conf = OmegaConf.merge(base_conf, second_conf, cli_conf)
>>> print(OmegaConf.to_yaml(conf))
server:
  port: 82
users:
- user1
- user2
log:
  file: log.txt
<BLANKLINE>
```

--------------------------------

### Select configuration value

Source: https://omegaconf.readthedocs.io/en/2.3_branch/_modules/omegaconf/omegaconf.html

Selects a value from a configuration node using a key, with options for default values and error handling.

```python
@staticmethod
def select(
    cfg: Container,
    key: str,
    *,
    default: Any = _DEFAULT_MARKER_,
    throw_on_resolution_failure: bool = True,
    throw_on_missing: bool = False,
) -> Any:
    """
    :param cfg: Config node to select from
    :param key: Key to select
    :param default: Default value to return if key is not found
    :param throw_on_resolution_failure: Raise an exception if an interpolation
           resolution error occurs, otherwise return None
    :param throw_on_missing: Raise an exception if an attempt to select a missing key (with the value '???')
           is made, otherwise return None
    :return: selected value or None if not found.
    """
    from ._impl import select_value

    try:
        return select_value(
            cfg=cfg,
            key=key,
            default=default,
            throw_on_resolution_failure=throw_on_resolution_failure,
            throw_on_missing=throw_on_missing,
        )
    except Exception as e:
        format_and_raise(node=cfg, key=key, value=None, cause=e, msg=str(e))
```

--------------------------------

### Manage configuration state with context managers

Source: https://omegaconf.readthedocs.io/en/2.3_branch/_modules/omegaconf/omegaconf.html

Use these context managers to temporarily modify the read-only or struct status of a configuration node.

```python
@contextmanager
def read_write(config: Node) -> Generator[Node, None, None]:
    prev_state = config._get_node_flag("readonly")
    try:
        OmegaConf.set_readonly(config, False)
        yield config
    finally:
        OmegaConf.set_readonly(config, prev_state)
```

```python
@contextmanager
def open_dict(config: Container) -> Generator[Container, None, None]:
    prev_state = config._get_node_flag("struct")
    try:
        OmegaConf.set_struct(config, False)
        yield config
    finally:
        OmegaConf.set_struct(config, prev_state)
```

--------------------------------

### Select configuration nodes or values

Source: https://omegaconf.readthedocs.io/en/2.3_branch/_sources/usage.rst.txt

Use dot-notation or brackets to retrieve values from a configuration object. Supports default values and mandatory value error handling.

```python
>>> cfg = OmegaConf.create({
...     "foo" : {
...         "missing" : "???",
...         "bar": {
...             "zonk" : 10,
...         }
...     }
... })
>>> assert OmegaConf.select(cfg, "foo") == {
...     "missing" : "???",    
...     "bar":  {
...         "zonk" : 10, 
...     }
... }
>>> assert OmegaConf.select(cfg, "foo.bar") == {
...     "zonk" : 10, 
... }
>>> assert OmegaConf.select(cfg, "foo.bar.zonk") == 10    # dots
>>> assert OmegaConf.select(cfg, "foo[bar][zonk]") == 10  # brackets
>>> assert OmegaConf.select(cfg, "no_such_key", default=99) == 99
>>> assert OmegaConf.select(cfg, "foo.missing") is None
>>> assert OmegaConf.select(cfg, "foo.missing", default=99) == 99
>>> OmegaConf.select(cfg,
...     "foo.missing", 
...     throw_on_missing=True
... )
Traceback (most recent call last):
...
omegaconf.errors.MissingMandatoryValue: missing node selected
    full_key: foo.missing
```

--------------------------------

### Define Nested Container Annotations

Source: https://omegaconf.readthedocs.io/en/2.3_branch/_sources/structured_config.rst.txt

Shows how to nest Dict and List annotations within a structured configuration.

```python
>>> @dataclass
... class NestedContainers:
...     dict_of_dict: Dict[str, Dict[str, int]]
...     list_of_list: List[List[int]] = field(default_factory=lambda: [[123]])
...     dict_of_list: Dict[str, List[int]] = MISSING
...     list_of_dict: List[Dict[str, int]] = MISSING
... 
... 
>>> cfg = OmegaConf.structured(NestedContainers(dict_of_dict={"foo": {"bar": 123}}))
>>> print(OmegaConf.to_yaml(cfg))
dict_of_dict:
  foo:
    bar: 123
list_of_list:
- - 123
dict_of_list: ???
list_of_dict: ???
<BLANKLINE>
>>> with raises(ValidationError):
...     cfg.list_of_dict = [["whoops"]]  # not a list of dicts
```

--------------------------------

### Define a structured config with simple types

Source: https://omegaconf.readthedocs.io/en/2.3_branch/_sources/structured_config.rst.txt

Defines a dataclass using various primitive types, Enums, and pathlib.Path to serve as a configuration schema.

```python
class Height(Enum):
    SHORT = 0
    TALL = 1

@dataclass
class SimpleTypes:
    num: int = 10
    pi: float = 3.1415
    is_awesome: bool = True
    height: Height = Height.SHORT
    description: str = "text"
    data: bytes = b"bin_data"
    path: pathlib.Path = pathlib.Path("hello.txt")
```

--------------------------------

### OmegaConf.save

Source: https://omegaconf.readthedocs.io/en/2.3_branch/api_reference.html

Saves a configuration object to a file.

```APIDOC
## OmegaConf.save(config: Any, f: Union[str, Path, IO], resolve: bool = False) -> None

### Description
Saves the provided configuration object to a file or file-like object.

### Parameters
- **config** (Any) - Required - The configuration object to save.
- **f** (Union[str, Path, IO]) - Required - The destination filename or file object.
- **resolve** (bool) - Optional - If True, saves the resolved version of the configuration.
```

--------------------------------

### Serialize configuration to YAML

Source: https://omegaconf.readthedocs.io/en/2.3_branch/_modules/omegaconf/omegaconf.html

Converts a configuration object into a YAML string representation.

```python
    @staticmethod
    def to_yaml(cfg: Any, *, resolve: bool = False, sort_keys: bool = False) -> str:
        """
        returns a yaml dump of this config object.

        :param cfg: Config object, Structured Config type or instance
        :param resolve: if True, will return a string with the interpolations resolved, otherwise
            interpolations are preserved
        :param sort_keys: If True, will print dict keys in sorted order. default False.
        :return: A string containing the yaml representation.
        """
        cfg = _ensure_container(cfg)
        container = OmegaConf.to_container(cfg, resolve=resolve, enum_to_str=True)
        return yaml.dump(  # type: ignore
            container,
            default_flow_style=False,
            allow_unicode=True,
            sort_keys=sort_keys,
            Dumper=get_omega_conf_dumper(),
        )
```

--------------------------------

### Using oc.dict.keys and oc.dict.values

Source: https://omegaconf.readthedocs.io/en/2.3_branch/custom_resolvers.html

Converts DictConfig keys or values into ListConfig objects for easier manipulation.

```python
>>> cfg = OmegaConf.create(
...     {
...         "workers": {
...             "node3": "10.0.0.2",
...             "node7": "10.0.0.9",
...         },
...         "nodes": "${oc.dict.keys: workers}",
...         "ips": "${oc.dict.values: workers}",
...     }
... )
>>> # Keys are copied from the DictConfig:
>>> show(cfg.nodes)
type: ListConfig, value: ['node3', 'node7']
>>> # Values are dynamically fetched through interpolations:
>>> show(cfg.ips)
type: ListConfig, value: ['${workers.node3}', '${workers.node7}']
>>> assert cfg.ips == ["10.0.0.2", "10.0.0.9"]
```

--------------------------------

### Convert to Object

Source: https://omegaconf.readthedocs.io/en/2.3_branch/_sources/usage.rst.txt

Recursively converts config objects to plain Python types, resolving interpolations and instantiating Structured Configs.

```python
>>> container = OmegaConf.to_object(conf)
>>> show(container)
type: dict, value: {'structured_config': MyConfig(port=80, host='localhost')}
>>> show(container["structured_config"])
type: MyConfig, value: MyConfig(port=80, host='localhost')
```

--------------------------------

### Set configuration value implementation

Source: https://omegaconf.readthedocs.io/en/2.3_branch/_modules/omegaconf/omegaconf.html

Internal logic for setting values in a configuration container, handling structural overrides and merging.

```python
        assert isinstance(
            root, Container
        ), f"Unexpected type for root: {type(root).__name__}"

        last_key: Union[str, int] = last
        if isinstance(root, ListConfig):
            last_key = int(last)

        ctx = flag_override(root, "struct", False) if force_add else nullcontext()
        with ctx:
            if merge and (OmegaConf.is_config(value) or is_primitive_container(value)):
                assert isinstance(root, BaseContainer)
                node = root._get_child(last_key)
                if OmegaConf.is_config(node):
                    assert isinstance(node, BaseContainer)
                    node.merge_with(value)
                    return

            if OmegaConf.is_dict(root):
                assert isinstance(last_key, str)
                root.__setattr__(last_key, value)
            elif OmegaConf.is_list(root):
                assert isinstance(last_key, int)
                root.__setitem__(last_key, value)
            else:
                assert False
```

--------------------------------

### Configure Read-Only and Struct Flags

Source: https://omegaconf.readthedocs.io/en/2.3_branch/_modules/omegaconf/omegaconf.html

Methods to toggle and check the read-only and struct status of configuration nodes.

```python
@staticmethod
def set_readonly(conf: Node, value: Optional[bool]) -> None:
    # noinspection PyProtectedMember
    conf._set_flag("readonly", value)
```

```python
@staticmethod
def is_readonly(conf: Node) -> Optional[bool]:
    # noinspection PyProtectedMember
    return conf._get_flag("readonly")
```

```python
@staticmethod
def set_struct(conf: Container, value: Optional[bool]) -> None:
    # noinspection PyProtectedMember
    conf._set_flag("struct", value)
```

```python
@staticmethod
def is_struct(conf: Container) -> Optional[bool]:
    # noinspection PyProtectedMember
    return conf._get_flag("struct")
```

--------------------------------

### Save configuration to file

Source: https://omegaconf.readthedocs.io/en/2.3_branch/_modules/omegaconf/omegaconf.html

Saves a DictConfig or ListConfig object to a file, optionally resolving variables.

```python
@staticmethod
def save(
    config: Any, f: Union[str, pathlib.Path, IO[Any]], resolve: bool = False
) -> None:
    """
    Save as configuration object to a file

    :param config: omegaconf.Config object (DictConfig or ListConfig).
    :param f: filename or file object
    :param resolve: True to save a resolved config (defaults to False)
    """
    if is_dataclass(config) or is_attr_class(config):
        config = OmegaConf.create(config)
    data = OmegaConf.to_yaml(config, resolve=resolve)
    if isinstance(f, (str, pathlib.Path)):
        with io.open(os.path.abspath(f), "w", encoding="utf-8") as file:
            file.write(data)
    elif hasattr(f, "write"):
        f.write(data)
        f.flush()
    else:
        raise TypeError("Unexpected file type")
```

--------------------------------

### omegaconf.open_dict

Source: https://omegaconf.readthedocs.io/en/2.3_branch/api_reference.html

Context manager to allow modification of a dictionary-like configuration container.

```APIDOC
## omegaconf.open_dict()

### Description
Allows adding new keys to a configuration container that is otherwise restricted.
```

--------------------------------

### OmegaConf.to_yaml

Source: https://omegaconf.readthedocs.io/en/2.3_branch/api_reference.html

Returns a YAML dump of the provided config object.

```APIDOC
## OmegaConf.to_yaml(cfg, resolve=False, sort_keys=False)

### Description
Returns a YAML dump of this config object.

### Parameters
- **cfg** (Any) - Required - Config object, Structured Config type or instance.
- **resolve** (bool) - Optional - If True, will return a string with the interpolations resolved, otherwise interpolations are preserved.
- **sort_keys** (bool) - Optional - If True, will print dict keys in sorted order. Default False.

### Returns
A string containing the yaml representation.
```

--------------------------------

### Accessing underlying configuration type

Source: https://omegaconf.readthedocs.io/en/2.3_branch/_sources/structured_config.rst.txt

Demonstrates how to retrieve the actual type of a configuration object using OmegaConf.get_type().

```python
>>> type(conf).__name__
'DictConfig'

>>> OmegaConf.get_type(conf).__name__
'SimpleTypes'
```

--------------------------------

### Working with Frozen Classes

Source: https://omegaconf.readthedocs.io/en/2.3_branch/_sources/structured_config.rst.txt

Frozen dataclasses are supported, making the configuration node and its children read-only.

```python
>>> from dataclasses import dataclass, field
>>> from typing import List
>>> @dataclass(frozen=True)
... class FrozenClass:
...     x: int = 10
...     list: List = field(default_factory=lambda: [1, 2, 3])

>>> conf = OmegaConf.structured(FrozenClass)
>>> with raises(ReadonlyConfigError):
...    conf.x = 20
```

```python
>>> with raises(ReadonlyConfigError):
...    conf.list[0] = 20
```

--------------------------------

### Define Interpolation Helpers

Source: https://omegaconf.readthedocs.io/en/2.3_branch/_modules/omegaconf/omegaconf.html

Utility functions for creating interpolation strings within configuration values.

```python
def II(interpolation: str) -> Any:
    """
    Equivalent to ``${interpolation}``

    :param interpolation:
    :return: input ``${node}`` with type Any
    """
    return "${" + interpolation + "}"
```

```python
def SI(interpolation: str) -> Any:
    """
    Use this for String interpolation, for example ``"http://${host}:${port}"``

    :param interpolation: interpolation string
    :return: input interpolation with type ``Any``
    """
    return interpolation
```

--------------------------------

### Apply Field Modifiers

Source: https://omegaconf.readthedocs.io/en/2.3_branch/_sources/structured_config.rst.txt

Demonstrates the use of MISSING and Optional modifiers within a structured configuration.

```python
>>> from typing import Optional
>>> from omegaconf import MISSING

>>> @dataclass
... class Modifiers:
...     num: int = 10
...     optional_num: Optional[int] = 10
...     another_num: int = MISSING
...     optional_dict: Optional[Dict[str, int]] = None
...     list_optional: List[Optional[int]] = field(default_factory=lambda: [10, MISSING, None])

>>> conf: Modifiers = OmegaConf.structured(Modifiers)
```

--------------------------------

### Saving and loading YAML

Source: https://omegaconf.readthedocs.io/en/2.3_branch/_sources/usage.rst.txt

Persists configuration to a YAML file; note that type information is not preserved.

```python
>>> conf = OmegaConf.create({"foo": 10, "bar": 20, 123: 456})
>>> with tempfile.NamedTemporaryFile() as fp:
...     OmegaConf.save(config=conf, f=fp.name)
...     loaded = OmegaConf.load(fp.name)
...     assert conf == loaded
```

--------------------------------

### Accessing Environment Variables with oc.env

Source: https://omegaconf.readthedocs.io/en/2.3_branch/_sources/custom_resolvers.rst.txt

Use oc.env to interpolate environment variables into configuration values, with optional fallback defaults.

```python
>>> conf = OmegaConf.load('source/env_interpolation.yaml')
>>> conf.user.name
'omry'
>>> conf.user.home
'/home/omry'
```

```python
>>> cfg = OmegaConf.create(
...     {
...         "database": {
...             "password1": "${oc.env:DB_PASSWORD,password}",
...             "password2": "${oc.env:DB_PASSWORD,12345}",
...             "password3": "${oc.env:DB_PASSWORD,null}",
...         },
...     }
... )
>>> # default is already a string
>>> show(cfg.database.password1)
type: str, value: 'password'
>>> # default is converted to a string automatically
>>> show(cfg.database.password2)
type: str, value: '12345'
>>> # unless it's None
>>> show(cfg.database.password3)
type: NoneType, value: None
```

--------------------------------

### Registering a new resolver

Source: https://omegaconf.readthedocs.io/en/2.3_branch/custom_resolvers.html

The signature for registering a custom resolver function.

```python
def register_new_resolver(
    name: str,
    resolver: Resolver,
    *,
    replace: bool = False,
    use_cache: bool = False,
) -> None
```

--------------------------------

### Retrieve missing keys

Source: https://omegaconf.readthedocs.io/en/2.3_branch/_sources/usage.rst.txt

Returns a set of all keys in the configuration that are currently set to missing (???).

```python
>>> missings = OmegaConf.missing_keys({
...     "foo": {"bar": "???"},
...     "missing": "???",
...      "list": ["a", None, "???"]
... })
>>> assert missings == {'list[2]', 'foo.bar', 'missing'}
```

--------------------------------

### Assign subclasses to structured fields

Source: https://omegaconf.readthedocs.io/en/2.3_branch/structured_config.html

Demonstrates that subclasses of the defined type can be assigned to structured configuration fields.

```python
>>> conf.manager = DuperUser()
>>> assert conf.manager.duper == True
```

--------------------------------

### oc.deprecated

Source: https://omegaconf.readthedocs.io/en/2.3_branch/custom_resolvers.html

Mark a configuration node as deprecated and provide a migration path.

```APIDOC
## oc.deprecated

### Description
Enables deprecation warnings when a specific config node is accessed.

### Syntax
`${oc.deprecated:key,message}`

### Parameters
- **key** (string) - Required - The new key to migrate to.
- **message** (string) - Optional - A custom warning message. Defaults to '$OLD_KEY is deprecated. Change your code and config to use '$NEW_KEY''.
```

--------------------------------

### OmegaConf.merge

Source: https://omegaconf.readthedocs.io/en/2.3_branch/api_reference.html

Merges multiple configuration objects into a single one.

```APIDOC
## OmegaConf.merge(*configs) -> Union[ListConfig, DictConfig]

### Description
Merges a list of previously created configuration objects into a single configuration.

### Parameters
- **configs** (Union[DictConfig, ListConfig, Dict, List, Any]) - Required - The input configurations to merge.

### Returns
- **Union[ListConfig, DictConfig]** - The resulting merged configuration object.
```

--------------------------------

### Runtime Type Validation

Source: https://omegaconf.readthedocs.io/en/2.3_branch/_sources/usage.rst.txt

Demonstrates how structured configs enforce type constraints.

```python
>>> conf.port = 42      # Ok, type matches
>>> conf.port = "1080"  # Ok! "1080" can be converted to an int
>>> conf.port = "oops"  # "oops" cannot be converted to an int
Traceback (most recent call last):
...
omegaconf.errors.ValidationError: Value 'oops' could not be converted to Integer
```

--------------------------------

### Caching resolver output

Source: https://omegaconf.readthedocs.io/en/2.3_branch/custom_resolvers.html

Use use_cache=True to cache results based on the input string literals.

```python
>>> import random
>>> random.seed(1234)
>>> OmegaConf.register_new_resolver(
...    "cached", random.randint, use_cache=True
... )
>>> OmegaConf.register_new_resolver("uncached", random.randint)
>>> c = OmegaConf.create(
...     {
...         "uncached": "${uncached:0,10000}",
...         "cached_1": "${cached:0,10000}",
...         "cached_2": "${cached:0, 10000}",
...         "cached_3": "${cached:0,${uncached}}",
...     }
... )
>>> # not the same since the cache is disabled by default
>>> assert c.uncached != c.uncached
>>> # same value on repeated access thanks to the cache
>>> assert c.cached_1 == c.cached_1 == 122
>>> # same input as `cached_1` => same value
>>> assert c.cached_2 == c.cached_1 == 122
>>> # same string literal "${uncached}" => same value
>>> assert c.cached_3 == c.cached_3 == 1192
```

--------------------------------

### omegaconf.II

Source: https://omegaconf.readthedocs.io/en/2.3_branch/api_reference.html

Creates an interpolation equivalent to ${interpolation}.

```APIDOC
## omegaconf.II(interpolation)

### Description
Equivalent to ${interpolation}.

### Parameters
- **interpolation** (Any) - Required - The interpolation string.

### Returns
- (Any) - The input node with type Any.
```

--------------------------------

### Define Structured Config with Dictionaries

Source: https://omegaconf.readthedocs.io/en/2.3_branch/structured_config.html

Use typing.Dict to define dictionary fields in a dataclass, ensuring keys and values adhere to supported types.

```python
>>> from dataclasses import dataclass, field
>>> from typing import Dict
>>> @dataclass
... class DictExample:
...     ints: Dict[str, int] = field(default_factory=lambda: {"a": 10, "b": 20, "c": 30})
...     bools: Dict[str, bool] = field(default_factory=lambda: {"Uno": True, "Zoro": False})
...     users: Dict[str, User] = field(default_factory=lambda: {"omry": User(name="omry")})
```

--------------------------------

### OmegaConf.missing_keys

Source: https://omegaconf.readthedocs.io/en/2.3_branch/_sources/usage.rst.txt

Returns a set of missing keys present in the input configuration.

```APIDOC
## OmegaConf.missing_keys(cfg)

### Description
Returns a set of missing keys present in the input cfg, represented as a dotlist style string.

### Parameters
- **cfg** (Any) - The configuration object to inspect.
```