ruamel.yaml anchors with Roundtriploader/Roundtripdumper

I am trying to load below example yaml file using the ruamel.yaml python package.

- database: dev_db
  <<: &defaults
    adapter: postgres
    host: localhost
    username: postgres
    password: password
  
- database: test_db
  <<: *defaults
- database: prod_db
  <<: *defaults

from pydantic import BaseModel
from ruamel.yaml import YAML

yaml = YAML(typ='rt')
with open('config.yaml', 'r') as file:
    envs = yaml.load(file)

for env in envs:
  print(c)

This generates below output which misses the aliased tags completely. But when I change the typ to 'safe', even the aliased tags are output correctly.

{'database': 'dev_db'}
{'database': 'test_db'}
{'database': 'prod_db'}

I am trying to create Pydantic data models with each entry in the YAML. How to get all the attributes with default loader(rt)?

Although you should be able to specify a mapping as value for a merge key (instead of an alias to
some previously anchored mapping, or a list of such aliases), this doesn't work properly
in ruamel.yaml's round-trip mode for ruamel.yaml<0.17.27:

import sys
import ruamel.yaml

yaml_str = """\
a: 42
<<: {b: 96}
"""
yaml = ruamel.yaml.YAML()
data = yaml.load(yaml_str)
print(data)

gives:

{'a': 42}

This is caused by an incorrect creation of that node, and although the anchor default in your example
is created (otherwise you could not use the alias), the value for that appears to be an empty dict.

The fix for this bug is rather small, but it is in a local function of the flatten_mapping method
of the RoundTripConstructor, so you'll have to provide the full replacement
for that:

from ruamel.yaml.nodes import MappingNode
from ruamel.yaml.constructor import ConstructorError, DuplicateKeyError, DuplicateKeyFutureWarning

def my_flatten_mapping(self, node):
    def constructed(value_node):
        if value_node in self.constructed_objects:
            value = self.constructed_objects[value_node]
        else:
            value = self.construct_object(value_node, deep=True) # << used to be deep=False
        return value

    merge_map_list = []
    index = 0
    while index < len(node.value):
        key_node, value_node = node.value[index]
        if key_node.tag == 'tag:yaml.org,2002:merge':
            if merge_map_list:  # double << key
                if self.allow_duplicate_keys:
                    del node.value[index]
                    index += 1
                    continue
                args = [
                    'while constructing a mapping',
                    node.start_mark,
                    f'found duplicate key "{key_node.value}"',
                    key_node.start_mark,
                    """
                    To suppress this check see:
                       http://yaml.readthedocs.io/en/latest/api.html#duplicate-keys
                    """,
                    """\
                    Duplicate keys will become an error in future releases, and are errors
                    by default when using the new API.
                    """,
                ]
                if self.allow_duplicate_keys is None:
                    warnings.warn(DuplicateKeyFutureWarning(*args), stacklevel=1)
                else:
                    raise DuplicateKeyError(*args)
            del node.value[index]
            if isinstance(value_node, MappingNode):
                merge_map_list.append((index, constructed(value_node)))
            elif isinstance(value_node, SequenceNode):
                for subnode in value_node.value:
                    if not isinstance(subnode, MappingNode):
                        raise ConstructorError(
                            'while constructing a mapping',
                            node.start_mark,
                            f'expected a mapping for merging, but found {subnode.id!s}',
                            subnode.start_mark,
                        )
                    merge_map_list.append((index, constructed(subnode)))
            else:
                raise ConstructorError(
                    'while constructing a mapping',
                    node.start_mark,
                    'expected a mapping or list of mappings for merging, '
                    f'but found {value_node.id!s}',
                    value_node.start_mark,
                )
        elif key_node.tag == 'tag:yaml.org,2002:value':
            key_node.tag = 'tag:yaml.org,2002:str'
            index += 1
        else:
            index += 1
    return merge_map_list


yaml = ruamel.yaml.YAML()
yaml.Constructor.flatten_mapping = my_flatten_mapping

data = yaml.load("""\
a: 42
<<: {b: 96}
""")
print(data)
print('=' * 20)

data = yaml.load("""\
- database: dev_db
  <<: &defaults
    adapter: postgres
    host: localhost
    username: postgres
    password: password
  
- database: test_db
  <<: *defaults
- database: prod_db
  <<: *defaults
""")

for d in data:
    print(d)

gives:

{'a': 42, 'b': 96}
====================
{'database': 'dev_db', 'adapter': 'postgres', 'host': 'localhost', 'username': 'postgres', 'password': 'password'}
{'database': 'test_db', 'adapter': 'postgres', 'host': 'localhost', 'username': 'postgres', 'password': 'password'}
{'database': 'prod_db', 'adapter': 'postgres', 'host': 'localhost', 'username': 'postgres', 'password': 'password'}

The fix for this is in ruamel.yaml>=0.17.27