How to validate yaml file from python script?

All three examples are valid YAML. They just represent different data. What you probably want to do is to validate the actual structure against your expectation. Since there is no standard way to define the structure of a YAML document (like e.g. JSON Schema for JSON), there is no standard way to validate it.

That being said, you can use Python's typing package for defining the desired structure and then the typeguard package to validate the loaded structure against it:

from typing import TypedDict, List
from typeguard import check_type
import yaml

class PipelineItem(TypedDict):
  name: str

class MyYamlSchema(TypedDict):
  pipelines: List[PipelineItem]

file = """
pipelines:
- name: some_name
"""

data = yaml.safe_load(file)
check_type("data", data, MyYamlSchema)

This will raise an exception for the second and third case.

This solution requires Python 3.8.