Create objects of custom class from YAML in Python

Question

I have Class definition:

Class definition

class Task(object):
    def __init__(self, name, *depends):
        self.__name    = name
        self.__depends = set(depends)

    @property
    def name(self):
        return self.__name

    @property
    def depends(self):
        return self.__depends

The rest of the code is using sample definition like this:

a = Task("a", "b")
b = Task("b")

nodes = (a, b)

I am trying to create this objects dynamically by parsing YAML in separate function which returns dictionary data and then I create all the objects:

Yaml File

a:
  foo:
   - "bar"
  graph_dep:
  - "b"
b:
  foo:
   - "bar"

I believe I was able to achieve creation of a and b objects of a Task class via function

def format_input(data):
    services = data.keys()
    holder = {Task(name=name) for name in services}
    return holder

q1: How do I combine them to get nodes object and have same result as using sample definitions? q2: How do I get values from graph_dep and add them in holder = {Task(name=name) for name in services} string?

Sorry newbie question in Python :)

Whats the foo: and - "bar" parts in the YAML? They don't occur in the Python code you show. — flyx
– flyx, Commented Dec 10, 2020 at 19:16
foo and bar are not used... All I need is to parse yaml and extract keys - those are services, graph_dep are dependencies, the script maps dependencies based on these values — Anton Teterine
– Anton Teterine, Commented Dec 10, 2020 at 19:27

flyx · Accepted Answer · 2020-12-10 19:59:53Z

Generally, if you don't need the YAML's original structure, it is unnecessary to load it into native Python types. Instead, you should only load them into a YAML node graph and then load the native structure you want to have from that:

import yaml

input = """
a:
  foo:
   - "bar"
  graph_dep:
  - "b"
b:
  foo:
   - "bar"
"""

class Task(object):
  def __init__(self, name, *depends):
    self.__name    = name
    self.__depends = set(depends)

  @property
  def name(self):
    return self.__name

  @property
  def depends(self):
    return self.__depends

  def __str__(self):
    return "Task({}, {})".format(self.__name, self.__depends)

def load_dependency_list(loader, node):
  for item in node.value:
    if item[0].value == "graph_dep":
      return loader.construct_sequence(item[1])
  return []

def load_tasks(loader, node):
  ret = ()
  for item in node.value:
    name = loader.construct_scalar(item[0])
    deplist = load_dependency_list(loader, item[1])
    ret += (Task(name, *deplist),)
  return ret

loader = yaml.SafeLoader(input)
nodes = load_tasks(loader, loader.get_single_node())
for node in nodes:
  print(node)

What this code does is:

getting the node graph of the single document in the YAML file via get_single_node()
on the returned root node, execute your loading code

load_tasks and load_dependency_list handle the levels in the YAML. PyYAML does provide facilities to register these functions as constructors, however that only makes sense if you have tags in your YAML input (like e.g. --- !nodes as first line). Without tags, PyYAML cannot automatically map constructors so you need to implement the construction process yourself.

In this case, load_dependency_list constructs the list contained in graph_dep or returns the empty list, ignoring other things in the YAML node. load_tasks iterates over the top level items, constructing Task object from each of them, and concatenates them into a tuple.

I added a __str__ method to Task for output. The output is

Task(a, {'b'})
Task(b, set())

This code has no safeguards whatsoever and I advise to check for proper node types whereever you access a YAML node so that the user gets a good error message if the structure is wrong.

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