Load YAML as nested objects instead of dictionary in Python

Question

I have a configuration file in YAML that is currently loaded as a dictionary using yaml.safe_load. For convenience in writing my code, I\'d prefer to load it as a set of nes

Answer 1

This can be done, relatively easily, and without changing the input file.

Since the dict PyYAML uses is hard-coded and cannot be patched, you not only have to provide a dict-like class that behaves as you want, you also have to go through the hoops to make PyYAML use that class. I.e. change the SafeConstructor that would normally construct a dict to use that new class, incorporate that in a new Loader and use PyYAML's load to use that Loader:

import sys
import yaml

from yaml.loader import Reader, Scanner, Parser, Composer, SafeConstructor, Resolver

class MyDict(dict):
   def __getattr__(self, name):
       return self[name]

class MySafeConstructor(SafeConstructor):
   def construct_yaml_map(self, node):
       data = MyDict()
       yield data
       value = self.construct_mapping(node)
       data.update(value)

MySafeConstructor.add_constructor(
  u'tag:yaml.org,2002:map', MySafeConstructor.construct_yaml_map)


class MySafeLoader(Reader, Scanner, Parser, Composer, MySafeConstructor, Resolver):
    def __init__(self, stream):
        Reader.__init__(self, stream)
        Scanner.__init__(self)
        Parser.__init__(self)
        Composer.__init__(self)
        MySafeConstructor.__init__(self)
        Resolver.__init__(self)


yaml_str = """\
a: 1
b:
- q: "foo"
  r: 99
  s: 98
- x: "bar"
  y: 97
  z: 96
c:
  d: 7
  e: 8
  f: [9,10,11]
"""

mydict = yaml.load(yaml_str, Loader=MySafeLoader)

print(mydict.b[0].r)

which gives:

99

If you need to be able to handle YAML1.2 you should use ruamel.yaml (disclaimer: I am the author of that package) which makes the above slightly simpler

import ruamel.yaml

# same definitions for yaml_str, MyDict

class MySafeConstructor(ruamel.yaml.constructor.SafeConstructor):
   def construct_yaml_map(self, node):
       data = MyDict()
       yield data
       value = self.construct_mapping(node)
       data.update(value)

MySafeConstructor.add_constructor(
  u'tag:yaml.org,2002:map', MySafeConstructor.construct_yaml_map)


yaml = ruamel.yaml.YAML(typ='safe')
yaml.Constructor = MySafeConstructor
mydict = yaml.load(yaml_str)

print(mydict.b[0].r)

which also gives:

99

(and if your real input is large, should load your data noticably faster)