How to Parse YAML Using PyYAML if there are '!' within the YAML

Question

I have a YAML file that I'd like to parse the description variable only; however, I know that the exclamation points in my CloudFormation template (YAML file) are giving PyYAML trouble.

I am receiving the following error:

yaml.constructor.ConstructorError: could not determine a constructor for the tag '!Equals'

The file has many !Ref and !Equals. How can I ignore these constructors and get a specific variable I'm looking for -- in this case, the description variable.

Answer 1

You can define a custom constructors using a custom yaml.SafeLoader

import yaml

doc = '''
Conditions: 
  CreateNewSecurityGroup: !Equals [!Ref ExistingSecurityGroup, NONE]
'''

class Equals(object):
    def __init__(self, data):
        self.data = data
    def __repr__(self):
        return "Equals(%s)" % self.data

class Ref(object):
    def __init__(self, data):
        self.data = data
    def __repr__(self):
        return "Ref(%s)" % self.data

def create_equals(loader,node):
    value = loader.construct_sequence(node)
    return Equals(value)

def create_ref(loader,node):
    value = loader.construct_scalar(node)
    return Ref(value)

class Loader(yaml.SafeLoader):
    pass

yaml.add_constructor(u'!Equals', create_equals, Loader)
yaml.add_constructor(u'!Ref', create_ref, Loader)
a = yaml.load(doc, Loader)
print(a)

Outputs:

{'Conditions': {'CreateNewSecurityGroup': Equals([Ref(ExistingSecurityGroup), 'NONE'])}}