disabling aliases in pyYaml

Lately I’ve been working with a python script to act as an external node classifier for Puppet. Puppet expects the output of the ENC to be in YAML, so naturally I reached for PyYAML. Everything was going great, but then I tried to put the same bit of information in both the classes AND parameters section. See this script for example:

import yaml

services = {
        'enabled': ['nginx', 'webapp'],
        'disabled': ['postgres'],
}

data = {'classes': {'services': services}, 'parameters': services}

print yaml.safe_dump(data, default_flow_style=False)

When you run this script you end up with the following output:

classes:
  services: &id001
    disabled:
    - postgres
    enabled:
    - nginx
    - webapp
parameters: \*id001

The issue with this is the use of aliases. Basically since the data I had in the ‘services’ dictionary was the same as what I had in parameters pyYAML recognized that and created the alias &id001 pointing at that data, then referenced it rather than copying that data into parameters.

Unfortunately the version of puppet that I am running (2.7.11, the default in Ubuntu 12.04) can’t handle aliases, even though they’re a definitely a part of the YAML syntax. Honestly in most cases I’d be super happy that PyYAML was smart enough to do this for me automatically. But in this case it was causing issues.

After digging around, I found out a way to work around it. Unfortunately it’s not as simple as an argument to safe_dump. Instead you need to provide your own instance of a Dumper object, and then override ignore_aliases so that it always returns True.

Here’s the updated script:

import yaml

services = {
        'enabled': ['nginx', 'webapp'],
        'disabled': ['postgres'],
}

data = {'classes': {'services': services}, 'parameters': services}

noalias_dumper = yaml.dumper.SafeDumper
noalias_dumper.ignore_aliases = lambda self, data: True
print yaml.dump(data, default_flow_style=False, Dumper=noalias_dumper)

All the changes are in the last three lines. First I create an object which is a copy of the SafeDumper class. Then I patch the ignore_aliases method on that object to always return True. Finally I use dump instead of safe_dump (safe_dump just calls dump, but specifies the SafeDumper) and pass along the newly created dumper which is based on SafeDumper. In the end, this is the new output:

classes:
  services:
    disabled:
    - postgres
    enabled:
    - nginx
    - webapp
parameters:
  disabled:
  - postgres
  enabled:
  - nginx
  - webapp

And now puppet can read it just fine.