Parse nested custom yaml tags

I have some yaml with application-specific tags (from an AWS Cloud Formation template, to be exact) that looks like this:

example_yaml = "Name: !Join [' ', ['EMR', !Ref 'Environment', !Ref 'Purpose']]"

I want to parse it so that I can do this:

>>> print(result)
>>> {'Name': 'EMR {Environment} {Purpose}'}

>>> name = result['name'].format(
...    Environment='Development',
...    Purpose='ETL'
... )
>>> print(name)
>>> EMR Development ETL

Currently my code looks like this:

import yaml
from pprint import pprint


def aws_join(loader, node):
    join_args = loader.construct_yaml_seq(node)
    delimiter = list(join_args)[0]
    joinables = list(join_args)[1]
    join_result = delimiter.join(joinables)
    return join_result

def aws_ref(loader, node):
    value = loader.construct_scalar(node)
    placeholder = '{'+value+'}'
    return placeholder

yaml.add_constructor('!Join', aws_join)
yaml.add_constructor('!Ref', aws_ref)

example_yaml = "Name: !Join [' ', ['EMR', !Ref 'Environment', !Ref 'Purpose']]"

pprint(yaml.load(example_yaml))

Unfortunately this results in an error.

...
   joinables = list(join_args)[1]
IndexError: list index out of range

Adding print('What I am: '+str(join_args)) to aws_join shows that I'm getting a generator:

What I am: <generator object SafeConstructor.construct_yaml_seq at 0x1082ece08>

That's why I tried to cast the generator as a list. The generator eventually populates correctly though, just not in time for me to use it. If I change my aws_join function to like this:

def aws_join(loader, node):
    join_args = loader.construct_yaml_seq(node)
    return join_args

Then the final result looks like this:

{'Name': [' ', ['EMR', '{Environment}', '{Purpose}']]}

So the required pieces to my function are there, just not when I need them in my function.

Solution

You are close, but the problem is that you are using the method construct_yaml_seq(). That method is actually a registered constructor for the normal YAML sequence (the one that eventually makes a Python list) and it calls the construct_sequence() method to handle the node that gets passed in, and that is what you should do as well.

As you are returning a string, which cannot deal with recursive data structures, you don't need to use the two step creation process (first yield-ing, then filling out) which the construct_yaml_seq() method follows. But this two step creation process is why you encountered a generator.

construct_sequence returns a simple list, but as you want the nodes underneath the !Join available when you start processing, make sure to specify the deep=True parameter, otherwise the second list element will be an empty list. And because construct_yaml_seq(), doesn't specify deep=True, you did not get the pieces in time in your function (otherwise you could have actually used that method).

import yaml
from pprint import pprint


def aws_join(loader, node):
    join_args = loader.construct_sequence(node, deep=True)
    # you can comment out next line
    assert join_args == [' ', ['EMR', '{Environment}', '{Purpose}']] 
    delimiter = join_args[0]
    joinables = join_args[1]
    return delimiter.join(joinables)

def aws_ref(loader, node):
    value = loader.construct_scalar(node)
    placeholder = '{'+value+'}'
    return placeholder

yaml.add_constructor('!Join', aws_join, Loader=yaml.SafeLoader)
yaml.add_constructor('!Ref', aws_ref, Loader=yaml.SafeLoader)

example_yaml = "Name: !Join [' ', ['EMR', !Ref 'Environment', !Ref 'Purpose']]"

pprint(yaml.safe_load(example_yaml))

which gives:

{'Name': 'EMR {Environment} {Purpose}'}

You should not use load(), it is documented to be potentially unsafe, and above all: it is not necessary here. Register with the SafeLoader and call safe_load()