PyYaml parsing of the Environment variable in the Yaml configuration file

Question

I need to read follow yaml-formatted configuration file:

version: 1
disable_existing_loggers: False
formatters:
  precise:
    format: "%(name)-15s # %(levelname)-8s # %(asctime)s # [Line: %(lineno)-3d]: %(message)s"
    datefmt: "%Y-%m-%d %H:%M:%S"
handlers:
  file:
    class:        logging.handlers.RotatingFileHandler
    filename:     <%= ENV['ENV_PROJECT'] %>/target/tracing.log
    encoding:     utf-8
    maxBytes :    10737418244
    backupCount:  7
    formatter:    precise
loggers:
  utility:
    handlers:     [file]
    level:        INFO
    propagate:    True
root:
  handlers:       [file]
  level:          INFO

But, instead off <%= ENV['ENV_PROJECT'] %> in the result string I need to get the relevant path, for example. For the loading this file I use follow code:

from yaml import load
with open('test.yml', 'rt') as stream:
    configuration = load(stream)

How can I get the required result? Tnx.

Oisin

Answer 1

You need to use 'resolver' and 'constructor' to achieve this. Here is one way to achieve this.

import yaml, os, re

#define the regex pattern that the parser will use to 'implicitely' tag your node
pattern = re.compile( r'^\<%= ENV\[\'(.*)\'\] %\>(.*)$' )

#now define a custom tag ( say pathex ) and associate the regex pattern we defined
yaml.add_implicit_resolver ( "!pathex", pattern )

#at this point the parser will associate '!pathex' tag whenever the node matches the pattern

#you need to now define a constructor that the parser will invoke
#you can do whatever you want with the node value
def pathex_constructor(loader,node):
  value = loader.construct_scalar(node)
  envVar, remainingPath = pattern.match(value).groups()
  return os.environ[envVar] + remainingPath

#'register' the constructor so that the parser will invoke 'pathex_constructor' for each node '!pathex'
yaml.add_constructor('!pathex', pathex_constructor)

#that is it

data = """
version: 1
disable_existing_loggers: False
formatters:
  precise:
    format: "%(name)-15s # %(levelname)-8s # %(asctime)s # [Line: %(lineno)-3d]: %(message)s"
    datefmt: "%Y-%m-%d %H:%M:%S"
handlers:
  file:
    class:        logging.handlers.RotatingFileHandler
    filename:     <%= ENV['ENV_PROJECT'] %>/target/tracing.log 
    encoding:     utf-8
    maxBytes :    10737418244
    backupCount:  7
    formatter:    precise
loggers:
  utility:
    handlers:     [file]
    level:        INFO
    propagate:    True
root:
  handlers:       [file]
  level:          INFO
"""

deSerializedData = yaml.load(data)

print(deSerializedData [ 'handlers'] [ 'file' ] ['filename'] )

Peltier

Thanks for answer, though I'm facing issues with int, where let say for backupCount in above example, it would come as '7' instead of 7. How did you resolve it? @Peltier – Athens