PyYaml parsing an environment variable in Yaml configuration file

Question

I need to read the following configuration file in yaml format:

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, <% = ENV ['ENV_PROJECT']%> in the result line I need, for example, to get the corresponding path. To download this file, I use the following code:

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

How can I get the desired result? Tnx.

Answer 1

For this you need to use "resolver" and "constructor". 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'] )