import json
try:
    #py3
    from urllib.request import urlopen

except ImportError:
    #py2
    from urllib2 import urlopen

class EC2InstanceIdentity(object):
    """
    Assists in discovery EC2 instance identity info
    Ref: http://docs.aws.amazon.com/AWSEC2/latest/WindowsGuide/instance-identity-documents.html
    """

    METADATA_URL = 'http://169.254.169.254/latest/dynamic/instance-identity/document'
    """What URL to hit for the instance identity document"""

    def __init__(self):
        self._json_document = None

    def __fetch_document(self):
        """
        Fetches the instance identity document & parses it
        """
        raw_document = urlopen(self.METADATA_URL).read()
        self._json_document = json.loads(raw_document)

    def __getattr__(self, attr_name):
        if not self._json_document:
            self.__fetch_document()

        return self._json_document[attr_name]