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Solve the following problem using Python, implementing the functions described below, one line at a time
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Description:
 def handle(cls, value, **kwargs):
 """Retrieve an environment variable. For example: # In stacker we would reference the environment variable like this: conf_key: ${envvar ENV_VAR_NAME} You can optionally store the value in a file, ie: $ cat envvar_value.txt ENV_VAR_NAME and reference it within stacker (NOTE: the path should be relative to the stacker config file):
 conf_key: ${envvar file://envvar_value.txt} # Both of the above would resolve to conf_key: ENV_VALUE """

        value = read_value_from_path(value)

        try:
            return os.environ[value]
        except KeyError:
            raise ValueError('EnvVar "{}" does not exist'.format(value))