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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 get_config_file():
 """ Returns the file name of the config file from which the environment variables are written. """

    import os
    from os.path import abspath, expanduser, join, exists

    __lib_name = get_library_name()

    assert __lib_name in ["sframe", "turicreate"]

    __default_config_path = join(expanduser("~"), ".%s" % __lib_name, "config")

    if "TURI_CONFIG_FILE" in os.environ:
        __default_config_path = abspath(expanduser(os.environ["TURI_CONFIG_FILE"]))

        if not exists(__default_config_path):
            print(("WARNING: Config file specified in environment variable "
                   "'TURI_CONFIG_FILE' as "
                   "'%s', but this path does not exist.") % __default_config_path)

    return __default_config_path