def setup_external_libs(libs):
    """
    Download external libraries from PyPI to SPARK_EC2_DIR/lib/ and prepend them to our PATH.
    """
    PYPI_URL_PREFIX = "https://pypi.python.org/packages/source"
    SPARK_EC2_LIB_DIR = os.path.join(SPARK_EC2_DIR, "lib")

    if not os.path.exists(SPARK_EC2_LIB_DIR):
        print("Downloading external libraries that spark-ec2 needs from PyPI to {path}...".format(
            path=SPARK_EC2_LIB_DIR
        ))
        print("This should be a one-time operation.")
        os.mkdir(SPARK_EC2_LIB_DIR)

    for lib in libs:
        versioned_lib_name = "{n}-{v}".format(n=lib["name"], v=lib["version"])
        lib_dir = os.path.join(SPARK_EC2_LIB_DIR, versioned_lib_name)

        if not os.path.isdir(lib_dir):
            tgz_file_path = os.path.join(SPARK_EC2_LIB_DIR, versioned_lib_name + ".tar.gz")
            print(" - Downloading {lib}...".format(lib=lib["name"]))
            download_stream = urlopen(
                "{prefix}/{first_letter}/{lib_name}/{lib_name}-{lib_version}.tar.gz".format(
                    prefix=PYPI_URL_PREFIX,
                    first_letter=lib["name"][:1],
                    lib_name=lib["name"],
                    lib_version=lib["version"]
                )
            )
            with open(tgz_file_path, "wb") as tgz_file:
                tgz_file.write(download_stream.read())
            with open(tgz_file_path, "rb") as tar:
                if hashlib.md5(tar.read()).hexdigest() != lib["md5"]:
                    print("ERROR: Got wrong md5sum for {lib}.".format(lib=lib["name"]), file=stderr)
                    sys.exit(1)
            tar = tarfile.open(tgz_file_path)
            tar.extractall(path=SPARK_EC2_LIB_DIR)
            tar.close()
            os.remove(tgz_file_path)
            print(" - Finished downloading {lib}.".format(lib=lib["name"]))
        sys.path.insert(1, lib_dir)
Download external libraries from PyPI to SPARK_EC2_DIR/lib/ and prepend them to our PATH.