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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 hkdf(master_secret, # type: bytes num_keys, # type: int hash_algo='SHA256', # type: str salt=None, # type: Optional[bytes] info=None, # type: Optional[bytes] key_size=DEFAULT_KEY_SIZE # type: int ):
 """ Executes the HKDF key derivation function as described in rfc5869 to derive `num_keys` keys of size `key_size` from the master_secret. :param master_secret: input keying material :param num_keys: the number of keys the kdf should produce :param hash_algo: The hash function used by HKDF for the internal HMAC calls. The choice of hash function defines the maximum length of the output key material. Output bytes <= 255 * hash digest size (in bytes). :param salt: HKDF is defined to operate with and without random salt. This is done to accommodate applications where a salt value is not available. We stress, however, that the use of salt adds significantly to themstrength of HKDF, ensuring independence between different uses of the hash function, supporting "source-independent" extraction, and strengthening the analytical results that back the HKDF design. Random salt differs fundamentally from the initial keying material in two ways: it is non-secret and can be re-used. Ideally, the salt value is a random (or pseudorandom) string of the length HashLen. Yet, even a salt value of less quality (shorter in size or with limited entropy) may still make a significant contribution to the security of the output keying material. :param info: While the 'info' value is optional in the definition of HKDF, it is often of great importance in applications. Its main objective is to bind the derived key material to application- and context-specific information. For example, 'info' may contain a protocol number, algorithm identifiers, user identities, etc. In particular, it may prevent the derivation of the same keying material for different contexts (when the same input key material (IKM) is used in such different contexts). It may also accommodate additional inputs to the key expansion part, if so desired (e.g., an application may want to bind the key material to its length L, thus making L part of the 'info' field). There is one technical requirement from 'info': it should be independent of the input key material value IKM. :param key_size: the size of the produced keys :return: Derived keys """

    try:
        hash_function = _HASH_FUNCTIONS[hash_algo]
    except KeyError:
        msg = "unsupported hash function '{}'".format(hash_algo)
        raise_from(ValueError(msg), None)

    hkdf = HKDF(algorithm=hash_function(),
                length=num_keys * key_size,
                salt=salt,
                info=info,
                backend=default_backend())
    # hkdf.derive returns a block of num_keys * key_size bytes which we
    # divide up into num_keys chunks, each of size key_size
    keybytes = hkdf.derive(master_secret)
    keys = tuple(keybytes[i * key_size:(i + 1) * key_size] for i in range(num_keys))
    return keys