<SYSTEM_TASK:>
Solve the following problem using Python, implementing the functions described below, one line at a time
<END_TASK>
<USER_TASK:>
Description:
 def _xdr_read_asset(unpacker):
 """Reads a stellar Asset from unpacker"""

    asset = messages.StellarAssetType(type=unpacker.unpack_uint())

    if asset.type == ASSET_TYPE_ALPHA4:
        asset.code = unpacker.unpack_fstring(4)
        asset.issuer = _xdr_read_address(unpacker)

    if asset.type == ASSET_TYPE_ALPHA12:
        asset.code = unpacker.unpack_fstring(12)
        asset.issuer = _xdr_read_address(unpacker)

    return asset