《区块链编程》第五章

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文章目录

《区块链编程》第五章

交易

交易的组成

  1. 版本号
  2. 输入
    i. 父交易的id
    ii. 父交易output的序号
    iii. 签名脚本
    iv. 序列号
  3. 输出
    i. amount
    ii. 公钥脚本
  4. 时间锁

练习1

p88

代码实现

from helper import little_endian_to_int
from io import BytesIO


class Tx:

    def __init__(self, version, tx_ins, tx_outs, locktime, testnet=False):
        self.version = version
        self.tx_ins = tx_ins  # <1>
        self.tx_outs = tx_outs
        self.locktime = locktime
        self.testnet = testnet  # <2>

    def __repr__(self):
        tx_ins = ''
        for tx_in in self.tx_ins:
            tx_ins += tx_in.__repr__() + 'n'
        tx_outs = ''
        for tx_out in self.tx_outs:
            tx_outs += tx_out.__repr__() + 'n'
        return 'tx: {}nversion: {}ntx_ins:n{}tx_outs:n{}locktime: {}'.format(
            self.id(),
            self.version,
            tx_ins,
            tx_outs,
            self.locktime,
        )

    def id(self):  # <3>
        '''Human-readable hexadecimal of the transaction hash'''
        return self.hash().hex()

    def hash(self):  # <4>
        '''Binary hash of the legacy serialization'''
        return hash256(self.serialize())[::-1]
    # end::source1[]

    @classmethod
    def parse(cls, s, testnet=False):
        '''Takes a byte stream and parses the transaction at the start
        return a Tx object
        '''
        version = little_endian_to_int(s.read(4))
        return cls(version, None, None, None, testnet=testnet)
if __name__ == '__main__':
    tx = '01000000'
    stream = BytesIO(bytes.fromhex(tx))
    print(Tx.parse(stream).version)

测试

1
[Finished in 306ms]

练习2

p92

代码实现

# -*- coding: utf-8 -*-
# @Author: 从化北(喵星人)
# @Date:   2022-01-04 15:38:18
# @Last Modified by:   从化北
# @Last Modified time: 2022-01-04 16:33:55
from helper import little_endian_to_int, read_varint
from io import BytesIO
from script import Script


class Tx:

    def __init__(self, version, tx_ins, tx_outs, locktime, testnet=False):
        self.version = version
        self.tx_ins = tx_ins  #
        self.tx_outs = tx_outs
        self.locktime = locktime
        self.testnet = testnet  # 


    @classmethod
    def parse(cls, s, testnet=False):
        '''Takes a byte stream and parses the transaction at the start
        return a Tx object
        '''
        # BytesIO的read会改变游标的位置。但不会更改BytesIO的数据
        version = little_endian_to_int(s.read(4))
        num_inputs = read_varint(s)
        inputs = []
        for _ in range(num_inputs):
            inputs.append(TxIn.parse(s))
        return cls(version, inputs, None, None, testnet=testnet)

class TxIn:
    def __init__(self, prev_tx, prev_index, script_sig=None, sequence=0xffffffff):
        self.prev_tx = prev_tx
        self.prev_index = prev_index
        if script_sig is None:  # <1>
            self.script_sig = Script()
        else:
            self.script_sig = script_sig
        self.sequence = sequence

    def __repr__(self):
        return '{}:{}'.format(
            self.prev_tx.hex(),
            self.prev_index,
        )
    # end::source2[]

    @classmethod
    def parse(cls, s):
        '''Takes a byte stream and parses the tx_input at the start
        return a TxIn object
        '''
        prev_tx = s.read(32)[::-1]
        prev_index = little_endian_to_int(s.read(4))
        script_sig = Script.parse(s)
        sequence = little_endian_to_int(s.read(4))
        return cls(prev_tx, prev_index, script_sig, sequence)


if __name__ == '__main__':
    raw_tx = bytes.fromhex('0100000001813f79011acb80925dfe69b3def355fe914bd1d96a3f5f71bf8303c6a989c7d1000000006b483045022100ed81ff192e75a3fd2304004dcadb746fa5e24c5031ccfcf21320b0277457c98f02207a986d955c6e0cb35d446a89d3f56100f4d7f67801c31967743a9c8e10615bed01210349fc4e631e3624a545de3f89f5d8684c7b8138bd94bdd531d2e213bf016b278afeffffff02a135ef01000000001976a914bc3b654dca7e56b04dca18f2566cdaf02e8d9ada88ac99c39800000000001976a9141c4bc762dd5423e332166702cb75f40df79fea1288ac19430600')
    stream = BytesIO(raw_tx)
    tx = Tx.parse(stream)
    print(tx.tx_ins)

测试

[d1c789a9c60383bf715f3f6ad9d14b91fe55f3deb369fe5d9280cb1a01793f81:0]
[Finished in 309ms]

练习3

p94

代码实现

# -*- coding: utf-8 -*-
# @Author: 从化北(喵星人)
# @Date:   2022-01-04 16:58:44
# @Last Modified by:   从化北
# @Last Modified time: 2022-01-04 17:03:18
from helper import little_endian_to_int, read_varint
from io import BytesIO
from script import Script


class Tx:

    def __init__(self, version, tx_ins, tx_outs, locktime, testnet=False):
        self.version = version
        self.tx_ins = tx_ins  #
        self.tx_outs = tx_outs
        self.locktime = locktime
        self.testnet = testnet  #

    @classmethod
    def parse(cls, s, testnet=False):
        '''Takes a byte stream and parses the transaction at the start
        return a Tx object
        '''
        # BytesIO的read会改变游标的位置。但不会更改BytesIO的数据
        version = little_endian_to_int(s.read(4))
        num_inputs = read_varint(s)
        inputs = []
        for _ in range(num_inputs):
            inputs.append(TxIn.parse(s))
        num_outputs = read_varint(s)
        outputs = []
        for _ in range(num_outputs):
            outputs.append(TxOut.parse(s))
        return cls(version, inputs, outputs, None, testnet=testnet)


class TxIn:
    def __init__(self, prev_tx, prev_index, script_sig=None, sequence=0xffffffff):
        self.prev_tx = prev_tx
        self.prev_index = prev_index
        if script_sig is None:  # <1>
            self.script_sig = Script()
        else:
            self.script_sig = script_sig
        self.sequence = sequence

    def __repr__(self):
        return '{}:{}'.format(
            self.prev_tx.hex(),
            self.prev_index,
        )
    # end::source2[]

    @classmethod
    def parse(cls, s):
        '''Takes a byte stream and parses the tx_input at the start
        return a TxIn object
        '''
        prev_tx = s.read(32)[::-1]
        prev_index = little_endian_to_int(s.read(4))
        script_sig = Script.parse(s)
        sequence = little_endian_to_int(s.read(4))
        return cls(prev_tx, prev_index, script_sig, sequence)


class TxOut:

    def __init__(self, amount, script_pubkey):
        self.amount = amount
        self.script_pubkey = script_pubkey

    def __repr__(self):
        return '{}:{}'.format(self.amount, self.script_pubkey)
    # end::source3[]

    @classmethod
    def parse(cls, s):
        '''Takes a byte stream and parses the tx_output at the start
        return a TxOut object
        '''
        amount = little_endian_to_int(s.read(8))
        script_pubkey = Script.parse(s)
        return cls(amount, script_pubkey)
        # amount is an integer in 8 bytes, little endian
        # use Script.parse to get the ScriptPubKey
        # return an instance of the class (see __init__ for args)
        # raise NotImplementedError

    # tag::source4[]
    def serialize(self):  # <1>
        '''Returns the byte serialization of the transaction output'''
        result = int_to_little_endian(self.amount, 8)
        result += self.script_pubkey.serialize()
        return result
    # end::source4[]


if __name__ == '__main__':
    raw_tx = bytes.fromhex('0100000001813f79011acb80925dfe69b3def355fe914bd1d96a3f5f71bf8303c6a989c7d1000000006b483045022100ed81ff192e75a3fd2304004dcadb746fa5e24c5031ccfcf21320b0277457c98f02207a986d955c6e0cb35d446a89d3f56100f4d7f67801c31967743a9c8e10615bed01210349fc4e631e3624a545de3f89f5d8684c7b8138bd94bdd531d2e213bf016b278afeffffff02a135ef01000000001976a914bc3b654dca7e56b04dca18f2566cdaf02e8d9ada88ac99c39800000000001976a9141c4bc762dd5423e332166702cb75f40df79fea1288ac19430600')
    stream = BytesIO(raw_tx)
    tx = Tx.parse(stream)
    # print(tx.tx_ins)
    print(tx.tx_outs)

测试

[32454049:OP_DUP OP_HASH160 bc3b654dca7e56b04dca18f2566cdaf02e8d9ada OP_EQUALVERIFY OP_CHECKSIG, 10011545:OP_DUP OP_HASH160 1c4bc762dd5423e332166702cb75f40df79fea12 OP_EQUALVERIFY OP_CHECKSIG]
[Finished in 319ms]

练习4

p95

代码实现

# -*- coding: utf-8 -*-
# @Author: 从化北(喵星人)
# @Date:   2022-01-04 17:04:41
# @Last Modified by:   从化北
# @Last Modified time: 2022-01-04 17:05:35
from helper import little_endian_to_int, read_varint
from io import BytesIO
from script import Script


class Tx:

    def __init__(self, version, tx_ins, tx_outs, locktime, testnet=False):
        self.version = version
        self.tx_ins = tx_ins  #
        self.tx_outs = tx_outs
        self.locktime = locktime
        self.testnet = testnet  #

    @classmethod
    def parse(cls, s, testnet=False):
        '''Takes a byte stream and parses the transaction at the start
        return a Tx object
        '''
        # BytesIO的read会改变游标的位置。但不会更改BytesIO的数据
        version = little_endian_to_int(s.read(4))
        num_inputs = read_varint(s)
        inputs = []
        for _ in range(num_inputs):
            inputs.append(TxIn.parse(s))
        num_outputs = read_varint(s)
        outputs = []
        for _ in range(num_outputs):
            outputs.append(TxOut.parse(s))
        locktime = little_endian_to_int(s.read(4))
        return cls(version, inputs, outputs, locktime, testnet=testnet)


class TxIn:
    def __init__(self, prev_tx, prev_index, script_sig=None, sequence=0xffffffff):
        self.prev_tx = prev_tx
        self.prev_index = prev_index
        if script_sig is None:  # <1>
            self.script_sig = Script()
        else:
            self.script_sig = script_sig
        self.sequence = sequence

    def __repr__(self):
        return '{}:{}'.format(
            self.prev_tx.hex(),
            self.prev_index,
        )
    # end::source2[]

    @classmethod
    def parse(cls, s):
        '''Takes a byte stream and parses the tx_input at the start
        return a TxIn object
        '''
        prev_tx = s.read(32)[::-1]
        prev_index = little_endian_to_int(s.read(4))
        script_sig = Script.parse(s)
        sequence = little_endian_to_int(s.read(4))
        return cls(prev_tx, prev_index, script_sig, sequence)


class TxOut:

    def __init__(self, amount, script_pubkey):
        self.amount = amount
        self.script_pubkey = script_pubkey

    def __repr__(self):
        return '{}:{}'.format(self.amount, self.script_pubkey)
    # end::source3[]

    @classmethod
    def parse(cls, s):
        '''Takes a byte stream and parses the tx_output at the start
        return a TxOut object
        '''
        amount = little_endian_to_int(s.read(8))
        script_pubkey = Script.parse(s)
        return cls(amount, script_pubkey)
        # amount is an integer in 8 bytes, little endian
        # use Script.parse to get the ScriptPubKey
        # return an instance of the class (see __init__ for args)
        # raise NotImplementedError

    # tag::source4[]
    def serialize(self):  # <1>
        '''Returns the byte serialization of the transaction output'''
        result = int_to_little_endian(self.amount, 8)
        result += self.script_pubkey.serialize()
        return result
    # end::source4[]


if __name__ == '__main__':
    raw_tx = bytes.fromhex('0100000001813f79011acb80925dfe69b3def355fe914bd1d96a3f5f71bf8303c6a989c7d1000000006b483045022100ed81ff192e75a3fd2304004dcadb746fa5e24c5031ccfcf21320b0277457c98f02207a986d955c6e0cb35d446a89d3f56100f4d7f67801c31967743a9c8e10615bed01210349fc4e631e3624a545de3f89f5d8684c7b8138bd94bdd531d2e213bf016b278afeffffff02a135ef01000000001976a914bc3b654dca7e56b04dca18f2566cdaf02e8d9ada88ac99c39800000000001976a9141c4bc762dd5423e332166702cb75f40df79fea1288ac19430600')
    stream = BytesIO(raw_tx)
    tx = Tx.parse(stream)
    # print(tx.tx_ins)
    print(tx.locktime)

运行结果

410393
[Finished in 320ms]

练习5

p95

代码实现

# -*- coding: utf-8 -*-
# @Author: 从化北(喵星人)
# @Date:   2022-01-04 17:06:15
# @Last Modified by:   从化北
# @Last Modified time: 2022-01-04 17:11:16
from helper import little_endian_to_int, read_varint
from io import BytesIO
from script import Script


class Tx:

    def __init__(self, version, tx_ins, tx_outs, locktime, testnet=False):
        self.version = version
        self.tx_ins = tx_ins  #
        self.tx_outs = tx_outs
        self.locktime = locktime
        self.testnet = testnet  #

    @classmethod
    def parse(cls, s, testnet=False):
        '''Takes a byte stream and parses the transaction at the start
        return a Tx object
        '''
        # BytesIO的read会改变游标的位置。但不会更改BytesIO的数据
        version = little_endian_to_int(s.read(4))
        num_inputs = read_varint(s)
        inputs = []
        for _ in range(num_inputs):
            inputs.append(TxIn.parse(s))
        num_outputs = read_varint(s)
        outputs = []
        for _ in range(num_outputs):
            outputs.append(TxOut.parse(s))
        locktime = little_endian_to_int(s.read(4))
        return cls(version, inputs, outputs, locktime, testnet=testnet)


class TxIn:
    def __init__(self, prev_tx, prev_index, script_sig=None, sequence=0xffffffff):
        self.prev_tx = prev_tx
        self.prev_index = prev_index
        if script_sig is None:  # <1>
            self.script_sig = Script()
        else:
            self.script_sig = script_sig
        self.sequence = sequence

    def __repr__(self):
        return '{}:{}'.format(
            self.prev_tx.hex(),
            self.prev_index,
        )
    # end::source2[]

    @classmethod
    def parse(cls, s):
        '''Takes a byte stream and parses the tx_input at the start
        return a TxIn object
        '''
        prev_tx = s.read(32)[::-1]
        prev_index = little_endian_to_int(s.read(4))
        script_sig = Script.parse(s)
        sequence = little_endian_to_int(s.read(4))
        return cls(prev_tx, prev_index, script_sig, sequence)


class TxOut:

    def __init__(self, amount, script_pubkey):
        self.amount = amount
        self.script_pubkey = script_pubkey

    def __repr__(self):
        return '{}:{}'.format(self.amount, self.script_pubkey)
    # end::source3[]

    @classmethod
    def parse(cls, s):
        '''Takes a byte stream and parses the tx_output at the start
        return a TxOut object
        '''
        amount = little_endian_to_int(s.read(8))
        script_pubkey = Script.parse(s)
        return cls(amount, script_pubkey)
        # amount is an integer in 8 bytes, little endian
        # use Script.parse to get the ScriptPubKey
        # return an instance of the class (see __init__ for args)
        # raise NotImplementedError

    # tag::source4[]
    def serialize(self):  # <1>
        '''Returns the byte serialization of the transaction output'''
        result = int_to_little_endian(self.amount, 8)
        result += self.script_pubkey.serialize()
        return result
    # end::source4[]


if __name__ == '__main__':
    raw_tx = bytes.fromhex('010000000456919960ac691763688d3d3bcea9ad6ecaf875df5339e
148a1fc61c6ed7a069e010000006a47304402204585bcdef85e6b1c6af5c2669d4830ff86e42dd
205c0e089bc2a821657e951c002201024a10366077f87d6bce1f7100ad8cfa8a064b39d4e8fe4e
a13a7b71aa8180f012102f0da57e85eec2934a82a585ea337ce2f4998b50ae699dd79f5880e253
dafafb7feffffffeb8f51f4038dc17e6313cf831d4f02281c2a468bde0fafd37f1bf882729e7fd
3000000006a47304402207899531a52d59a6de200179928ca900254a36b8dff8bb75f5f5d71b1c
dc26125022008b422690b8461cb52c3cc30330b23d574351872b7c361e9aae3649071c1a716012
1035d5c93d9ac96881f19ba1f686f15f009ded7c62efe85a872e6a19b43c15a2937feffffff567
bf40595119d1bb8a3037c356efd56170b64cbcc160fb028fa10704b45d775000000006a4730440
2204c7c7818424c7f7911da6cddc59655a70af1cb5eaf17c69dadbfc74ffa0b662f02207599e08
bc8023693ad4e9527dc42c34210f7a7d1d1ddfc8492b654a11e7620a0012102158b46fbdff65d0
172b7989aec8850aa0dae49abfb84c81ae6e5b251a58ace5cfeffffffd63a5e6c16e620f86f375
925b21cabaf736c779f88fd04dcad51d26690f7f345010000006a47304402200633ea0d3314bea
0d95b3cd8dadb2ef79ea8331ffe1e61f762c0f6daea0fabde022029f23b3e9c30f080446150b23
852028751635dcee2be669c2a1686a4b5edf304012103ffd6f4a67e94aba353a00882e563ff272
2eb4cff0ad6006e86ee20dfe7520d55feffffff0251430f00000000001976a914ab0c0b2e98b1a
b6dbf67d4750b0a56244948a87988ac005a6202000000001976a9143c82d7df364eb6c75be8c80
df2b3eda8db57397088ac46430600')
    stream = BytesIO(raw_tx)
    tx = Tx.parse(stream)
    # print(tx.tx_ins)
    print("第二个input的签名脚本：")
    print(tx.tx_ins[1].script_sig)
    print("第一个output的公钥脚本：")
    print(tx.tx_outs[0].script_pubkey)
    print("第二个output的数量：")
    print(tx.tx_outs[1].amount)

运行结果

第二个input的签名脚本：
304402207899531a52d59a6de200179928ca900254a36b8dff8bb75f5f5d71b1cdc26125022008b422690b8461cb52c3cc30330b23d574351872b7c361e9aae3649071c1a71601 035d5c93d9ac96881f19ba1f686f15f009ded7c62efe85a872e6a19b43c15a2937
第一个output的公钥脚本：
OP_DUP OP_HASH160 ab0c0b2e98b1ab6dbf67d4750b0a56244948a879 OP_EQUALVERIFY OP_CHECKSIG
第二个output的数量：
40000000
[Finished in 319ms]

练习6

p98

代码实现

# -*- coding: utf-8 -*-
# @Author: 从化北(喵星人)
# @Date:   2022-01-04 17:12:19
# @Last Modified by:   从化北
# @Last Modified time: 2022-01-04 17:15:35


class Tx:
    ...
    def fee(self, testnet=False):
        input_sum, output_sum = 0, 0
        for tx_in in self.tx_ins:
            input_sum += tx_in.value(testnet=testnet)
        for tx_out in self.tx_outs:
            output_sum += tx_out.amount
        return input_sum - output_sum

运行结果

这部分的fee不给测试了。

无

本章中涉及的tx完整代码

书中给的tx

from io import BytesIO
from unittest import TestCase

import json
import requests

from helper import (
    encode_varint,
    hash256,
    int_to_little_endian,
    little_endian_to_int,
    read_varint,
    run,
)
from script import Script


# tag::source7[]
class TxFetcher:
    cache = {}

    @classmethod
    def get_url(cls, testnet=False):
        if testnet:
            return 'https://blockstream.info/testnet/api/'
        else:
            return 'https://blockstream.info/api/'

    @classmethod
    def fetch(cls, tx_id, testnet=False, fresh=False):
        if fresh or (tx_id not in cls.cache):
            url = '{}/tx/{}/hex'.format(cls.get_url(testnet), tx_id)
            response = requests.get(url)
            try:
                raw = bytes.fromhex(response.text.strip())
            except ValueError:
                raise ValueError('unexpected response: {}'.format(response.text))
            if raw[4] == 0:
                raw = raw[:4] + raw[6:]
                tx = Tx.parse(BytesIO(raw), testnet=testnet)
                tx.locktime = little_endian_to_int(raw[-4:])
            else:
                tx = Tx.parse(BytesIO(raw), testnet=testnet)
            if tx.id() != tx_id:  # <1>
                raise ValueError('not the same id: {} vs {}'.format(tx.id(),
                                                                    tx_id))
            cls.cache[tx_id] = tx
        cls.cache[tx_id].testnet = testnet
        return cls.cache[tx_id]
    # end::source7[]

    @classmethod
    def load_cache(cls, filename):
        disk_cache = json.loads(open(filename, 'r').read())
        for k, raw_hex in disk_cache.items():
            raw = bytes.fromhex(raw_hex)
            if raw[4] == 0:
                raw = raw[:4] + raw[6:]
                tx = Tx.parse(BytesIO(raw))
                tx.locktime = little_endian_to_int(raw[-4:])
            else:
                tx = Tx.parse(BytesIO(raw))
            cls.cache[k] = tx

    @classmethod
    def dump_cache(cls, filename):
        with open(filename, 'w') as f:
            to_dump = {k: tx.serialize().hex() for k, tx in cls.cache.items()}
            s = json.dumps(to_dump, sort_keys=True, indent=4)
            f.write(s)


# tag::source1[]
class Tx:

    def __init__(self, version, tx_ins, tx_outs, locktime, testnet=False):
        self.version = version
        self.tx_ins = tx_ins  # <1>
        self.tx_outs = tx_outs
        self.locktime = locktime
        self.testnet = testnet  # <2>

    def __repr__(self):
        tx_ins = ''
        for tx_in in self.tx_ins:
            tx_ins += tx_in.__repr__() + 'n'
        tx_outs = ''
        for tx_out in self.tx_outs:
            tx_outs += tx_out.__repr__() + 'n'
        return 'tx: {}nversion: {}ntx_ins:n{}tx_outs:n{}locktime: {}'.format(
            self.id(),
            self.version,
            tx_ins,
            tx_outs,
            self.locktime,
        )

    def id(self):  # <3>
        '''Human-readable hexadecimal of the transaction hash'''
        return self.hash().hex()

    def hash(self):  # <4>
        '''Binary hash of the legacy serialization'''
        return hash256(self.serialize())[::-1]
    # end::source1[]

    @classmethod
    def parse(cls, s, testnet=False):
        '''Takes a byte stream and parses the transaction at the start
        return a Tx object
        '''
        version = little_endian_to_int(s.read(4))
        num_inputs = read_varint(s)
        inputs = []
        for _ in range(num_inputs):
            inputs.append(TxIn.parse(s))
        num_outputs = read_varint(s)
        outputs = []
        for _ in range(num_outputs):
            outputs.append(TxOut.parse(s))
        locktime = little_endian_to_int(s.read(4))
        return cls(version, inputs, outputs, locktime, testnet=testnet)

        # s.read(n) will return n bytes
        # version is an integer in 4 bytes, little-endian
        # num_inputs is a varint, use read_varint(s)
        # parse num_inputs number of TxIns
        # num_outputs is a varint, use read_varint(s)
        # parse num_outputs number of TxOuts
        # locktime is an integer in 4 bytes, little-endian
        # return an instance of the class (see __init__ for args)
        # raise NotImplementedError

    # tag::source6[]
    def serialize(self):
        '''Returns the byte serialization of the transaction'''
        result = int_to_little_endian(self.version, 4)
        result += encode_varint(len(self.tx_ins))
        for tx_in in self.tx_ins:
            result += tx_in.serialize()
        result += encode_varint(len(self.tx_outs))
        for tx_out in self.tx_outs:
            result += tx_out.serialize()
        result += int_to_little_endian(self.locktime, 4)
        return result
    # end::source6[]

    def fee(self):
        '''Returns the fee of this transaction in satoshi'''
        # initialize input sum and output sum
        # use TxIn.value() to sum up the input amounts
        # use TxOut.amount to sum up the output amounts
        # fee is input sum - output sum
        input_sum, output_sum = 0, 0
        for tx_in in self.tx_ins:
            input_sum += tx_in.value(testnet=testnet)
        for tx_out in self.tx_outs:
            output_sum += tx_out.amount
        return input_sum - output_sum


# tag::source2[]
class TxIn:
    def __init__(self, prev_tx, prev_index, script_sig=None, sequence=0xffffffff):
        self.prev_tx = prev_tx
        self.prev_index = prev_index
        if script_sig is None:  # <1>
            self.script_sig = Script()
        else:
            self.script_sig = script_sig
        self.sequence = sequence

    def __repr__(self):
        return '{}:{}'.format(
            self.prev_tx.hex(),
            self.prev_index,
        )
    # end::source2[]

    @classmethod
    def parse(cls, s):
        '''Takes a byte stream and parses the tx_input at the start
        return a TxIn object
        '''
        prev_tx = s.read(32)[::-1]
        prev_index = little_endian_to_int(s.read(4))
        script_sig = Script.parse(s)
        sequence = little_endian_to_int(s.read(4))
        return cls(prev_tx, prev_index, script_sig, sequence)

        # prev_tx is 32 bytes, little endian
        # prev_index is an integer in 4 bytes, little endian
        # use Script.parse to get the ScriptSig
        # sequence is an integer in 4 bytes, little-endian
        # return an instance of the class (see __init__ for args)
        # raise NotImplementedError

    # tag::source5[]
    def serialize(self):
        '''Returns the byte serialization of the transaction input'''
        result = self.prev_tx[::-1]
        result += int_to_little_endian(self.prev_index, 4)
        result += self.script_sig.serialize()
        result += int_to_little_endian(self.sequence, 4)
        return result
    # end::source5[]

    # tag::source8[]
    def fetch_tx(self, testnet=False):
        return TxFetcher.fetch(self.prev_tx.hex(), testnet=testnet)

    def value(self, testnet=False):
        '''Get the output value by looking up the tx hash.
        Returns the amount in satoshi.
        '''
        tx = self.fetch_tx(testnet=testnet)
        return tx.tx_outs[self.prev_index].amount

    def script_pubkey(self, testnet=False):
        '''Get the ScriptPubKey by looking up the tx hash.
        Returns a Script object.
        '''
        tx = self.fetch_tx(testnet=testnet)
        return tx.tx_outs[self.prev_index].script_pubkey
    # end::source8[]


# tag::source3[]
class TxOut:

    def __init__(self, amount, script_pubkey):
        self.amount = amount
        self.script_pubkey = script_pubkey

    def __repr__(self):
        return '{}:{}'.format(self.amount, self.script_pubkey)
    # end::source3[]

    @classmethod
    def parse(cls, s):
        '''Takes a byte stream and parses the tx_output at the start
        return a TxOut object
        '''
        amount = little_endian_to_int(s.read(8))
        script_pubkey = Script.parse(s)
        return cls(amount, script_pubkey)
        # amount is an integer in 8 bytes, little endian
        # use Script.parse to get the ScriptPubKey
        # return an instance of the class (see __init__ for args)
        raise NotImplementedError

    # tag::source4[]
    def serialize(self):  # <1>
        '''Returns the byte serialization of the transaction output'''
        result = int_to_little_endian(self.amount, 8)
        result += self.script_pubkey.serialize()
        return result
    # end::source4[]

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