python用700行代码实现http客户端

本文用python在TCP的基础上实现一个HTTP客户端, 该客户端能够复用TCP连接, 使用HTTP1.1协议.

一. 创建HTTP请求

　　HTTP是基于TCP连接的, 它的请求报文格式如下:

　　因此, 我们只需要创建一个到服务器的TCP连接, 然后按照上面的格式写好报文并发给服务器, 就实现了一个HTTP请求.

1. HTTPConnection类

　　基于以上的分析, 我们首先定义一个HTTPConnection类来管理连接和请求内容:

class HTTPConnection:
  default_port = 80
  _http_vsn = 11
  _http_vsn_str = 'HTTP/1.1'

  def __init__(self, host: str, port: int = None) -> None:
    self.sock = None
    self._buffer = []
    self.host = host
    self.port = port if port is not None else self.default_port
    self._state = _CS_IDLE
    self._response = None
    self._method = None
    self.block_size = 8192

  def _output(self, s: Union[str, bytes]) -> None:
    if hasattr(s, 'encode'):
      s = s.encode('latin-1')
    self._buffer.append(s)

  def connect(self) -> None:
    self.sock = socket.create_connection((self.host, self.port))

　　对于这个HTTPConnection对象, 我们只需要创建TCP连接, 然后按照HTTP协议的格式把请求数据写入buffer中, 最后把buffer中的数据发送出去就行了.

2. 编写请求行

　　请求行的内容比较简单, 就是说明请求方法, 请求路径和HTTP协议. 使用下面的方法来编写一个请求行:

def put_request(self, method: str, url: str) -> None:
  self._method = method

  url = url or '/'

  request = f'{method} {url} {self._http_vsn_str}'
  self._output(request)

3. 添加请求头

　　HTTP请求头和python的字典类似, 每行都是一个字段名与值的映射关系. HTTP协议并不要求设置所有合法的请求头的值, 我们只需要按照需要, 设置特定的请求头即可. 使用如下代码添加请求头:

def put_header(self, header: Union[bytes, str], value: Union[bytes, str, int]) -> None:
  if hasattr(header, 'encode'):
    header = header.encode('ascii')

  if hasattr(value, 'encode'):
    value = value.encode('latin-1')
  elif isinstance(value, int):
    value = str(value).encode('ascii')

  header = header + b': ' + value
  self._output(header)

　　此外, 在HTTP请求中, Host请求头字段是必须的, 否则网站可能会拒绝响应. 因此, 如果用户没有设置这个字段, 这里就应该主动把它加上去:

def _add_host(self, url: str) -> None:
  # 所有HTTP / 1.1请求报文中必须包含一个Host头字段
  # 如果用户没给,就调用这个函数来生成
  netloc = ''
  if url.startswith('http'):
    nil, netloc, nil, nil, nil = urllib.parse.urlsplit(url)

  if netloc:
    try:
      netloc_enc = netloc.encode('ascii')
    except UnicodeEncodeError:
      netloc_enc = netloc.encode('idna')
    self.put_header('Host', netloc_enc)
  else:
    host = self.host
    port = self.port

    try:
      host_enc = host.encode('ascii')
    except UnicodeEncodeError:
      host_enc = host.encode('idna')

    # 对IPv6的地址进行额外处理
    if host.find(':') >= 0:
      host_enc = b'[' + host_enc + b']'

    if port == self.default_port:
      self.put_header('Host', host_enc)
    else:
      host_enc = host_enc.decode('ascii')
      self.put_header('Host', f'{host_enc}:{port}')

4. 发送请求正文

　　我们接受两种形式的body数据: 一个基于io.IOBase的可读文件对象, 或者是一个能通过迭代得到数据的对象. 在传输数据之前, 我们首先要确定数据是否采用分块传输:

def request(self, method: str, url: str, headers: dict = None, body: Union[io.IOBase, Iterable] = None,
      encode_chunked: bool = False) -> None:
  ...
  if 'content-length' not in header_names:
    if 'transfer-encoding' not in header_names:
      encode_chunked = False
      content_length = self._get_content_length(body, method)
      if content_length is None:
        if body is not None:
          # 在这种情况下, body一般是个生成器或者可读文件之类的东西,应该分块传输
          encode_chunked = True
          self.put_header('Transfer-Encoding', 'chunked')
      else:
        self.put_header('Content-Length', str(content_length))
    else:
      # 如果设置了transfer-encoding,则根据用户给的encode_chunked参数决定是否分块
      pass
  else:
    # 只要给了content-length,那么一定不是分块传输
    encode_chunked = False
  ...

@staticmethod
def _get_content_length(body: Union[str, bytes, bytearray, Iterable, io.IOBase], method: str) -> Optional[int]:
  if body is None:
    # PUT,POST,PATCH三个方法默认是有body的
    if method.upper() in _METHODS_EXPECTING_BODY:
      return 0
    else:
      return None

  if hasattr(body, 'read'):
    return None

  try:
    # 对于bytes或者bytearray格式的数据,通过memoryview获取它的长度
    return memoryview(body).nbytes
  except TypeError:
    pass

  if isinstance(body, str):
    return len(body)

  return None

　　 在确定了是否分块之后, 就可以把正文发出去了. 如果body是一个可读文件的话, 就调用_read_readable方法把它封装为一个生成器:

def _send_body(self, message_body: Union[str, bytes, bytearray, Iterable, io.IOBase], encode_chunked: bool) -> None:
  if hasattr(message_body, 'read'):
    chunks = self._read_readable(message_body)
  else:
    try:
      memoryview(message_body)
    except TypeError:
      try:
        chunks = iter(message_body)
      except TypeError:
        raise TypeError(
          f'message_body should be a bytes-like object or an iterable, got {repr(type(message_body))}')
    else:
      # 如果是字节类型的,通过一次迭代把它发出去
      chunks = (message_body,)

  for chunk in chunks:
    if not chunk:
      continue

    if encode_chunked:
      chunk = f'{len(chunk):X}\r\n'.encode('ascii') + chunk + b'\r\n'
    self.send(chunk)

  if encode_chunked:
    self.send(b'0\r\n\r\n')

def _read_readable(self, readable: io.IOBase) -> Generator[bytes, None, None]:
  need_encode = False
  if isinstance(readable, io.TextIOBase):
    need_encode = True
  while True:
    data_block = readable.read(self.block_size)
    if not data_block:
      break
    if need_encode:
      data_block = data_block.encode('utf-8')
    yield data_block

二. 获取响应数据

　　HTTP响应报文的格式与请求报文大同小异, 它大致是这样的:

　　因此, 我们只要用HTTPConnection的socket对象读取服务器发送的数据, 然后按照上面的格式对数据进行解析就行了.

1. HTTPResponse类

　　我们首先定义一个简单的HTTPResponse类. 它的属性大致上就是socket的文件对象以及一些请求的信息等等, 调用它的begin方法来解析响应行和响应头的数据, 然后调用read方法读取响应正文:

class HTTPResponse:

  def __init__(self, sock: socket.socket, method: str = None) -> None:
    self.fp = sock.makefile('rb')
    self._method = method
    self.headers = None
    self.version = _UNKNOWN
    self.status = _UNKNOWN
    self.reason = _UNKNOWN
    self.chunked = _UNKNOWN
    self.chunk_left = _UNKNOWN
    self.length = _UNKNOWN
    self.will_close = _UNKNOWN

  def begin(self) -> None:
    ...

  def read(self, amount: int = None) -> bytes:
    ...

2. 解析状态行

　　状态行的解析比较简单, 我们只需要读取响应的第一行数据, 然后把它解析为HTTP协议版本,状态码和原因短语三部分就行了:

def _read_status(self) -> Tuple[str, int, str]:
  line = str(self._read_line(), 'latin-1')
  if not line:
    raise RemoteDisconnected('Remote end closed connection without response')
  try:
    version, status, reason = line.split(None, 2)
  except ValueError:
    # reason只是给人看的, 一般和status对应, 所以它有可能不存在
    try:
      version, status = line.split(None, 1)
      reason = ''
    except ValueError:
      version, status, reason = '', '', ''
  if not version.startswith('HTTP/'):
    self._close_conn()
    raise BadStatusLine(line)

  try:
    status = int(status)
    if status < 100 or status > 999:
      raise BadStatusLine(line)
  except ValueError:
    raise BadStatusLine(line)
  return version, status, reason.strip()

　　如果状态码为100, 则客户端需要解析多个响应状态行. 它的原理是这样的: 在请求数据过大的时候, 有的客户端会先不发送请求数据, 而是先在header中添加一个Expect: 100-continue, 如果服务器愿意接收数据, 会返回100的状态码, 这时候客户端再把数据发过去. 因此, 如果读取到100的状态码, 那么后面往往还会收到一个正式的响应数据, 应该继续读取响应头. 这部分的代码如下:

def begin(self) -> None:
  while True:
    version, status, reason = self._read_status()
    if status != HTTPStatus.CONTINUE:
      break
    # 跳过100状态码部分的响应头
    while True:
      skip = self._read_line().strip()
      if not skip:
        breakself.status = status
  self.reason = reason
  if version in ('HTTP/1.0', 'HTTP/0.9'):
    self.version = 10
  elif version.startswith('HTTP/1.'):
    self.version = 11
  else:
    # HTTP2还没研究, 这里就不写了
    raise UnknownProtocol(version)

  ...

3. 解析响应头

　　解析响应头比响应行还要简单. 因为每个header字段占一行, 我们只需要一直调用read_line方法读取字段, 直到读完header为止就行了.

def _parse_header(self) -> None:
  headers = {}
  while True:
    line = self._read_line()
    if len(headers) > _MAX_HEADERS:
      raise HTTPException('got more than %d headers' % _MAX_HEADERS)
    if line in _EMPTY_LINE:
      break
    line = line.decode('latin-1')
    i = line.find(':')
    if i == -1:
      raise BadHeaderLine(line)
    # 这里默认没有重名的情况
    key, value = line[:i].lower(), line[i + 1:].strip()
    headers[key] = value
  self.headers = headers

4. 接收响应正文

　　在接收响应正文之前, 首先要确定它的传输方式和长度:

def _set_chunk(self) -> None:
  transfer_encoding = self.get_header('transfer-encoding')
  if transfer_encoding and transfer_encoding.lower() == 'chunked':
    self.chunked = True
    self.chunk_left = None
  else:
    self.chunked = False

def _set_length(self) -> None:
  # 首先要知道数据是否是分块传输的
  if self.chunked == _UNKNOWN:
    self._set_chunk()

  # 如果状态码是1xx或者204(无响应内容)或者304(使用上次缓存的内容),则没有响应正文
  # 如果这是个HEAD请求,那么也不能有响应正文
  if (self.status == HTTPStatus.NO_CONTENT or
      self.status == HTTPStatus.NOT_MODIFIED or
      100 <= self.status < 200 or
      self._method == 'HEAD'):
    self.length = 0
    return

  length = self.get_header('content-length')
  if length and not self.chunked:
    try:
      self.length = int(length)
    except ValueError:
      self.length = None
    else:
      if self.length < 0:
        self.length = None
  else:
    self.length = None

　　然后, 我们实现一个read方法, 从body中读取指定大小的数据:

def read(self, amount: int = None) -> bytes:
  if self.is_closed():
    return b''
  if self._method == 'HEAD':
    self.close()
    return b''
  if amount is None:
    return self._read_all()
  return self._read_amount(amount)

　　如果没有指定需要的数据大小, 就默认读取所有数据:

def _read_all(self) -> bytes:
  if self.chunked:
    return self._read_all_chunk()
  if self.length is None:
    s = self.fp.read()
  else:
    try:
      s = self._read_bytes(self.length)
    except IncompleteRead:
      self.close()
      raise
    self.length = 0
  self.close()
  return s

def _read_all_chunk(self) -> bytes:
  assert self.chunked != _UNKNOWN
  value = []
  try:
    while True:
      chunk = self._read_chunk()
      if chunk is None:
        break
      value.append(chunk)
    return b''.join(value)
  except IncompleteRead:
    raise IncompleteRead(b''.join(value))

def _read_chunk(self) -> Optional[bytes]:
  try:
    chunk_size = self._read_chunk_size()
  except ValueError:
    raise IncompleteRead(b'')
  if chunk_size == 0:
    self._read_and_discard_trailer()
    self.close()
    return None
  chunk = self._read_bytes(chunk_size)
  # 每块的结尾会有一个\r\n,这里把它读掉
  self._read_bytes(2)
  return chunk

def _read_chunk_size(self) -> int:
  line = self._read_line(error_message='chunk size')
  i = line.find(b';')
  if i >= 0:
    line = line[:i]
  try:
    return int(line, 16)
  except ValueError:
    self.close()
    raise

def _read_and_discard_trailer(self) -> None:
  # chunk的尾部可能会挂一些额外的信息,比如MD5值,过期时间等等,一般会在header中用trailer字段说明
  # 当chunk读完之后调用这个函数, 这些信息就先舍弃掉得了
  while True:
    line = self._read_line(error_message='chunk size')
    if line in _EMPTY_LINE:
      break

　　否则的话, 就读取部分数据, 如果正好是分块数据的话, 就比较复杂了. 简单来说, 就是用bytearray制造一个所需大小的数组, 然后依次读取chunk把数据往里面填, 直到填满或者没数据为止.  然后用chunk_left记录下当前块剩余的量, 以便下次读取.

def _read_amount(self, amount: int) -> bytes:
  if self.chunked:
    return self._read_amount_chunk(amount)
  if isinstance(self.length, int) and amount > self.length:
    amount = self.length
  container = bytearray(amount)
  n = self.fp.readinto(container)
  if not n and container:
    # 如果读不到字节了,也就可以关了
    self.close()
  elif self.length is not None:
    self.length -= n
    if not self.length:
      self.close()
  return memoryview(container)[:n].tobytes()

def _read_amount_chunk(self, amount: int) -> bytes:
  # 调用这个方法,读取amount大小的chunk类型数据,不足就全部读取
  assert self.chunked != _UNKNOWN
  total_bytes = 0
  container = bytearray(amount)
  mvb = memoryview(container)
  try:
    while True:
      # mvb可以理解为容器的空的那一部分
      # 这里一直调用_full_readinto把数据填进去,让mvb越来越小,同时记录填入的量
      # 等没数据或者当前数据足够把mvb填满之后,跳出循环
      chunk_left = self._get_chunk_left()
      if chunk_left is None:
        break
      if len(mvb) <= chunk_left:
        n = self._full_readinto(mvb)
        self.chunk_left = chunk_left - n
        total_bytes += n
        break
      temp_mvb = mvb[:chunk_left]
      n = self._full_readinto(temp_mvb)
      mvb = mvb[n:]
      total_bytes += n
      self.chunk_left = 0

  except IncompleteRead:
    raise IncompleteRead(bytes(container[:total_bytes]))

  return memoryview(container)[:total_bytes].tobytes()

def _full_readinto(self, container: memoryview) -> int:
  # 返回读取的量.如果没能读满,这个方法会报警
  amount = len(container)
  n = self.fp.readinto(container)
  if n < amount:
    raise IncompleteRead(bytes(container[:n]), amount - n)
  return n

def _get_chunk_left(self) -> Optional[int]:
  # 如果当前块读了一半,那么直接返回self.chunk_left就行了
  # 否则,有三种情况
  # 1). chunk_left为None,说明body压根没开始读,于是返回当前这一整块的长度
  # 2). chunk_left为0,说明这块读完了,于是返回下一块的长度
  # 3). body数据读完了,返回None,顺便做好善后工作
  chunk_left = self.chunk_left
  if not chunk_left:
    if chunk_left == 0:
      # 如果剩余零,说明上一块已经读完了,这里把\r\n读掉
      # 如果是None,就说明chunk压根没开始读
      self._read_bytes(2)
    try:
      chunk_left = self._read_chunk_size()
    except ValueError:
      raise IncompleteRead(b'')
    if chunk_left == 0:
      self._read_and_discard_trailer()
      self.close()
      chunk_left = None
    self.chunk_left = chunk_left
  return chunk_left

三. 复用TCP连接

　　HTTP通信本质上是基于TCP连接发送和接收HTTP请求和响应, 因此, 只要TCP连接不断开, 我们就可以继续用它进行HTTP请求, 这样就避免了创建和销毁TCP连接产生的消耗.

1. 判断连接是否会断开

　　在下面几种情况中, 服务端会自动断开连接:

• HTTP协议小于1.1且没有在头部设置了keep-alive
• HTTP协议大于等于1.1但是在头部设置了connection: close
• 数据没有分块传输, 也没有说明数据的长度, 这种情况下, 服务器一般会在发送完成后断开连接, 让客户端知道数据发完了

　　根据上面列出来的几种情况, 通过下面的代码来判断连接是否会断开:

def _check_close(self) -> bool:
  conn = self.get_header('connection')

  if not self.chunked and self.length is None:
    return True

  if self.version == 11:
    if conn and 'close' in conn.lower():
      return True
    return False
  else:
    if self.headers.get('keep-alive'):
      return False

    if conn and 'keep-alive' in conn.lower():
      return False

  return True

2. 正确地关闭HTTPResponse对象

　　由于TCP连接的复用, 一个HTTPConnection可以产生多个HTTPResponse对象, 而这些对象在同一个TCP连接上, 会共用这个连接的读缓冲区. 这就导致, 如果上一个HTTPResponse对象没有把它的那部分数据读完, 就会对下一个响应产生影响.

　　另一方面来看, 我们也需要及时地关闭与这个TCP关联的文件对象来避免占用资源. 因此, 我们定义如下的close方法关闭一个HTTPResponse对象:

def close(self) -> None:
  if self.is_closed():
    return
  fp = self.fp
  self.fp = None
  fp.close()

def is_closed(self) -> bool:
  return self.fp is None

　　用户调用HTTPResponse对象的read方法, 把缓冲区数据读完之后, 就会自动调用close方法(具体实现见上一章的第四节: 读取响应数据这部分). 因此, 在获取下一个响应数据之前, 我们只需要调用这个对象的is_closed方法, 就能判断读缓冲区是否已经读完, 能否继续接收响应了.

3. HTTP请求的生命周期

　　不使用管道机制的话, 不同的HTTP请求必须按次序进行, 相互之间不能重叠. 基于这个原因, 我们为HTTPConnection对象设置IDLE, REQ_STARTED和REQ_SENT三种状态, 一个完整的请求应该经历这几种状态:

　　根据上面的流程, 对HTTPConnection中对应的方法进行修改:

def get_response(self) -> HTTPResponse:
  if self._response and self._response.is_closed():
    self._response = None
  if self._state != _CS_REQ_SENT or self._response:
    raise ResponseNotReady(self._state)

  response = HTTPResponse(self.sock, method=self._method)

  try:
    try:
      response.begin()
    except ConnectionError:
      self.close()
      raise
    assert response.will_close != _UNKNOWN
    self._state = _CS_IDLE

    if response.will_close:
      self.close()
    else:
      self._response = response

    return response
  except Exception as _:
    response.close()
    raise

def put_request(self, method: str, url: str) -> None:
  # 调用这个函数开始新一轮的请求,它负责写好请求行输出到缓存里面去
  # 调用它的前提是当前处于空闲状态
  # 如果之前的response还在并且已结束,会自动把它消除掉
  if self._response and self._response.is_closed():
    self._response = None

  if self._state == _CS_IDLE:
    self._state = _CS_REQ_STARTED
  else:
    raise CannotSendRequest(self._state)

  ...

def put_header(self, header: Union[bytes, str], value: Union[bytes, str, int]) -> None:
  if self._state != _CS_REQ_STARTED:
    raise CannotSendHeader()

  ...

def end_headers(self, message_body=None, encode_chunked=False) -> None:
  if self._state == _CS_REQ_STARTED:
    self._state = _CS_REQ_SENT
  else:
    raise CannotSendHeader()
  ...

　　需要注意的是, 如果第二个请求已经进入到获取响应的阶段了, 而上一个请求的响应还没关闭, 那么就应该直接报错, 否则读取到的会是上一个请求剩余的响应部分数据, 导致解析响应出现问题.

事实上, HTTP1.1开始支持管道化技术, 也就是一次提交多个HTTP请求, 然后等待响应, 而不是在接收到上一个请求的响应后, 才发送后面的请求.
基于这种处理模式, 管道化技术理论上可以减少IO时间的损耗, 提升效率, 不过, 需要服务端的支持, 而且会增加程序的复杂程度, 这里就不实现了.

四. 总结

1. 完整代码

　　HTTPConnection的完整代码如下:

class HTTPConnection:
  default_port = 80
  _http_vsn = 11
  _http_vsn_str = 'HTTP/1.1'

  def __init__(self, host: str, port: int = None) -> None:
    self.sock = None
    self._buffer = []
    self.host = host
    self.port = port if port is not None else self.default_port
    self._state = _CS_IDLE
    self._response = None
    self._method = None
    self.block_size = 8192

  def request(self, method: str, url: str, headers: dict = None, body: Union[io.IOBase, Iterable] = None,
        encode_chunked: bool = False) -> None:
    self.put_request(method, url)
    headers = headers or {}
    header_names = frozenset(k.lower() for k in headers.keys())
    if 'host' not in header_names:
      self._add_host(url)

    if 'content-length' not in header_names:
      if 'transfer-encoding' not in header_names:
        encode_chunked = False
        content_length = self._get_content_length(body, method)
        if content_length is None:
          if body is not None:
            encode_chunked = True
            self.put_header('Transfer-Encoding', 'chunked')
        else:
          self.put_header('Content-Length', str(content_length))
      else:
        # 如果设置了transfer-encoding,则根据用户给的encode_chunked参数决定是否分块
        pass
    else:
      # 只要给了content-length,那么一定不是分块传输
      encode_chunked = False

    for hdr, value in headers.items():
      self.put_header(hdr, value)
    if isinstance(body, str):
      body = _encode(body)
    self.end_headers(body, encode_chunked=encode_chunked)

  def send(self, data: bytes) -> None:
    if self.sock is None:
      self.connect()

    self.sock.sendall(data)

  def get_response(self) -> HTTPResponse:
    if self._response and self._response.is_closed():
      self._response = None
    if self._state != _CS_REQ_SENT or self._response:
      raise ResponseNotReady(self._state)

    response = HTTPResponse(self.sock, method=self._method)

    try:
      try:
        response.begin()
      except ConnectionError:
        self.close()
        raise
      assert response.will_close != _UNKNOWN
      self._state = _CS_IDLE

      if response.will_close:
        self.close()
      else:
        self._response = response

      return response
    except Exception as _:
      response.close()
      raise

  def connect(self) -> None:
    self.sock = socket.create_connection((self.host, self.port))

  def close(self) -> None:
    self._state = _CS_IDLE
    try:
      sock = self.sock
      if sock:
        self.sock = None
        sock.close()
    finally:
      response = self._response
      if response:
        self._response = None
        response.close()

  def put_request(self, method: str, url: str) -> None:
    # 调用这个函数开始新一轮的请求,它负责写好请求行输出到缓存里面去
    # 调用它的前提是当前处于空闲状态
    # 如果之前的response还在并且已结束,会自动把它消除掉
    if self._response and self._response.is_closed():
      self._response = None

    if self._state == _CS_IDLE:
      self._state = _CS_REQ_STARTED
    else:
      raise CannotSendRequest(self._state)

    self._method = method

    url = url or '/'

    request = f'{method} {url} {self._http_vsn_str}'
    self._output(request)

  def put_header(self, header: Union[bytes, str], value: Union[bytes, str, int]) -> None:
    if self._state != _CS_REQ_STARTED:
      raise CannotSendHeader()

    if hasattr(header, 'encode'):
      header = header.encode('ascii')

    if hasattr(value, 'encode'):
      value = value.encode('latin-1')
    elif isinstance(value, int):
      value = str(value).encode('ascii')

    header = header + b': ' + value
    self._output(header)

  def end_headers(self, message_body=None, encode_chunked=False) -> None:
    if self._state == _CS_REQ_STARTED:
      self._state = _CS_REQ_SENT
    else:
      raise CannotSendHeader()
    self._send_output(message_body, encode_chunked=encode_chunked)

  def _add_host(self, url: str) -> None:
    # 所有HTTP / 1.1请求报文中必须包含一个Host头字段
    # 如果用户没给,就调用这个函数来生成
    netloc = ''
    if url.startswith('http'):
      nil, netloc, nil, nil, nil = urlsplit(url)

    if netloc:
      try:
        netloc_enc = netloc.encode('ascii')
      except UnicodeEncodeError:
        netloc_enc = netloc.encode('idna')
      self.put_header('Host', netloc_enc)
    else:
      host = self.host
      port = self.port

      try:
        host_enc = host.encode('ascii')
      except UnicodeEncodeError:
        host_enc = host.encode('idna')

      # 对IPv6的地址进行额外处理
      if host.find(':') >= 0:
        host_enc = b'[' + host_enc + b']'

      if port == self.default_port:
        self.put_header('Host', host_enc)
      else:
        host_enc = host_enc.decode('ascii')
        self.put_header('Host', f'{host_enc}:{port}')

  def _output(self, s: Union[str, bytes]) -> None:
    # 将数据添加到缓冲区
    if hasattr(s, 'encode'):
      s = s.encode('latin-1')
    self._buffer.append(s)

  def _send_output(self, message_body=None, encode_chunked=False) -> None:
    # 发送并清空缓冲数据.然后,如果有请求正文,就也顺便发送

    self._buffer.extend((b'', b''))
    msg = b'\r\n'.join(self._buffer)
    self._buffer.clear()
    self.send(msg)

    if message_body is not None:
      self._send_body(message_body, encode_chunked)

  def _send_body(self, message_body: Union[bytes, str, bytearray, Iterable, io.IOBase], encode_chunked: bool) -> None:
    if hasattr(message_body, 'read'):
      chunks = self._read_readable(message_body)
    else:
      try:
        memoryview(message_body)
      except TypeError:
        try:
          chunks = iter(message_body)
        except TypeError:
          raise TypeError(
            f'message_body should be a bytes-like object or an iterable, got {repr(type(message_body))}')
      else:
        # 如果是字节类型的,通过一次迭代把它发出去
        chunks = (message_body,)

    for chunk in chunks:
      if not chunk:
        continue

      if encode_chunked:
        chunk = f'{len(chunk):X}\r\n'.encode('ascii') + chunk + b'\r\n'
      self.send(chunk)

    if encode_chunked:
      self.send(b'0\r\n\r\n')

  def _read_readable(self, readable: io.IOBase) -> Generator[bytes, None, None]:
    need_encode = False
    if isinstance(readable, io.TextIOBase):
      need_encode = True
    while True:
      data_block = readable.read(self.block_size)
      if not data_block:
        break
      if need_encode:
        data_block = data_block.encode('utf-8')
      yield data_block

  @staticmethod
  def _get_content_length(body: Union[str, bytes, bytearray, Iterable, io.IOBase], method: str) -> Optional[int]:
    if body is None:
      # PUT,POST,PATCH三个方法默认是有body的
      if method.upper() in _METHODS_EXPECTING_BODY:
        return 0
      else:
        return None

    if hasattr(body, 'read'):
      return None

    try:
      # 对于bytes或者bytearray格式的数据,通过memoryview获取它的长度
      return memoryview(body).nbytes
    except TypeError:
      pass

    if isinstance(body, str):
      return len(body)

    return None

　　HTTPResponse的完整代码如下:

class HTTPResponse:

  def __init__(self, sock: socket.socket, method: str = None) -> None:
    self.fp = sock.makefile('rb')
    self._method = method
    self.headers = None
    self.version = _UNKNOWN
    self.status = _UNKNOWN
    self.reason = _UNKNOWN
    self.chunked = _UNKNOWN
    self.chunk_left = _UNKNOWN
    self.length = _UNKNOWN
    self.will_close = _UNKNOWN

  def begin(self) -> None:
    if self.headers is not None:
      return
    self._parse_status_line()
    self._parse_header()
    self._set_chunk()
    self._set_length()
    self.will_close = self._check_close()

  def _read_line(self, limit: int = _MAX_LINE + 1, error_message: str = '') -> bytes:
    # 注意,这个方法默认不去除line尾部的\r\n
    line = self.fp.readline(limit)
    if len(line) > _MAX_LINE:
      raise LineTooLong(error_message)
    return line

  def _read_bytes(self, amount: int) -> bytes:
    data = self.fp.read(amount)
    if len(data) < amount:
      raise IncompleteRead(data, amount - len(data))
    return data

  def _parse_status_line(self) -> None:
    while True:
      version, status, reason = self._read_status()
      if status != HTTPStatus.CONTINUE:
        break
      while True:
        skip = self._read_line(error_message='header line').strip()
        if not skip:
          break

    self.status = status
    self.reason = reason
    if version in ('HTTP/1.0', 'HTTP/0.9'):
      self.version = 10
    elif version.startswith('HTTP/1.'):
      self.version = 11
    else:
      raise UnknownProtocol(version)

  def _read_status(self) -> Tuple[str, int, str]:
    line = str(self._read_line(error_message='status line'), 'latin-1')
    if not line:
      raise RemoteDisconnected('Remote end closed connection without response')
    try:
      version, status, reason = line.split(None, 2)
    except ValueError:
      # reason只是给人看的, 和status对应, 所以它有可能不存在
      try:
        version, status = line.split(None, 1)
        reason = ''
      except ValueError:
        version, status, reason = '', '', ''
    if not version.startswith('HTTP/'):
      self.close()
      raise BadStatusLine(line)

    try:
      status = int(status)
      if status < 100 or status > 999:
        raise BadStatusLine(line)
    except ValueError:
      raise BadStatusLine(line)
    return version, status, reason.strip()

  def _parse_header(self) -> None:
    headers = {}
    while True:
      line = self._read_line(error_message='header line')
      if len(headers) > _MAX_HEADERS:
        raise HTTPException('got more than %d headers' % _MAX_HEADERS)
      if line in _EMPTY_LINE:
        break
      line = line.decode('latin-1')
      i = line.find(':')
      if i == -1:
        raise BadHeaderLine(line)
      # 这里默认没有重名的情况
      key, value = line[:i].lower(), line[i + 1:].strip()
      headers[key] = value
    self.headers = headers

  def _set_chunk(self) -> None:
    transfer_encoding = self.get_header('transfer-encoding')
    if transfer_encoding and transfer_encoding.lower() == 'chunked':
      self.chunked = True
      self.chunk_left = None
    else:
      self.chunked = False

  def _set_length(self) -> None:
    # 首先要知道数据是否是分块传输的
    if self.chunked == _UNKNOWN:
      self._set_chunk()

    # 如果状态码是1xx或者204(无响应内容)或者304(使用上次缓存的内容),则没有响应正文
    # 如果这是个HEAD请求,那么也不能有响应正文
    assert isinstance(self.status, int)
    if (self.status == HTTPStatus.NO_CONTENT or
        self.status == HTTPStatus.NOT_MODIFIED or
        100 <= self.status < 200 or
        self._method == 'HEAD'):
      self.length = 0
      return

    length = self.get_header('content-length')
    if length and not self.chunked:
      try:
        self.length = int(length)
      except ValueError:
        self.length = None
      else:
        if self.length < 0:
          self.length = None
    else:
      self.length = None

  def _check_close(self) -> bool:
    conn = self.get_header('connection')

    if not self.chunked and self.length is None:
      return True

    if self.version == 11:
      if conn and 'close' in conn.lower():
        return True
      return False
    else:
      if self.headers.get('keep-alive'):
        return False

      if conn and 'keep-alive' in conn.lower():
        return False

    return True

  def close(self) -> None:
    if self.is_closed():
      return
    fp = self.fp
    self.fp = None
    fp.close()

  def is_closed(self) -> bool:
    return self.fp is None

  def read(self, amount: int = None) -> bytes:
    if self.is_closed():
      return b''
    if self._method == 'HEAD':
      self.close()
      return b''
    if amount is None:
      return self._read_all()
    print(amount, amount is None)
    return self._read_amount(amount)

  def _read_all(self) -> bytes:
    if self.chunked:
      return self._read_all_chunk()
    if self.length is None:
      s = self.fp.read()
    else:
      try:
        s = self._read_bytes(self.length)
      except IncompleteRead:
        self.close()
        raise
      self.length = 0
    self.close()
    return s

  def _read_all_chunk(self) -> bytes:
    assert self.chunked != _UNKNOWN
    value = []
    try:
      while True:
        chunk = self._read_chunk()
        if chunk is None:
          break
        value.append(chunk)
      return b''.join(value)
    except IncompleteRead:
      raise IncompleteRead(b''.join(value))

  def _read_chunk(self) -> Optional[bytes]:
    try:
      chunk_size = self._read_chunk_size()
    except ValueError:
      raise IncompleteRead(b'')
    if chunk_size == 0:
      self._read_and_discard_trailer()
      self.close()
      return None
    chunk = self._read_bytes(chunk_size)
    # 每块的结尾会有一个\r\n,这里把它读掉
    self._read_bytes(2)
    return chunk

  def _read_chunk_size(self) -> int:
    line = self._read_line(error_message='chunk size')
    i = line.find(b';')
    if i >= 0:
      line = line[:i]
    try:
      return int(line, 16)
    except ValueError:
      self.close()
      raise

  def _read_and_discard_trailer(self) -> None:
    # chunk的尾部可能会挂一些额外的信息,比如MD5值,过期时间等等,一般会在header中用trailer字段说明
    # 当chunk读完之后调用这个函数, 这些信息就先舍弃掉得了
    while True:
      line = self._read_line(error_message='chunk size')
      if line in _EMPTY_LINE:
        break

  def _read_amount(self, amount: int) -> bytes:
    if self.chunked:
      return self._read_amount_chunk(amount)
    if isinstance(self.length, int) and amount > self.length:
      amount = self.length
    container = bytearray(amount)
    n = self.fp.readinto(container)
    if not n and container:
      # 如果读不到字节了,也就可以关了
      self.close()
    elif self.length is not None:
      self.length -= n
      if not self.length:
        self.close()
    return memoryview(container)[:n].tobytes()

  def _read_amount_chunk(self, amount: int) -> bytes:
    # 调用这个方法,读取amount大小的chunk类型数据,不足就全部读取
    assert self.chunked != _UNKNOWN
    total_bytes = 0
    container = bytearray(amount)
    mvb = memoryview(container)
    try:
      while True:
        # mvb可以理解为容器的空的那一部分
        # 这里一直调用_full_readinto把数据填进去,让mvb越来越小,同时记录填入的量
        # 等没数据或者当前数据足够把mvb填满之后,跳出循环
        chunk_left = self._get_chunk_left()
        if chunk_left is None:
          break
        if len(mvb) <= chunk_left:
          n = self._full_readinto(mvb)
          self.chunk_left = chunk_left - n
          total_bytes += n
          break
        temp_mvb = mvb[:chunk_left]
        n = self._full_readinto(temp_mvb)
        mvb = mvb[n:]
        total_bytes += n
        self.chunk_left = 0

    except IncompleteRead:
      raise IncompleteRead(bytes(container[:total_bytes]))

    return memoryview(container)[:total_bytes].tobytes()

  def _full_readinto(self, container: memoryview) -> int:
    # 返回读取的量.如果没能读满,这个方法会报警
    amount = len(container)
    n = self.fp.readinto(container)
    if n < amount:
      raise IncompleteRead(bytes(container[:n]), amount - n)
    return n

  def _get_chunk_left(self) -> Optional[int]:
    # 如果当前块读了一半,那么直接返回self.chunk_left就行了
    # 否则,有三种情况
    # 1). chunk_left为None,说明body压根没开始读,于是返回当前这一整块的长度
    # 2). chunk_left为0,说明这块读完了,于是返回下一块的长度
    # 3). body数据读完了,返回None,顺便做好善后工作
    chunk_left = self.chunk_left
    if not chunk_left:
      if chunk_left == 0:
        # 如果剩余零,说明上一块已经读完了,这里把\r\n读掉
        # 如果是None,就说明chunk压根没开始读
        self._read_bytes(2)
      try:
        chunk_left = self._read_chunk_size()
      except ValueError:
        raise IncompleteRead(b'')
      if chunk_left == 0:
        self._read_and_discard_trailer()
        self.close()
        chunk_left = None
      self.chunk_left = chunk_left
    return chunk_left

  def get_header(self, name, default: str = None) -> Optional[str]:
    if self.headers is None:
      raise ResponseNotReady()
    return self.headers.get(name, default)

  @property
  def info(self) -> str:
    return repr(self.headers)

　　这两个类应该放到同一个py文件中, 同时这个文件内还有其他一些辅助性质的代码:

import io
import socket
from typing import Generator, Iterable, Optional, Tuple, Union
from urllib.parse import urlsplit

_CS_IDLE = 'Idle'
_CS_REQ_STARTED = 'Request-started'
_CS_REQ_SENT = 'Request-sent'

_METHODS_EXPECTING_BODY = {'PATCH', 'POST', 'PUT'}
_UNKNOWN = 'UNKNOWN'

_MAX_LINE = 65536
_MAX_HEADERS = 100

_EMPTY_LINE = (b'\r\n', b'\n', b'')

class HTTPStatus:
  CONTINUE = 100
  SWITCHING_PROTOCOLS = 101
  PROCESSING = 102
  OK = 200
  CREATED = 201
  ACCEPTED = 202
  NON_AUTHORITATIVE_INFORMATION = 203
  NO_CONTENT = 204
  RESET_CONTENT = 205
  PARTIAL_CONTENT = 206
  MULTI_STATUS = 207
  ALREADY_REPORTED = 208
  IM_USED = 226
  MULTIPLE_CHOICES = 300
  MOVED_PERMANENTLY = 301
  FOUND = 302
  SEE_OTHER = 303
  NOT_MODIFIED = 304
  USE_PROXY = 305
  TEMPORARY_REDIRECT = 307
  PERMANENT_REDIRECT = 308
  BAD_REQUEST = 400
  UNAUTHORIZED = 401
  PAYMENT_REQUIRED = 402
  FORBIDDEN = 403
  NOT_FOUND = 404
  METHOD_NOT_ALLOWED = 405
  NOT_ACCEPTABLE = 406
  PROXY_AUTHENTICATION_REQUIRED = 407
  REQUEST_TIMEOUT = 408
  CONFLICT = 409
  GONE = 410
  LENGTH_REQUIRED = 411
  PRECONDITION_FAILED = 412
  REQUEST_ENTITY_TOO_LARGE = 413
  REQUEST_URI_TOO_LONG = 414
  UNSUPPORTED_MEDIA_TYPE = 415
  REQUESTED_RANGE_NOT_SATISFIABLE = 416
  EXPECTATION_FAILED = 417
  MISDIRECTED_REQUEST = 421
  UNPROCESSABLE_ENTITY = 422
  LOCKED = 423
  FAILED_DEPENDENCY = 424
  UPGRADE_REQUIRED = 426
  PRECONDITION_REQUIRED = 428
  TOO_MANY_REQUESTS = 429
  REQUEST_HEADER_FIELDS_TOO_LARGE = 431
  UNAVAILABLE_FOR_LEGAL_REASONS = 451
  INTERNAL_SERVER_ERROR = 500
  NOT_IMPLEMENTED = 501
  BAD_GATEWAY = 502
  SERVICE_UNAVAILABLE = 503
  GATEWAY_TIMEOUT = 504
  HTTP_VERSION_NOT_SUPPORTED = 505
  VARIANT_ALSO_NEGOTIATES = 506
  INSUFFICIENT_STORAGE = 507
  LOOP_DETECTED = 508
  NOT_EXTENDED = 510
  NETWORK_AUTHENTICATION_REQUIRED = 511

class HTTPResponse:
  ...

class HTTPConnection:
  ...

def _encode(data: str, encoding: str = 'latin-1', name: str = 'data') -> bytes:
  # 给请求正文等不知道能怎么转码的东西转码时用这个,默认使用latin-1编码
  # 它的好处是,转码失败后能抛出详细的错误信息,一目了然
  try:
    return data.encode(encoding)
  except UnicodeEncodeError as err:
    raise UnicodeEncodeError(
      err.encoding,
      err.object,
      err.start,
      err.end,
      "{} ({:.20!r}) is not valid {}. Use {}.encode('utf-8') if you want to send it encoded in UTF-8.".format(
        name.title(), data[err.start:err.end], encoding, name)
    ) from None

class HTTPException(Exception):
  pass

class ImproperConnectionState(HTTPException):
  pass

class CannotSendRequest(ImproperConnectionState):
  pass

class CannotSendHeader(ImproperConnectionState):
  pass

class CannotCloseStream(ImproperConnectionState):
  pass

class ResponseNotReady(ImproperConnectionState):
  pass

class LineTooLong(HTTPException):
  def __init__(self, line_type):
    HTTPException.__init__(self, 'got more than %d bytes when reading %s'
                % (_MAX_LINE, line_type))

class BadStatusLine(HTTPException):
  def __init__(self, line):
    if not line:
      line = repr(line)
    self.args = line,
    self.line = line

class BadHeaderLine(HTTPException):
  def __init__(self, line):
    if not line:
      line = repr(line)
    self.args = line,
    self.line = line

class RemoteDisconnected(ConnectionResetError, BadStatusLine):
  def __init__(self, *args, **kwargs):
    BadStatusLine.__init__(self, '')
    ConnectionResetError.__init__(self, *args, **kwargs)

class UnknownProtocol(HTTPException):
  def __init__(self, version):
    self.args = version,
    self.version = version

class UnknownTransferEncoding(HTTPException):
  pass

class IncompleteRead(HTTPException):
  def __init__(self, partial, expected=None):
    self.args = partial,
    self.partial = partial
    self.expected = expected

  def __repr__(self):
    if self.expected is not None:
      e = f', {self.expected} more expected'
    else:
      e = ''
    return f'{self.__class__.__name__}({len(self.partial)} bytes read{e})'

  __str__ = object.__str__

2. 需要注意的点

　　总的来说, 本文的内容不算复杂, 毕竟HTTP属于不难理解, 但知识点很多很杂的类型. 这里把本文中一些需要注意的点总结一下:

• 请求和响应数据的结构大致相同, 都是状态行+头部+正文, 状态行和头部的每个字段都用一个\r\n分割, 与正文之间用两个分割;
• 状态行是必须的, 请求头则最少需要host这个字段, 同时为了大家的方便, 你最好也设置一下Accept-encoding和Accept来限制服务器返回给你的数据内容和格式;
• 正文不是必须的, 特别是对于除了3P(PATCH, POST, PUT)之外的方法来说. 如果你有正文, 你最好在header中使用Content-Length说明正文的长度, 如果是分块发送, 则使用Transfer-Encoding字段说明;
• 如果对正文使用分块传输, 每块的格式是: 16进制的数据长度+\r\n+数据+\r\n, 使用0\r\n\r\n来收尾. 收尾之后, 你还可以放一个trailer, 里面放数据的MD5值或者过期时间什么的, 这时候最好在header中设置trailer字段;
• 在一个请求的生命周期完成后, TCP连接是否会断开取决于三点: 响应数据的HTTP版本, 响应头中的Connection和Keep-Alive字段, 是否知道响应正文的长度;
• 最最重要的一点, HTTP协议只是一个约定而非限制, 这就和矿泉水的建议零售价差不多, 你可以选择遵守, 也可以不遵守, 后果自负.

3. 结果测试

　　首先, 我们用tornado写一个简单的服务器, 它会显示客户端的地址和接口;

import tornado.web
import tornado.ioloop

class IndexHandler(tornado.web.RequestHandler):

  def get(self) -> None:
    print(f'new connection from {self.request.connection.context.address}')
    self.write('hello world')

app = tornado.web.Application([(r'/', IndexHandler)])
app.listen(8888)
tornado.ioloop.IOLoop.current().start()

　　然后, 使用我们刚写好的客户端进行测试:

from client import HTTPConnection

def fetch(conn: HTTPConnection, url: str = '') -> None:
  conn.request('GET', url)
  res = conn.get_response()
  print(res.read())

connection = HTTPConnection('127.0.0.1', 8888)
for i in range(10):
  fetch(connection)

　　结果如下:

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