*http.client* --- HTTP协议客户端
********************************

**源码:** Lib/http/client.py

======================================================================

这个模块定义了实现 HTTP 和 HTTPS 协议客户端的类。 它通常不直接使用 ---
模块 "urllib.request" 用它来处理使用 HTTP 和 HTTPS 的 URL。

参见: Requests 包 推荐用于更高层级的 HTTP 客户端接口。

注解: HTTPS 支持仅在 Python 启动了 SSL 支持进行编译时（通过 "ssl" 模
  块）可 用。

该模块支持以下类：

class http.client.HTTPConnection(host, port=None[, timeout], source_address=None, blocksize=8192)

   "HTTPConnection" 的实例代表与 HTTP 的一个连接事务。 它的实例化应当
   传入一个主机和可选的端口号。 如果没有传入端口号，如果主机字符串的形
   式为 "主机:端口" 则会从中提取端口，否则将使用默认的 HTTP 端口（80）
   。 如果给出了可选的 *timeout* 参数，则阻塞操作（例如连接尝试）将在
   指定的秒数之后超时（如果未给出，则使用全局默认超时设置）。 可选的
   *source_address* 参数可以为一个 (主机, 端口) 元组，用作进行 HTTP 连
   接的源地址。 可选的 *blocksize* 参数可以字节为单位设置缓冲区的大小
   ，用来发送文件类消息体。

   举个例子，以下调用都是创建连接到同一主机和端口的服务器的实例：

      >>> h1 = http.client.HTTPConnection('www.python.org')
      >>> h2 = http.client.HTTPConnection('www.python.org:80')
      >>> h3 = http.client.HTTPConnection('www.python.org', 80)
      >>> h4 = http.client.HTTPConnection('www.python.org', 80, timeout=10)

   在 3.2 版更改: 添加了 *source_address*。

   在 3.4 版更改: 删除了 *strict* 参数，不再支持 HTTP 0.9 风格的“简单
   响应”。

   在 3.7 版更改: 添加了 *blocksize* 参数。

class http.client.HTTPSConnection(host, port=None, key_file=None, cert_file=None[, timeout], source_address=None, *, context=None, check_hostname=None, blocksize=8192)

   "HTTPConnection" 的子类，使用 SSL 与安全服务器进行通信。 默认端口为
   "443"。 如果指定了 *context*，它必须为一个描述 SSL 各选项的
   "ssl.SSLContext" 实例。

   请参阅 Security considerations 了解有关最佳实践的更多信息。

   在 3.2 版更改: 添加了 *source_address*, *context* 和
   *check_hostname*。

   在 3.2 版更改: 这个类目前会在可能的情况下（即如果 "ssl.HAS_SNI" 为
   真值）支持 HTTPS 虚拟主机。

   在 3.4 版更改: 删除了 *strict* 参数，不再支持 HTTP 0.9 风格的“简单
   响应”。

   在 3.4.3 版更改: 目前这个类在默认情况下会执行所有必要的证书和主机检
   查。 要回复到先前的非验证行为，可以将
   "ssl._create_unverified_context()" 传递给 *context* 参数。

   在 3.8 版更改: This class now enables TLS 1.3
   "ssl.SSLContext.post_handshake_auth" for the default *context* or
   when *cert_file* is passed with a custom *context*.

   3.6 版后已移除: *key_file* 和 *cert_file* 已弃用并转而推荐
   *context*。 请改用 "ssl.SSLContext.load_cert_chain()" 或让
   "ssl.create_default_context()" 为你选择系统所信任的 CA 证书。
   *check_hostname* 参数也已弃用；应当改用 *context* 的
   "ssl.SSLContext.check_hostname" 属性。

class http.client.HTTPResponse(sock, debuglevel=0, method=None, url=None)

   在成功连接后返回类的实例，而不是由用户直接实例化。

   在 3.4 版更改: 删除了 *strict* 参数，不再支持HTTP 0.9 风格的“简单响
   应”。

This module provides the following function:

http.client.parse_headers(fp)

   Parse the headers from a file pointer *fp* representing a HTTP
   request/response. The file has to be a "BufferedIOBase" reader
   (i.e. not text) and must provide a valid **RFC 2822** style header.

   This function returns an instance of "http.client.HTTPMessage" that
   holds the header fields, but no payload (the same as
   "HTTPResponse.msg" and
   "http.server.BaseHTTPRequestHandler.headers"). After returning, the
   file pointer *fp* is ready to read the HTTP body.

   注解: "parse_headers()" does not parse the start-line of a HTTP
     message; it only parses the "Name: value" lines. The file has to
     be ready to read these field lines, so the first line should
     already be consumed before calling the function.

下列异常可以适当地被引发:

exception http.client.HTTPException

   此模块中其他异常的基类。 它是 "Exception" 的一个子类。

exception http.client.NotConnected

   "HTTPException" 的一个子类。

exception http.client.InvalidURL

   "HTTPException" 的一个子类，如果给出了一个非数字或为空值的端口就会
   被引发。

exception http.client.UnknownProtocol

   "HTTPException" 的一个子类。

exception http.client.UnknownTransferEncoding

   "HTTPException" 的一个子类。

exception http.client.UnimplementedFileMode

   "HTTPException" 的一个子类。

exception http.client.IncompleteRead

   "HTTPException" 的一个子类。

exception http.client.ImproperConnectionState

   "HTTPException" 的一个子类。

exception http.client.CannotSendRequest

   "ImproperConnectionState" 的一个子类。

exception http.client.CannotSendHeader

   "ImproperConnectionState" 的一个子类。

exception http.client.ResponseNotReady

   "ImproperConnectionState" 的一个子类。

exception http.client.BadStatusLine

   "HTTPException" 的一个子类。 如果服务器反馈了一个我们不理解的 HTTP
   状态码就会被引发。

exception http.client.LineTooLong

   "HTTPException" 的一个子类。 如果在 HTTP 协议中从服务器接收到过长的
   行就会被引发。

exception http.client.RemoteDisconnected

   "ConnectionResetError" 和 "BadStatusLine" 的一个子类。 当尝试读取响
   应时的结果是未从连接读取到数据时由 "HTTPConnection.getresponse()"
   引发，表明远端已关闭连接。

   3.5 新版功能: 在此之前引发的异常为 "BadStatusLine""('')"。

此模块中定义的常量为：

http.client.HTTP_PORT

   HTTP 协议默认的端口号 (总是 "80")。

http.client.HTTPS_PORT

   HTTPS 协议默认的端口号 (总是 "443")。

http.client.responses

   This dictionary maps the HTTP 1.1 status codes to the W3C names.

   Example: "http.client.responses[http.client.NOT_FOUND]" is "'Not
   Found'".

See HTTP 状态码 for a list of HTTP status codes that are available in
this module as constants.


HTTPConnection Objects
======================

"HTTPConnection" instances have the following methods:

HTTPConnection.request(method, url, body=None, headers={}, *, encode_chunked=False)

   This will send a request to the server using the HTTP request
   method *method* and the selector *url*.

   If *body* is specified, the specified data is sent after the
   headers are finished.  It may be a "str", a *bytes-like object*, an
   open *file object*, or an iterable of "bytes".  If *body* is a
   string, it is encoded as ISO-8859-1, the default for HTTP.  If it
   is a bytes-like object, the bytes are sent as is.  If it is a *file
   object*, the contents of the file is sent; this file object should
   support at least the "read()" method.  If the file object is an
   instance of "io.TextIOBase", the data returned by the "read()"
   method will be encoded as ISO-8859-1, otherwise the data returned
   by "read()" is sent as is.  If *body* is an iterable, the elements
   of the iterable are sent as is until the iterable is exhausted.

   The *headers* argument should be a mapping of extra HTTP headers to
   send with the request.

   If *headers* contains neither Content-Length nor Transfer-Encoding,
   but there is a request body, one of those header fields will be
   added automatically.  If *body* is "None", the Content-Length
   header is set to "0" for methods that expect a body ("PUT", "POST",
   and "PATCH").  If *body* is a string or a bytes-like object that is
   not also a *file*, the Content-Length header is set to its length.
   Any other type of *body* (files and iterables in general) will be
   chunk-encoded, and the Transfer-Encoding header will automatically
   be set instead of Content-Length.

   The *encode_chunked* argument is only relevant if Transfer-Encoding
   is specified in *headers*.  If *encode_chunked* is "False", the
   HTTPConnection object assumes that all encoding is handled by the
   calling code.  If it is "True", the body will be chunk-encoded.

   注解: Chunked transfer encoding has been added to the HTTP
     protocol version 1.1.  Unless the HTTP server is known to handle
     HTTP 1.1, the caller must either specify the Content-Length, or
     must pass a "str" or bytes-like object that is not also a file as
     the body representation.

   3.2 新版功能: *body* can now be an iterable.

   在 3.6 版更改: If neither Content-Length nor Transfer-Encoding are
   set in *headers*, file and iterable *body* objects are now chunk-
   encoded. The *encode_chunked* argument was added. No attempt is
   made to determine the Content-Length for file objects.

HTTPConnection.getresponse()

   Should be called after a request is sent to get the response from
   the server. Returns an "HTTPResponse" instance.

   注解: Note that you must have read the whole response before you
     can send a new request to the server.

   在 3.5 版更改: If a "ConnectionError" or subclass is raised, the
   "HTTPConnection" object will be ready to reconnect when a new
   request is sent.

HTTPConnection.set_debuglevel(level)

   Set the debugging level.  The default debug level is "0", meaning
   no debugging output is printed.  Any value greater than "0" will
   cause all currently defined debug output to be printed to stdout.
   The "debuglevel" is passed to any new "HTTPResponse" objects that
   are created.

   3.1 新版功能.

HTTPConnection.set_tunnel(host, port=None, headers=None)

   Set the host and the port for HTTP Connect Tunnelling. This allows
   running the connection through a proxy server.

   The host and port arguments specify the endpoint of the tunneled
   connection (i.e. the address included in the CONNECT request, *not*
   the address of the proxy server).

   The headers argument should be a mapping of extra HTTP headers to
   send with the CONNECT request.

   For example, to tunnel through a HTTPS proxy server running locally
   on port 8080, we would pass the address of the proxy to the
   "HTTPSConnection" constructor, and the address of the host that we
   eventually want to reach to the "set_tunnel()" method:

      >>> import http.client
      >>> conn = http.client.HTTPSConnection("localhost", 8080)
      >>> conn.set_tunnel("www.python.org")
      >>> conn.request("HEAD","/index.html")

   3.2 新版功能.

HTTPConnection.connect()

   Connect to the server specified when the object was created.  By
   default, this is called automatically when making a request if the
   client does not already have a connection.

HTTPConnection.close()

   Close the connection to the server.

HTTPConnection.blocksize

   Buffer size in bytes for sending a file-like message body.

   3.7 新版功能.

As an alternative to using the "request()" method described above, you
can also send your request step by step, by using the four functions
below.

HTTPConnection.putrequest(method, url, skip_host=False, skip_accept_encoding=False)

   This should be the first call after the connection to the server
   has been made. It sends a line to the server consisting of the
   *method* string, the *url* string, and the HTTP version
   ("HTTP/1.1").  To disable automatic sending of "Host:" or "Accept-
   Encoding:" headers (for example to accept additional content
   encodings), specify *skip_host* or *skip_accept_encoding* with non-
   False values.

HTTPConnection.putheader(header, argument[, ...])

   Send an **RFC 822**-style header to the server.  It sends a line to
   the server consisting of the header, a colon and a space, and the
   first argument.  If more arguments are given, continuation lines
   are sent, each consisting of a tab and an argument.

HTTPConnection.endheaders(message_body=None, *, encode_chunked=False)

   Send a blank line to the server, signalling the end of the headers.
   The optional *message_body* argument can be used to pass a message
   body associated with the request.

   If *encode_chunked* is "True", the result of each iteration of
   *message_body* will be chunk-encoded as specified in **RFC 7230**,
   Section 3.3.1.  How the data is encoded is dependent on the type of
   *message_body*.  If *message_body* implements the buffer interface
   the encoding will result in a single chunk. If *message_body* is a
   "collections.abc.Iterable", each iteration of *message_body* will
   result in a chunk.  If *message_body* is a *file object*, each call
   to ".read()" will result in a chunk. The method automatically
   signals the end of the chunk-encoded data immediately after
   *message_body*.

   注解: Due to the chunked encoding specification, empty chunks
     yielded by an iterator body will be ignored by the chunk-encoder.
     This is to avoid premature termination of the read of the request
     by the target server due to malformed encoding.

   3.6 新版功能: Chunked encoding support.  The *encode_chunked*
   parameter was added.

HTTPConnection.send(data)

   Send data to the server.  This should be used directly only after
   the "endheaders()" method has been called and before
   "getresponse()" is called.


HTTPResponse Objects
====================

An "HTTPResponse" instance wraps the HTTP response from the server.
It provides access to the request headers and the entity body.  The
response is an iterable object and can be used in a with statement.

在 3.5 版更改: The "io.BufferedIOBase" interface is now implemented
and all of its reader operations are supported.

HTTPResponse.read([amt])

   Reads and returns the response body, or up to the next *amt* bytes.

HTTPResponse.readinto(b)

   Reads up to the next len(b) bytes of the response body into the
   buffer *b*. Returns the number of bytes read.

   3.3 新版功能.

HTTPResponse.getheader(name, default=None)

   Return the value of the header *name*, or *default* if there is no
   header matching *name*.  If there is more than one  header with the
   name *name*, return all of the values joined by ', '.  If 'default'
   is any iterable other than a single string, its elements are
   similarly returned joined by commas.

HTTPResponse.getheaders()

   Return a list of (header, value) tuples.

HTTPResponse.fileno()

   Return the "fileno" of the underlying socket.

HTTPResponse.msg

   A "http.client.HTTPMessage" instance containing the response
   headers.  "http.client.HTTPMessage" is a subclass of
   "email.message.Message".

HTTPResponse.version

   HTTP protocol version used by server.  10 for HTTP/1.0, 11 for
   HTTP/1.1.

HTTPResponse.status

   Status code returned by server.

HTTPResponse.reason

   Reason phrase returned by server.

HTTPResponse.debuglevel

   A debugging hook.  If "debuglevel" is greater than zero, messages
   will be printed to stdout as the response is read and parsed.

HTTPResponse.closed

   Is "True" if the stream is closed.


示例
====

Here is an example session that uses the "GET" method:

   >>> import http.client
   >>> conn = http.client.HTTPSConnection("www.python.org")
   >>> conn.request("GET", "/")
   >>> r1 = conn.getresponse()
   >>> print(r1.status, r1.reason)
   200 OK
   >>> data1 = r1.read()  # This will return entire content.
   >>> # The following example demonstrates reading data in chunks.
   >>> conn.request("GET", "/")
   >>> r1 = conn.getresponse()
   >>> while not r1.closed:
   ...     print(r1.read(200))  # 200 bytes
   b'<!doctype html>\n<!--[if"...
   ...
   >>> # Example of an invalid request
   >>> conn = http.client.HTTPSConnection("docs.python.org")
   >>> conn.request("GET", "/parrot.spam")
   >>> r2 = conn.getresponse()
   >>> print(r2.status, r2.reason)
   404 Not Found
   >>> data2 = r2.read()
   >>> conn.close()

Here is an example session that uses the "HEAD" method.  Note that the
"HEAD" method never returns any data.

   >>> import http.client
   >>> conn = http.client.HTTPSConnection("www.python.org")
   >>> conn.request("HEAD", "/")
   >>> res = conn.getresponse()
   >>> print(res.status, res.reason)
   200 OK
   >>> data = res.read()
   >>> print(len(data))
   0
   >>> data == b''
   True

Here is an example session that shows how to "POST" requests:

   >>> import http.client, urllib.parse
   >>> params = urllib.parse.urlencode({'@number': 12524, '@type': 'issue', '@action': 'show'})
   >>> headers = {"Content-type": "application/x-www-form-urlencoded",
   ...            "Accept": "text/plain"}
   >>> conn = http.client.HTTPConnection("bugs.python.org")
   >>> conn.request("POST", "", params, headers)
   >>> response = conn.getresponse()
   >>> print(response.status, response.reason)
   302 Found
   >>> data = response.read()
   >>> data
   b'Redirecting to <a href="http://bugs.python.org/issue12524">http://bugs.python.org/issue12524</a>'
   >>> conn.close()

Client side "HTTP PUT" requests are very similar to "POST" requests.
The difference lies only the server side where HTTP server will allow
resources to be created via "PUT" request. It should be noted that
custom HTTP methods +are also handled in "urllib.request.Request" by
sending the appropriate +method attribute.Here is an example session
that shows how to do "PUT" request using http.client:

   >>> # This creates an HTTP message
   >>> # with the content of BODY as the enclosed representation
   >>> # for the resource http://localhost:8080/file
   ...
   >>> import http.client
   >>> BODY = "***filecontents***"
   >>> conn = http.client.HTTPConnection("localhost", 8080)
   >>> conn.request("PUT", "/file", BODY)
   >>> response = conn.getresponse()
   >>> print(response.status, response.reason)
   200, OK


HTTPMessage Objects
===================

An "http.client.HTTPMessage" instance holds the headers from an HTTP
response.  It is implemented using the "email.message.Message" class.
