Requests源码分析

这一节我们来看看requests是如何发送一个request的，这一节内容可能比较多，有很多底层代码，我自己也看的头疼，建议阅读前先喝瓶酸奶以保持轻松的心情。如果你准备好了，请往下看。

我们在Pycharm中按住win点击get，会来到get方法的源码：

def get(url, params=None, **kwargs):
    r"""Sends a GET request.

    :param url: URL for the new :class:`Request` object.
    :param params: (optional) Dictionary, list of tuples or bytes to send
        in the body of the :class:`Request`.
    :param \*\*kwargs: Optional arguments that ``request`` takes.
    :return: :class:`Response <Response>` object
    :rtype: requests.Response
    """

    kwargs.setdefault('allow_redirects', True)
    return request('get', url, params=params, **kwargs)

发现调用了request方法，继续往下看

def request(method, url, **kwargs):
    """Constructs and sends a :class:`Request <Request>`.

    :param method: method for the new :class:`Request` object.
    :param url: URL for the new :class:`Request` object.
    :param params: (optional) Dictionary, list of tuples or bytes to send
        in the body of the :class:`Request`.
    :param data: (optional) Dictionary, list of tuples, bytes, or file-like
        object to send in the body of the :class:`Request`.
    :param json: (optional) A JSON serializable Python object to send in the body of the :class:`Request`.
    :param headers: (optional) Dictionary of HTTP Headers to send with the :class:`Request`.
    :param cookies: (optional) Dict or CookieJar object to send with the :class:`Request`.
    :param files: (optional) Dictionary of ``'name': file-like-objects`` (or ``{'name': file-tuple}``) for multipart encoding upload.
        ``file-tuple`` can be a 2-tuple ``('filename', fileobj)``, 3-tuple ``('filename', fileobj, 'content_type')``
        or a 4-tuple ``('filename', fileobj, 'content_type', custom_headers)``, where ``'content-type'`` is a string
        defining the content type of the given file and ``custom_headers`` a dict-like object containing additional headers
        to add for the file.
    :param auth: (optional) Auth tuple to enable Basic/Digest/Custom HTTP Auth.
    :param timeout: (optional) How many seconds to wait for the server to send data
        before giving up, as a float, or a :ref:`(connect timeout, read
        timeout) <timeouts>` tuple.
    :type timeout: float or tuple
    :param allow_redirects: (optional) Boolean. Enable/disable GET/OPTIONS/POST/PUT/PATCH/DELETE/HEAD redirection. Defaults to ``True``.
    :type allow_redirects: bool
    :param proxies: (optional) Dictionary mapping protocol to the URL of the proxy.
    :param verify: (optional) Either a boolean, in which case it controls whether we verify
            the server's TLS certificate, or a string, in which case it must be a path
            to a CA bundle to use. Defaults to ``True``.
    :param stream: (optional) if ``False``, the response content will be immediately downloaded.
    :param cert: (optional) if String, path to ssl client cert file (.pem). If Tuple, ('cert', 'key') pair.
    :return: :class:`Response <Response>` object
    :rtype: requests.Response

    Usage::

      >>> import requests
      >>> req = requests.request('GET', 'https://httpbin.org/get')
      <Response [200]>
    """

    # By using the 'with' statement we are sure the session is closed, thus we
    # avoid leaving sockets open which can trigger a ResourceWarning in some
    # cases, and look like a memory leak in others.
    with sessions.Session() as session:
        return session.request(method=method, url=url, **kwargs)

这里就对所有requests可以使用的参数做了一一解释，在公众号文章有翻译：大佬的话（一）。看到，这里是启用了一个session(在HTTP协议中讲的会话)，继续往下看：

    def request(self, method, url,
            params=None, data=None, headers=None, cookies=None, files=None,
            auth=None, timeout=None, allow_redirects=True, proxies=None,
            hooks=None, stream=None, verify=None, cert=None, json=None):
        """Constructs a :class:`Request <Request>`, prepares it and sends it.
        Returns :class:`Response <Response>` object.

        :param method: method for the new :class:`Request` object.
        :param url: URL for the new :class:`Request` object.
        :param params: (optional) Dictionary or bytes to be sent in the query
            string for the :class:`Request`.
        :param data: (optional) Dictionary, list of tuples, bytes, or file-like
            object to send in the body of the :class:`Request`.
        :param json: (optional) json to send in the body of the
            :class:`Request`.
        :param headers: (optional) Dictionary of HTTP Headers to send with the
            :class:`Request`.
        :param cookies: (optional) Dict or CookieJar object to send with the
            :class:`Request`.
        :param files: (optional) Dictionary of ``'filename': file-like-objects``
            for multipart encoding upload.
        :param auth: (optional) Auth tuple or callable to enable
            Basic/Digest/Custom HTTP Auth.
        :param timeout: (optional) How long to wait for the server to send
            data before giving up, as a float, or a :ref:`(connect timeout,
            read timeout) <timeouts>` tuple.
        :type timeout: float or tuple
        :param allow_redirects: (optional) Set to True by default.
        :type allow_redirects: bool
        :param proxies: (optional) Dictionary mapping protocol or protocol and
            hostname to the URL of the proxy.
        :param stream: (optional) whether to immediately download the response
            content. Defaults to ``False``.
        :param verify: (optional) Either a boolean, in which case it controls whether we verify
            the server's TLS certificate, or a string, in which case it must be a path
            to a CA bundle to use. Defaults to ``True``.
        :param cert: (optional) if String, path to ssl client cert file (.pem).
            If Tuple, ('cert', 'key') pair.
        :rtype: requests.Response
        """
        # Create the Request.
        req = Request(
            method=method.upper(),
            url=url,
            headers=headers,
            files=files,
            data=data or {},
            json=json,
            params=params or {},
            auth=auth,
            cookies=cookies,
            hooks=hooks,
        )
        prep = self.prepare_request(req)

        proxies = proxies or {}

        settings = self.merge_environment_settings(
            prep.url, proxies, stream, verify, cert
        )

        # Send the request.
        send_kwargs = {
            'timeout': timeout,
            'allow_redirects': allow_redirects,
        }
        send_kwargs.update(settings)
        resp = self.send(prep, **send_kwargs)

        return resp

把所有的参数加起来，创建了一个Request，发送这个Request，调用了self.send方法

    def send(self, request, **kwargs):
        """Send a given PreparedRequest.

        :rtype: requests.Response
        """
        # Set defaults that the hooks can utilize to ensure they always have
        # the correct parameters to reproduce the previous request.
        kwargs.setdefault('stream', self.stream)
        kwargs.setdefault('verify', self.verify)
        kwargs.setdefault('cert', self.cert)
        kwargs.setdefault('proxies', self.proxies)

        # It's possible that users might accidentally send a Request object.
        # Guard against that specific failure case.
        if isinstance(request, Request):
            raise ValueError('You can only send PreparedRequests.')

        # Set up variables needed for resolve_redirects and dispatching of hooks
        allow_redirects = kwargs.pop('allow_redirects', True)
        stream = kwargs.get('stream')
        hooks = request.hooks

        # Get the appropriate adapter to use
        adapter = self.get_adapter(url=request.url)

        # Start time (approximately) of the request
        start = preferred_clock()

        # Send the request
        r = adapter.send(request, **kwargs)

        # Total elapsed time of the request (approximately)
        elapsed = preferred_clock() - start
        r.elapsed = timedelta(seconds=elapsed)

        # Response manipulation hooks
        r = dispatch_hook('response', hooks, r, **kwargs)

        # Persist cookies
        if r.history:

            # If the hooks create history then we want those cookies too
            for resp in r.history:
                extract_cookies_to_jar(self.cookies, resp.request, resp.raw)

        extract_cookies_to_jar(self.cookies, request, r.raw)

        # Redirect resolving generator.
        gen = self.resolve_redirects(r, request, **kwargs)

        # Resolve redirects if allowed.
        history = [resp for resp in gen] if allow_redirects else []

        # Shuffle things around if there's history.
        if history:
            # Insert the first (original) request at the start
            history.insert(0, r)
            # Get the last request made
            r = history.pop()
            r.history = history

        # If redirects aren't being followed, store the response on the Request for Response.next().
        if not allow_redirects:
            try:
                r._next = next(self.resolve_redirects(r, request, yield_requests=True, **kwargs))
            except StopIteration:
                pass

        if not stream:
            r.content

        return r

发送request是调用r = adapter.send(request, **kwargs)这一行，继续往下看，

    def send(self, request, stream=False, timeout=None, verify=True, cert=None, proxies=None):
        """Sends PreparedRequest object. Returns Response object.

        :param request: The :class:`PreparedRequest <PreparedRequest>` being sent.
        :param stream: (optional) Whether to stream the request content.
        :param timeout: (optional) How long to wait for the server to send
            data before giving up, as a float, or a :ref:`(connect timeout,
            read timeout) <timeouts>` tuple.
        :type timeout: float or tuple or urllib3 Timeout object
        :param verify: (optional) Either a boolean, in which case it controls whether
            we verify the server's TLS certificate, or a string, in which case it
            must be a path to a CA bundle to use
        :param cert: (optional) Any user-provided SSL certificate to be trusted.
        :param proxies: (optional) The proxies dictionary to apply to the request.
        :rtype: requests.Response
        """

        try:
            conn = self.get_connection(request.url, proxies)
        except LocationValueError as e:
            raise InvalidURL(e, request=request)

        self.cert_verify(conn, request.url, verify, cert)
        url = self.request_url(request, proxies)
        self.add_headers(request, stream=stream, timeout=timeout, verify=verify, cert=cert, proxies=proxies)

        chunked = not (request.body is None or 'Content-Length' in request.headers)

        if isinstance(timeout, tuple):
            try:
                connect, read = timeout
                timeout = TimeoutSauce(connect=connect, read=read)
            except ValueError as e:
                # this may raise a string formatting error.
                err = ("Invalid timeout {}. Pass a (connect, read) "
                       "timeout tuple, or a single float to set "
                       "both timeouts to the same value".format(timeout))
                raise ValueError(err)
        elif isinstance(timeout, TimeoutSauce):
            pass
        else:
            timeout = TimeoutSauce(connect=timeout, read=timeout)

        try:
            if not chunked:
                resp = conn.urlopen(
                    method=request.method,
                    url=url,
                    body=request.body,
                    headers=request.headers,
                    redirect=False,
                    assert_same_host=False,
                    preload_content=False,
                    decode_content=False,
                    retries=self.max_retries,
                    timeout=timeout
                )

            # Send the request.
            else:
                if hasattr(conn, 'proxy_pool'):
                    conn = conn.proxy_pool

                low_conn = conn._get_conn(timeout=DEFAULT_POOL_TIMEOUT)

                try:
                    low_conn.putrequest(request.method,
                                        url,
                                        skip_accept_encoding=True)

                    for header, value in request.headers.items():
                        low_conn.putheader(header, value)

                    low_conn.endheaders()

                    for i in request.body:
                        low_conn.send(hex(len(i))[2:].encode('utf-8'))
                        low_conn.send(b'\r\n')
                        low_conn.send(i)
                        low_conn.send(b'\r\n')
                    low_conn.send(b'0\r\n\r\n')

                    # Receive the response from the server
                    try:
                        # For Python 2.7, use buffering of HTTP responses
                        r = low_conn.getresponse(buffering=True)
                    except TypeError:
                        # For compatibility with Python 3.3+
                        r = low_conn.getresponse()

                    resp = HTTPResponse.from_httplib(
                        r,
                        pool=conn,
                        connection=low_conn,
                        preload_content=False,
                        decode_content=False
                    )
                except:
                    # If we hit any problems here, clean up the connection.
                    # Then, reraise so that we can handle the actual exception.
                    low_conn.close()
                    raise

        except (ProtocolError, socket.error) as err:
            raise ConnectionError(err, request=request)

        except MaxRetryError as e:
            if isinstance(e.reason, ConnectTimeoutError):
                # TODO: Remove this in 3.0.0: see #2811
                if not isinstance(e.reason, NewConnectionError):
                    raise ConnectTimeout(e, request=request)

            if isinstance(e.reason, ResponseError):
                raise RetryError(e, request=request)

            if isinstance(e.reason, _ProxyError):
                raise ProxyError(e, request=request)

            if isinstance(e.reason, _SSLError):
                # This branch is for urllib3 v1.22 and later.
                raise SSLError(e, request=request)

            raise ConnectionError(e, request=request)

        except ClosedPoolError as e:
            raise ConnectionError(e, request=request)

        except _ProxyError as e:
            raise ProxyError(e)

        except (_SSLError, _HTTPError) as e:
            if isinstance(e, _SSLError):
                # This branch is for urllib3 versions earlier than v1.22
                raise SSLError(e, request=request)
            elif isinstance(e, ReadTimeoutError):
                raise ReadTimeout(e, request=request)
            else:
                raise

        return self.build_response(request, resp)

继续看conn = self.get_connection(request.url, proxies)这一段，再一层层往下看，看到这里

pool_classes_by_scheme = {
    'http': HTTPConnectionPool,
    'https': HTTPSConnectionPool,
}

发现这里有两个连接池，一个是处理HTTP的、一个是处理HTTPS的，再点进去，发现HTTPSConnectionPool是继承HTTPConnectionPool的，所以我们只需要查看HTTPConnectionPool的就行了，最终的地址：connectionpool.py

注意看这里：urlopen

        """
        Get a connection from the pool and perform an HTTP request. This is the
        lowest level call for making a request, so you'll need to specify all
        the raw details.
                """

从池中获取连接并执行HTTP请求。这就是请求的最低级别调用，因此需要指定原始细节。

最终找到创建连接的地方：connection.py

# This function is copied from socket.py in the Python 2.7 standard
# library test suite. Added to its signature is only `socket_options`.
# One additional modification is that we avoid binding to IPv6 servers
# discovered in DNS if the system doesn't have IPv6 functionality.
def create_connection(address, timeout=socket._GLOBAL_DEFAULT_TIMEOUT,
                      source_address=None, socket_options=None):
    """Connect to *address* and return the socket object.

    Convenience function.  Connect to *address* (a 2-tuple ``(host,
    port)``) and return the socket object.  Passing the optional
    *timeout* parameter will set the timeout on the socket instance
    before attempting to connect.  If no *timeout* is supplied, the
    global default timeout setting returned by :func:`getdefaulttimeout`
    is used.  If *source_address* is set it must be a tuple of (host, port)
    for the socket to bind as a source address before making the connection.
    An host of '' or port 0 tells the OS to use the default.
    """

    host, port = address
    if host.startswith('['):
        host = host.strip('[]')
    err = None

    # Using the value from allowed_gai_family() in the context of getaddrinfo lets
    # us select whether to work with IPv4 DNS records, IPv6 records, or both.
    # The original create_connection function always returns all records.
    family = allowed_gai_family()

    for res in socket.getaddrinfo(host, port, family, socket.SOCK_STREAM):
        af, socktype, proto, canonname, sa = res
        sock = None
        try:
            sock = socket.socket(af, socktype, proto)

            # If provided, set socket level options before connecting.
            _set_socket_options(sock, socket_options)

            if timeout is not socket._GLOBAL_DEFAULT_TIMEOUT:
                sock.settimeout(timeout)
            if source_address:
                sock.bind(source_address)
            sock.connect(sa)
            return sock

        except socket.error as e:
            err = e
            if sock is not None:
                sock.close()
                sock = None

    if err is not None:
        raise err

    raise socket.error("getaddrinfo returns an empty list")

注释这样写：

此函数从Python 2.7标准库测试套件中的socket.py复制。 添加到它的签名只是socket_options。 另一个修改是，如果系统没有IPv6功能，我们将避免绑定到在DNS中发现的IPv6服务器。

其中关键代码是：

1. sock = socket.socket(af, socktype, proto)
2. sock.bind(source_address)
3. sock.connect(sa)

底层是Python的socket编程，Python 中，我们用 socket（）函数来创建套接字，语法格式如下：

socket.socket([family[, type[, proto]]])

更多细节参考：Python3 网络编程

关于发送请求的细节：

    def request_chunked(self, method, url, body=None, headers=None):
        """
        Alternative to the common request method, which sends the
        body with chunked encoding and not as one block
        """
        headers = HTTPHeaderDict(headers if headers is not None else {})
        skip_accept_encoding = 'accept-encoding' in headers
        skip_host = 'host' in headers
        self.putrequest(
            method,
            url,
            skip_accept_encoding=skip_accept_encoding,
            skip_host=skip_host
        )
        for header, value in headers.items():
            self.putheader(header, value)
        if 'transfer-encoding' not in headers:
            self.putheader('Transfer-Encoding', 'chunked')
        self.endheaders()

        if body is not None:
            stringish_types = six.string_types + (six.binary_type,)
            if isinstance(body, stringish_types):
                body = (body,)
            for chunk in body:
                if not chunk:
                    continue
                if not isinstance(chunk, six.binary_type):
                    chunk = chunk.encode('utf8')
                len_str = hex(len(chunk))[2:]
                self.send(len_str.encode('utf-8'))
                self.send(b'\r\n')
                self.send(chunk)
                self.send(b'\r\n')

        # After the if clause, to always have a closed body
        self.send(b'0\r\n\r\n')

获取response

    def read(self, amt=None):
        if self.fp is None:
            return b""

        if self._method == "HEAD":
            self._close_conn()
            return b""

        if amt is not None:
            # Amount is given, implement using readinto
            b = bytearray(amt)
            n = self.readinto(b)
            return memoryview(b)[:n].tobytes()
        else:
            # Amount is not given (unbounded read) so we must check self.length
            # and self.chunked

            if self.chunked:
                return self._readall_chunked()

            if self.length is None:
                s = self.fp.read()
            else:
                try:
                    s = self._safe_read(self.length)
                except IncompleteRead:
                    self._close_conn()
                    raise
                self.length = 0
            self._close_conn()        # we read everything
            return s

看到这里，越来越感觉到自己的无知，什么样的人才能写出这么牛逼的代码啊。给大佬跪了。。。