Python requests上传文件实现步骤

官方文档：https://2.python-requests.org//en/master/

工作中涉及到一个功能，需要上传附件到一个接口，接口参数如下：

使用http post提交附件 multipart/form-data 格式，url : http://test.com/flow/upload，

字段列表：
md5:      //md5加密（随机值_当时时间戳）
filesize:  //文件大小
file:       //文件内容(须含文件名)
返回值：
{"success":true,"uploadName":"tmp.xml","uploadPath":"uploads\/201311\/758e875fb7c7a508feef6b5036119b9f"}

由于工作中主要用python，并且项目中已有使用requests库的地方，所以计划使用requests来实现，本来以为是很简单的一个小功能，结果花费了大量的时间，requests官方的例子只提到了上传文件，并不需要传额外的参数：

https://2.python-requests.org//en/master/user/quickstart/#post-a-multipart-encoded-file

>>> url = 'https://httpbin.org/post'
>>> files = {'file': ('report.xls', open('report.xls', 'rb'), 'application/vnd.ms-excel', {'Expires': '0'})}

>>> r = requests.post(url, files=files)
>>> r.text
{
 ...
 "files": {
  "file": "<censored...binary...data>"
 },
 ...
}

但是如果涉及到了参数的传递时，其实就要用到requests的两个参数：data、files，将要上传的文件传入files，将其他参数传入data，request库会将两者合并到一起做一个multi part，然后发送给服务器。

最终实现的代码是这样的：

with open(file_name) as f:
　　content = f.read()
request_data = {
  'md5':md5.md5('%d_%d' % (0, int(time.time()))).hexdigest(),
  'filesize':len(content),
}
files = {'file':(file_name, open(file_name, 'rb'))}
MyLogger().getlogger().info('url:%s' % (request_url))
resp = requests.post(request_url, data=request_data, files=files)

虽然最终代码可能看起来很简单，但是其实我费了好大功夫才确认这样是OK的，中间还翻了requests的源码，下面记录一下翻阅源码的过程：

首先，找到post方法的实现，在requests.api.py中：

def post(url, data=None, json=None, **kwargs):
  r"""Sends a POST request.

  :param url: URL for the new :class:`Request` object.
  :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 data to send in the body of the :class:`Request`.
  :param \*\*kwargs: Optional arguments that ``request`` takes.
  :return: :class:`Response <Response>` object
  :rtype: requests.Response
  """

  return request('post', url, data=data, json=json, **kwargs)

这里可以看到它调用了request方法，咱们继续跟进request方法，在requests.api.py中：

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

  :param method: method for the new :class:`Request` object: ``GET``, ``OPTIONS``, ``HEAD``, ``POST``, ``PUT``, ``PATCH``, or ``DELETE``.
  :param url: URL for the new :class:`Request` object.
  :param params: (optional) Dictionary, list of tuples or bytes to send
    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) 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)

这个方法的注释比较多，从注释里其实已经可以看到files参数使用传送文件，但是还是无法知道当需要同时传递参数和文件时该如何处理，继续跟进session.request方法，在requests.session.py中：

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

先大概看一下这个方法，先是准备request，最后一步是调用send，推测应该是发送请求了，所以我们需要跟进到prepare_request方法中，在requests.session.py中：

def prepare_request(self, request):
    """Constructs a :class:`PreparedRequest <PreparedRequest>` for
    transmission and returns it. The :class:`PreparedRequest` has settings
    merged from the :class:`Request <Request>` instance and those of the
    :class:`Session`.

    :param request: :class:`Request` instance to prepare with this
      session's settings.
    :rtype: requests.PreparedRequest
    """
    cookies = request.cookies or {}

    # Bootstrap CookieJar.
    if not isinstance(cookies, cookielib.CookieJar):
      cookies = cookiejar_from_dict(cookies)

    # Merge with session cookies
    merged_cookies = merge_cookies(
      merge_cookies(RequestsCookieJar(), self.cookies), cookies)

    # Set environment's basic authentication if not explicitly set.
    auth = request.auth
    if self.trust_env and not auth and not self.auth:
      auth = get_netrc_auth(request.url)

    p = PreparedRequest()
    p.prepare(
      method=request.method.upper(),
      url=request.url,
      files=request.files,
      data=request.data,
      json=request.json,
      headers=merge_setting(request.headers, self.headers, dict_class=CaseInsensitiveDict),
      params=merge_setting(request.params, self.params),
      auth=merge_setting(auth, self.auth),
      cookies=merged_cookies,
      hooks=merge_hooks(request.hooks, self.hooks),
    )
    return p

在prepare_request中，生成了一个PreparedRequest对象，并调用其prepare方法，跟进到prepare方法中，在requests.models.py中：

def prepare(self,
      method=None, url=None, headers=None, files=None, data=None,
      params=None, auth=None, cookies=None, hooks=None, json=None):
    """Prepares the entire request with the given parameters."""

    self.prepare_method(method)
    self.prepare_url(url, params)
    self.prepare_headers(headers)
    self.prepare_cookies(cookies)
    self.prepare_body(data, files, json)
    self.prepare_auth(auth, url)

    # Note that prepare_auth must be last to enable authentication schemes
    # such as OAuth to work on a fully prepared request.

    # This MUST go after prepare_auth. Authenticators could add a hook
    self.prepare_hooks(hooks)

这里调用许多prepare_xx方法，这里我们只关心处理了data、files、json的方法，跟进到prepare_body中，在requests.models.py中：

def prepare_body(self, data, files, json=None):
    """Prepares the given HTTP body data."""

    # Check if file, fo, generator, iterator.
    # If not, run through normal process.

    # Nottin' on you.
    body = None
    content_type = None

    if not data and json is not None:
      # urllib3 requires a bytes-like body. Python 2's json.dumps
      # provides this natively, but Python 3 gives a Unicode string.
      content_type = 'application/json'
      body = complexjson.dumps(json)
      if not isinstance(body, bytes):
        body = body.encode('utf-8')

    is_stream = all([
      hasattr(data, '__iter__'),
      not isinstance(data, (basestring, list, tuple, Mapping))
    ])

    try:
      length = super_len(data)
    except (TypeError, AttributeError, UnsupportedOperation):
      length = None

    if is_stream:
      body = data

      if getattr(body, 'tell', None) is not None:
        # Record the current file position before reading.
        # This will allow us to rewind a file in the event
        # of a redirect.
        try:
          self._body_position = body.tell()
        except (IOError, OSError):
          # This differentiates from None, allowing us to catch
          # a failed `tell()` later when trying to rewind the body
          self._body_position = object()

      if files:
        raise NotImplementedError('Streamed bodies and files are mutually exclusive.')

      if length:
        self.headers['Content-Length'] = builtin_str(length)
      else:
        self.headers['Transfer-Encoding'] = 'chunked'
    else:
      # Multi-part file uploads.
      if files:
        (body, content_type) = self._encode_files(files, data)
      else:
        if data:
          body = self._encode_params(data)
          if isinstance(data, basestring) or hasattr(data, 'read'):
            content_type = None
          else:
            content_type = 'application/x-www-form-urlencoded'

      self.prepare_content_length(body)

      # Add content-type if it wasn't explicitly provided.
      if content_type and ('content-type' not in self.headers):
        self.headers['Content-Type'] = content_type

    self.body = body

这个函数比较长，需要重点关注L52，这里调用了_encode_files方法，我们跟进这个方法：

def _encode_files(files, data):
    """Build the body for a multipart/form-data request.

    Will successfully encode files when passed as a dict or a list of
    tuples. Order is retained if data is a list of tuples but arbitrary
    if parameters are supplied as a dict.
    The tuples may be 2-tuples (filename, fileobj), 3-tuples (filename, fileobj, contentype)
    or 4-tuples (filename, fileobj, contentype, custom_headers).
    """
    if (not files):
      raise ValueError("Files must be provided.")
    elif isinstance(data, basestring):
      raise ValueError("Data must not be a string.")

    new_fields = []
    fields = to_key_val_list(data or {})
    files = to_key_val_list(files or {})

    for field, val in fields:
      if isinstance(val, basestring) or not hasattr(val, '__iter__'):
        val = [val]
      for v in val:
        if v is not None:
          # Don't call str() on bytestrings: in Py3 it all goes wrong.
          if not isinstance(v, bytes):
            v = str(v)

          new_fields.append(
            (field.decode('utf-8') if isinstance(field, bytes) else field,
             v.encode('utf-8') if isinstance(v, str) else v))

    for (k, v) in files:
      # support for explicit filename
      ft = None
      fh = None
      if isinstance(v, (tuple, list)):
        if len(v) == 2:
          fn, fp = v
        elif len(v) == 3:
          fn, fp, ft = v
        else:
          fn, fp, ft, fh = v
      else:
        fn = guess_filename(v) or k
        fp = v

      if isinstance(fp, (str, bytes, bytearray)):
        fdata = fp
      elif hasattr(fp, 'read'):
        fdata = fp.read()
      elif fp is None:
        continue
      else:
        fdata = fp

      rf = RequestField(name=k, data=fdata, filename=fn, headers=fh)
      rf.make_multipart(content_type=ft)
      new_fields.append(rf)

    body, content_type = encode_multipart_formdata(new_fields)

    return body, content_type

OK，到此为止，仔细阅读完这个段代码，就可以搞明白requests.post方法传入的data、files两个参数的作用了，其实requests在这里把它俩合并在一起了，作为post的body。

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