requests 是用 python 处理 HTTP 请求时毋庸置疑的第一选择，其中的一个原因就是它简洁的 API 接口。那么这些接口下面隐藏的是什么样的东东呢，今天我们来分析一下 requests 是如何处理 multipart/form-data 格式的请求的。
multipart/form-data 编码格式
废话少说，我也懒得介绍 multipart/form-data 是啥了。先抓包看一下浏览器在 form 表单的 enctype="multipart/form-data 时，构造的 POST 请求时什么样。
分析一下，基本的格式长这样：
POST / HTTP/1.1[换行]
Host: localhost[换行]
Content-Type: multipart/form-data; boundary=boundary[换行]
[换行]
--boundary[换行]
Content-Disposition: form-data; name="参数名"[换行]
[换行]
内容[换行]
--boundary[换行]
Content-Disposition: form-data; name="图片名"; filename="图片文件名"[换行]
Content-Type: 类型[换行]
[换行]
二进制内容[换行]
--boundary--
在 Content-Disposition 中，如果有 filename 参数，则 PHP（或其他后端框架）会将此参数解析到 $_FILES 中。
requests 如何编码？
r = requests.post(url, data={'data': 'd'}, files={'files': '1'})
当提供了 files 参数时，requests 也会将 data 做 multipart/form-data 编码，但是不添加 filename 参数，所以后端会解析到 $_POST 中，files 参数任然解析到 $_FILES 中。
r = requests.port(url, files={'file': (None, 'file')})
files={'file': (None, 'file')} 这样将 files 参数写成二元组，第一项表示此参数的 filename 属性。这里手动置为 None，则表示不包含 filename 属性，所以后端会将此参数解析到 $_POST 中。
这种方式既保证了使用 multipart/form-data 编码，也不会被解析到 $_FILES 中，在编写 POC 时常用。
requests 编码分析
编码部分的代码在 requests.models.PreparedRequest.prepare_body() 中，在 PreparedRequests.prepare() 方法中会调用 .prepare_body()，生成 HTTP 请求的 body 部分：
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
# 如果只提供了 json 参数，则使用 application/json 编码
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')
# data 是否为一个可迭代对象（是否使用 chunked 方式上传）
is_stream = all([
hasattr(data, '__iter__'),
not isinstance(data, (basestring, list, tuple, collections.Mapping))
])
try:
length = super_len(data)
except (TypeError, AttributeError, UnsupportedOperation):
length = None
# 使用 chunked 方式上传
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.
# 如果有 files 参数，则使用 _encode_files 方法，进行 multi-part 编码
if files:
(body, content_type) = self._encode_files(files, data)
# 否则按照 urlencoded 方式编码 data 部分
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
下面看一下 _encode_files() 和 _encode_params() 方法：
class RequestEncodingMixin(object):
...
@staticmethod
def _encode_params(data):
"""Encode parameters in a piece of data.
Will successfully encode parameters when passed as a dict or a list of
2-tuples. Order is retained if data is a list of 2-tuples but arbitrary
if parameters are supplied as a dict.
"""
if isinstance(data, (str, bytes)):
return data
elif hasattr(data, 'read'):
return data
elif hasattr(data, '__iter__'):
result = []
for k, vs in to_key_val_list(data):
if isinstance(vs, basestring) or not hasattr(vs, '__iter__'):
vs = [vs]
for v in vs:
if v is not None:
result.append(
(k.encode('utf-8') if isinstance(k, str) else k,
v.encode('utf-8') if isinstance(v, str) else v))
# 直接使用 urlencode 进行编码
return urlencode(result, doseq=True)
else:
return data
@staticmethod
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 = []
# 全部转为 [(key, val)] 的形式，统一处理
fields = to_key_val_list(data or {})
files = to_key_val_list(files or {})
for field, val in fields:
# 从这里看 data 参数还接受 list of string?
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
# 分别处理各种大小的元组
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:
# 如果不是元组形式，则从 file_obj 中猜测文件名；或者直接使用内容字符串作为文件名
fn = guess_filename(v) or k
fp = v
if isinstance(fp, (str, bytes, bytearray)):
fdata = fp
else:
fdata = fp.read()
# 使用的 urllib3.fields.RequestFile
rf = RequestField(name=k, data=fdata, filename=fn, headers=fh)
rf.make_multipart(content_type=ft)
new_fields.append(rf)
# 使用 urllib3.filepost 中的方法
body, content_type = encode_multipart_formdata(new_fields)
return body, content_type
可以看到，在处理 multipart 的编码时，requests 直接使用 urllib3 提供的方法，那我们就看一下 urllib3 是如何实现的。
def encode_multipart_formdata(fields, boundary=None):
"""
Encode a dictionary of ``fields`` using the multipart/form-data MIME format.
:param fields:
Dictionary of fields or list of (key, :class:`~urllib3.fields.RequestField`).
:param boundary:
If not specified, then a random boundary will be generated using
:func:`mimetools.choose_boundary`.
"""
body = BytesIO()
if boundary is None:
boundary = choose_boundary()
for field in iter_field_objects(fields):
body.write(b('--%s\r\n' % (boundary)))
writer(body).write(field.render_headers())
data = field.data
if isinstance(data, int):
data = str(data) # Backwards compatibility
if isinstance(data, six.text_type):
writer(body).write(data)
else:
body.write(data)
body.write(b'\r\n')
body.write(b('--%s--\r\n' % (boundary)))
content_type = str('multipart/form-data; boundary=%s' % boundary)
return body.getvalue(), content_type
def iter_field_objects(fields):
"""
Iterate over fields.
Supports list of (k, v) tuples and dicts, and lists of
:class:`~urllib3.fields.RequestField`.
"""
if isinstance(fields, dict):
i = six.iteritems(fields)
else:
i = iter(fields)
for field in i:
if isinstance(field, RequestField):
yield field
else:
yield RequestField.from_tuples(*field)
代码比较直白，就不多说啦~