.. module:: aiohttp
aiohttp supports a full featured multipart reader and writer. Both
are designed with steaming processing in mind to avoid unwanted
footprint which may be significant if you're dealing with large
payloads, but this also means that most I/O operation are only
possible to be executed a single time.
Assume you made a request, as usual, and want to process the response multipart data:
async with aiohttp.request(...) as resp:
pass
First, you need to wrap the response with a :meth:`MultipartReader.from_response`. This needs to keep the implementation of :class:`MultipartReader` separated from the response and the connection routines which makes it more portable:
reader = aiohttp.MultipartReader.from_response(resp)
Let's assume with this response you'd received some JSON document and multiple files for it, but you don't need all of them, just a specific one.
So first you need to enter into a loop where the multipart body will be processed:
metadata = None
filedata = None
while True:
part = await reader.next()
The returned type depends on what the next part is: if it's a simple body part
then you'll get :class:`BodyPartReader` instance here, otherwise, it will
be another :class:`MultipartReader` instance for the nested multipart. Remember,
that multipart format is recursive and supports multiple levels of nested body
parts. When there are no more parts left to fetch, None value will be
returned - that's the signal to break the loop:
if part is None:
break
Both :class:`BodyPartReader` and :class:`MultipartReader` provides access to body part headers: this allows you to filter parts by their attributes:
if part.headers[aiohttp.hdrs.CONTENT_TYPE] == 'application/json':
metadata = await part.json()
continue
Nor :class:`BodyPartReader` or :class:`MultipartReader` instances does not read the whole body part data without explicitly asking for. :class:`BodyPartReader` provides a set of helpers methods to fetch popular content types in friendly way:
- :meth:`BodyPartReader.text` for plain text data;
- :meth:`BodyPartReader.json` for JSON;
- :meth:`BodyPartReader.form` for application/www-urlform-encode
Each of these methods automatically recognizes if content is compressed by using gzip and deflate encoding (while it respects identity one), or if transfer encoding is base64 or quoted-printable - in each case the result will get automatically decoded. But in case you need to access to raw binary data as it is, there are :meth:`BodyPartReader.read` and :meth:`BodyPartReader.read_chunk` coroutine methods as well to read raw binary data as it is all-in-single-shot or by chunks respectively.
When you have to deal with multipart files, the :attr:`BodyPartReader.filename` property comes to help. It's a very smart helper which handles Content-Disposition handler right and extracts the right filename attribute from it:
if part.filename != 'secret.txt':
continue
If current body part does not matches your expectation and you want to skip it
- just continue a loop to start a next iteration of it. Here is where magic
happens. Before fetching the next body part await reader.next() it
ensures that the previous one was read completely. If it was not, all its content
sends to the void in term to fetch the next part. So you don't have to care
about cleanup routines while you're within a loop.
Once you'd found a part for the file you'd searched for, just read it. Let's handle it as it is without applying any decoding magic:
filedata = await part.read(decode=False)
Later you may decide to decode the data. It's still simple and possible to do:
filedata = part.decode(filedata)
Once you are done with multipart processing, just break a loop:
break
:class:`MultipartWriter` provides an interface to build multipart payload from
the Python data and serialize it into chunked binary stream. Since multipart
format is recursive and supports deeply nesting, you can use with statement
to design your multipart data closer to how it will be:
with aiohttp.MultipartWriter('mixed') as mpwriter:
...
with aiohttp.MultipartWriter('related') as subwriter:
...
mpwriter.append(subwriter)
with aiohttp.MultipartWriter('related') as subwriter:
...
with aiohttp.MultipartWriter('related') as subsubwriter:
...
subwriter.append(subsubwriter)
mpwriter.append(subwriter)
with aiohttp.MultipartWriter('related') as subwriter:
...
mpwriter.append(subwriter)
The :meth:`MultipartWriter.append` is used to join new body parts into a single stream. It accepts various inputs and determines what default headers should be used for.
For text data default Content-Type is :mimetype:`text/plain; charset=utf-8`:
mpwriter.append('hello')
For binary data :mimetype:`application/octet-stream` is used:
mpwriter.append(b'aiohttp')
You can always override these default by passing your own headers with the second argument:
mpwriter.append(io.BytesIO(b'GIF89a...'),
{'CONTENT-TYPE': 'image/gif'})
For file objects Content-Type will be determined by using Python's mod:mimetypes module and additionally Content-Disposition header will include the file's basename:
part = root.append(open(__file__, 'rb'))
If you want to send a file with a different name, just handle the :class:`Payload` instance which :meth:`MultipartWriter.append` will always return and set Content-Disposition explicitly by using the :meth:`Payload.set_content_disposition` helper:
part.set_content_disposition('attachment', filename='secret.txt')
Additionally, you may want to set other headers here:
part.headers[aiohttp.hdrs.CONTENT_ID] = 'X-12345'
If you'd set Content-Encoding, it will be automatically applied to the data on serialization (see below):
part.headers[aiohttp.hdrs.CONTENT_ENCODING] = 'gzip'
There are also :meth:`MultipartWriter.append_json` and :meth:`MultipartWriter.append_form` helpers which are useful to work with JSON and form urlencoded data, so you don't have to encode it every time manually:
mpwriter.append_json({'test': 'passed'})
mpwriter.append_form([('key', 'value')])
When it's done, to make a request just pass a root :class:`MultipartWriter`
instance as :meth:`aiohttp.ClientSession.request` data argument:
await session.post('http://example.com', data=mpwriter)
Behind the scenes :meth:`MultipartWriter.write` will yield chunks of every part and if body part has Content-Encoding or Content-Transfer-Encoding they will be applied on streaming content.
Please note, that on :meth:`MultipartWriter.write` all the file objects will be read until the end and there is no way to repeat a request without rewinding their pointers to the start.
Example MJPEG Streaming multipart/x-mixed-replace. By default
:meth:`MultipartWriter.write` appends closing --boundary-- and breaks your
content. Providing close_boundary = False prevents this.:
my_boundary = 'some-boundary'
response = web.StreamResponse(
status=200,
reason='OK',
headers={
'Content-Type': 'multipart/x-mixed-replace;boundary=--%s' % my_boundary
}
)
while True:
frame = get_jpeg_frame()
with MultipartWriter('image/jpeg', boundary=my_boundary) as mpwriter:
mpwriter.append(frame, {
'Content-Type': 'image/jpeg'
})
await mpwriter.write(response, close_boundary=False)
await response.drain()
The Internet is full of terror and sometimes you may find a server which implements multipart support in strange ways when an oblivious solution does not work.
For instance, is server used :class:`cgi.FieldStorage` then you have to ensure that no body part contains a Content-Length header:
for part in mpwriter:
part.headers.pop(aiohttp.hdrs.CONTENT_LENGTH, None)
On the other hand, some server may require to specify Content-Length for the whole multipart request. aiohttp does not do that since it sends multipart using chunked transfer encoding by default. To overcome this issue, you have to serialize a :class:`MultipartWriter` by our own in the way to calculate its size:
class Writer:
def __init__(self):
self.buffer = bytearray()
async def write(self, data):
self.buffer.extend(data)
writer = Writer()
mpwriter.writer(writer)
await aiohttp.post('http://example.com',
data=writer.buffer, headers=mpwriter.headers)
Sometimes the server response may not be well formed: it may or may not contains nested parts. For instance, we request a resource which returns JSON documents with the files attached to it. If the document has any attachments, they are returned as a nested multipart. If it has not it responds as plain body parts:
CONTENT-TYPE: multipart/mixed; boundary=--:
--:
CONTENT-TYPE: application/json
{"_id": "foo"}
--:
CONTENT-TYPE: multipart/related; boundary=----:
----:
CONTENT-TYPE: application/json
{"_id": "bar"}
----:
CONTENT-TYPE: text/plain
CONTENT-DISPOSITION: attachment; filename=bar.txt
bar! bar! bar!
----:--
--:
CONTENT-TYPE: application/json
{"_id": "boo"}
--:
CONTENT-TYPE: multipart/related; boundary=----:
----:
CONTENT-TYPE: application/json
{"_id": "baz"}
----:
CONTENT-TYPE: text/plain
CONTENT-DISPOSITION: attachment; filename=baz.txt
baz! baz! baz!
----:--
--:--
Reading such kind of data in single stream is possible, but is not clean at all:
result = []
while True:
part = await reader.next()
if part is None:
break
if isinstance(part, aiohttp.MultipartReader):
# Fetching files
while True:
filepart = await part.next()
if filepart is None:
break
result[-1].append((await filepart.read()))
else:
# Fetching document
result.append([(await part.json())])
Let's hack a reader in the way to return pairs of document and reader of the related files on each iteration:
class PairsMultipartReader(aiohttp.MultipartReader):
# keep reference on the original reader
multipart_reader_cls = aiohttp.MultipartReader
async def next(self):
"""Emits a tuple of document object (:class:`dict`) and multipart
reader of the followed attachments (if any).
:rtype: tuple
"""
reader = await super().next()
if self._at_eof:
return None, None
if isinstance(reader, self.multipart_reader_cls):
part = await reader.next()
doc = await part.json()
else:
doc = await reader.json()
return doc, reader
And this gives us a more cleaner solution:
reader = PairsMultipartReader.from_response(resp)
result = []
while True:
doc, files_reader = await reader.next()
if doc is None:
break
files = []
while True:
filepart = await files_reader.next()
if file.part is None:
break
files.append((await filepart.read()))
result.append((doc, files))
.. seealso:: :ref:`aiohttp-multipart-reference`