I need to get the raw content of POST request body (as a string) yet when I try to access request.body
I'm getting an exception:
django.http.request.RawPostDataException:
You cannot access body after reading from request's data stream
I am aware that it is adviced to use request.data
instead of request.body
when using Django Rest Framework, yet for the purpose of validating digital signature I have to have the request body in a raw and "untouched" form, since this is what 3rd-party signed and what I need to validate.
Pseudocode:
3rd_party_sign(json_data + secret_key) != validate_sign(json.dumps(request.data) + secret_key)
3rd_party_sign(json_data + secret_key) == validate_sign(request.body + secret_key)
I have found interesting topic on DRFs GitHub, but it does not fully cover the problem. I have investigated the case and came up with a neat solution. Surprisingly there was no such question on SO, so I decided to add it for public following the SO self-answer guidelines.
The key for understanding the problem and solution is how the HttpRequest.body
(source) works:
@property
def body(self):
if not hasattr(self, '_body'):
if self._read_started:
raise RawPostDataException("You cannot access body after reading from request's data stream")
# (...)
try:
self._body = self.read()
except IOError as e:
raise UnreadablePostError(*e.args) from e
self._stream = BytesIO(self._body)
return self._body
When accessing body
- if the self._body
is already set its simply returned, otherwise the internal request stream is being read and assigned to _body: self._body = self.read()
. Since then any further access to body
falls back to return self._body
. In addition before reading the internal request stream there is a if self._read_started
check which raises an exception if "read has started".
The self._read_started
flague is being set by the read()
method (source):
def read(self, *args, **kwargs):
self._read_started = True
try:
return self._stream.read(*args, **kwargs)
except IOError as e:
six.reraise(UnreadablePostError, ...)
Now it should be clear that the RawPostDataException
will be raised after accessing the request.body
if only the read()
method has been called without assigning its result to requests self._body
.
Now lets have a look at DRF JSONParser
class (source):
class JSONParser(BaseParser):
media_type = 'application/json'
renderer_class = renderers.JSONRenderer
def parse(self, stream, media_type=None, parser_context=None):
parser_context = parser_context or {}
encoding = parser_context.get('encoding', settings.DEFAULT_CHARSET)
try:
data = stream.read().decode(encoding)
return json.loads(data)
except ValueError as exc:
raise ParseError('JSON parse error - %s' % six.text_type(exc))
(I have chosen slightly older version o DRF source, cause after May 2017 there have been some performance improvements that obscure the key line for understanding our problem)
Now it should be clear that the stream.read()
call sets the _read_started
flague and therefore it is impossible for the body
property to access the stream once again (after the parser).
The "no request.body" approach is a DRF intention (I guess) so despite it is technically possible to enable access to request.body
globally (via custom middleware) - it should NOT be done without deep understanding of all its consequences.
The access to the request.body
property may be explicitly and locally granted in the following manner:
You need to define custom parser:
import json
from django.conf import settings
from rest_framework.exceptions import ParseError
from rest_framework import renderers
from rest_framework.parsers import BaseParser
class MyJSONParser(BaseParser):
media_type = 'application/json'
renderer_class = renderers.JSONRenderer
def parse(self, stream, media_type=None, parser_context=None):
parser_context = parser_context or {}
encoding = parser_context.get('encoding', settings.DEFAULT_CHARSET)
request = parser_context.get('request')
try:
data = stream.read().decode(encoding)
setattr(request, 'raw_body', data) # setting a 'body' alike custom attr with raw POST content
return json.loads(data)
except ValueError as exc:
raise ParseError('JSON parse error - %s' % six.text_type(exc))
Then it can be used when it is necessary to access raw request content:
@api_view(['POST'])
@parser_classes((MyJSONParser,))
def example_view(request, format=None):
return Response({'received data': request.raw_body})
While request.body
still remains globally inaccessible (as DRF authors intended).