Dealing with Request Data¶
The most important rule about web development is “Do not trust the user”. This is especially true for incoming request data on the input stream. With WSGI this is actually a bit harder than you would expect. Because of that Werkzeug wraps the request stream for you to save you from the most prominent problems with it.
Missing EOF Marker on Input Stream¶
The input stream has no end-of-file marker. If you would call the
read() method on the wsgi.input stream you would cause your
application to hang on conforming servers. This is actually intentional
however painful. Werkzeug solves that problem by wrapping the input
stream in a special
LimitedStream. The input stream is exposed
on the request objects as
stream. This one is either
an empty stream (if the form data was parsed) or a limited stream with
the contents of the input stream.
When does Werkzeug Parse?¶
Werkzeug parses the incoming data under the following situations:
- you access either
streamand the request method was POST or PUT.
- if you call
These calls are not interchangeable. If you invoke
you must not use the request object or at least not the attributes that
trigger the parsing process.
This is also true if you read from the wsgi.input stream before the parsing.
General rule: Leave the WSGI input stream alone. Especially in WSGI middlewares. Use either the parsing functions or the request object. Do not mix multiple WSGI utility libraries for form data parsing or anything else that works on the input stream.
How does it Parse?¶
The standard Werkzeug parsing behavior handles three cases:
- input content type was multipart/form-data. In this situation the
streamwill be empty and
formwill contain the regular POST / PUT data,
fileswill contain the uploaded files as
- input content type was application/x-www-form-urlencoded. Then the
streamwill be empty and
formwill contain the regular POST / PUT data and
fileswill be empty.
- the input content type was neither of them,
streampoints to a
LimitedStreamwith the input data for further processing.
Special note on the
get_data method: Calling this
loads the full request data into memory. This is only safe to do if the
max_content_length is set. Also you can either
read the stream or call
Limiting Request Data¶
To avoid being the victim of a DDOS attack you can set the maximum
accepted content length and request field sizes. The
class has two attributes for that:
The first one can be used to limit the total content length. For example
by setting it to
1024 * 1024 * 16 the request won’t accept more than
16MB of transmitted data.
Because certain data can’t be moved to the hard disk (regular post data)
whereas temporary files can, there is a second limit you can set. The
max_form_memory_size limits the size of POST
transmitted form data. By setting it to
1024 * 1024 * 2 you can make
sure that all in memory-stored fields is not more than 2MB in size.
This however does not affect in-memory stored files if the stream_factory used returns a in-memory file.
How to extend Parsing?¶
Modern web applications transmit a lot more than multipart form data or
url encoded data. Extending the parsing capabilities by subclassing
BaseRequest is simple. The following example implements
parsing for incoming JSON data:
from werkzeug.utils import cached_property from werkzeug.wrappers import Request from simplejson import loads class JSONRequest(Request): # accept up to 4MB of transmitted data. max_content_length = 1024 * 1024 * 4 @cached_property def json(self): if self.headers.get('content-type') == 'application/json': return loads(self.data)