Parsers are responsible for taking the content of the request body as a bytestream, and transforming it into a native Python data representation.

Flask API includes a few built-in parser classes and also provides support for defining your own custom parsers.

How the parser is determined

The set of valid parsers for a view is always defined as a list of classes. When any of the properties, request.form or request.files are accessed, Flask API will examine the Content-Type header on the incoming request, and determine which parser to use to handle the request content.

Note: When developing client applications always remember to make sure you're setting the Content-Type header when sending data in an HTTP request.

If you don't set the content type, most clients will default to using 'application/x-www-form-urlencoded', which may not be what you wanted.

As an example, if you are sending json encoded data using jQuery with the .ajax() method, you should make sure to include the contentType: 'application/json' setting.

Setting the parsers

The default set of parsers may be set globally, using the DEFAULT_PARSERS configuration key. The default configuration will deal with parsing either JSON or form encoded requests.

app.config['DEFAULT_PARSERS'] = [

You can also set the parsers used for an individual view, using the set_parsers decorator.

from flask.ext.api.decorators import set_parsers
from flask.ext.api.parsers import JSONParser


@set_parsers(JSONParser, MyCustomXMLParser)
def example():
    return {
        'example': 'Setting renderers on a per-view basis',
        'request data':

API Reference


Parses JSON request content and populates

media_type: application/json


Parses HTML form content. will be populated with a MultiDict of data.

You will typically want to use both FormParser and MultiPartParser together in order to fully support HTML form data.

media_type: application/x-www-form-urlencoded


Parses multipart HTML form content, which supports file uploads. Both and request.files will be populated with a MultiDict.

You will typically want to use both FormParser and MultiPartParser together in order to fully support HTML form data.

media_type: multipart/form-data

Custom parsers

To implement a custom parser, you should override BaseParser, set the .media_type property, and implement the .parse(self, stream, media_type, **options) method.

The method should return the data that will be used to populate the property.

The arguments passed to .parse() are:


A bytestream representing the body of the request.


An instance of MediaType indicating media type of the incoming request.

Depending on the request's Content-Type: header, this may be more specific than the renderer's media_type attribute, and may include media type parameters. For example "text/plain; charset=utf-8".


Any additional contextual arguments that may be required in order to parse the request. By default this includes a single keyword argument:


The following is an example plaintext parser that will populate the property with a string representing the body of the request.

class PlainTextParser(BaseParser):
    Plain text parser.
    media_type = 'text/plain'

    def parse(self, stream, media_type, **options):
        Simply return a string representing the body of the request.