Armin Ronacher's Thoughts and Writings

Pro/Cons about Werkzeug, WebOb and Django

written on Wednesday, August 5, 2009

Yesterday I had a discussion with Ben Bangert from the Pylons team, Philip Jenvey and zepolen from the pylons IRC channel. The topic of the discussion was why we have Request and Response objects in Werkzeug, WebOb and Django and what we could to to improve the situation a bit.

We decided on writing down what we like or dislike on these three systems in order to find out in which direction to go, so this is my attempt. Please keep in mind that this are my opinions only!


Let's start with WebOb which is the smallest of the three libraries in question. WebOb really just sticks to the basics and provides request and response objects and some data structures required.

The philosophy of WebOb is to stay as compatible to paste as possible and that modifications on the request object appear in the WSGI environment. That basically means that when you do anything on the request object and you create another one later from the same environment you will see your modifications again.

This is without doubt something that neither Werkzeug or Django do. Both Werkzeug and Django consider the incoming request something you should not modify, after all it came from the client. If you need to create a request or WSGI environment in Werkzeug you get a separate utility for, that is designed for exactly that purpose.

While I have to admit that the idea of a reflecting request object is tempting, I don't think it's a good idea. Using the WSGI environment as a communication channel seems wrong to me. The main problem with it is that WebOb cannot achieve what it's doing with standard environment keys. There are currently five WebOb keys in the environment for “caching” purposes and for compatibility with paste it also understands a couple of paste environment keys.

The idea is that other applications can get a request again at a completely different point, but I'm not sure if WSGI is the correct solution for that particular problem. Reusable applications based on the complex WSGI middleware system seems to be the wrong layer to me.

Some other parts where I don't agree with the WebOb concepts:

  • The parsing of the data is implemented either in private functions or directly in the request object. I strongly prefer giving the user the choice to access the parser separately. Sometimes you really just need a cookie parsed, why create a full request object then?
  • WebOb uses request.GET and request.POST for URL parameters and form data. Because you can have URL parameters in non-GET requests as well this is misleading, for POST data it's wrong as well because form data is available in more than just POST requests. Accessing request.POST to get form data in a PUT request seems wrong.
  • WebOb still uses cgi.FieldStorage and not only internally but also it puts those objects into the POST dict. This is not the best idea for multiple reasons. First of all users are encouraged to trust their submitted data and blindly expect a field storage object if they have a upload field in their form. One could easily cause trouble by sending forged requests to the application. If logging is set up the administrator is sent tons of error mails instantly. I strongly prefer storing uploaded files in a separate dictionary like Django and Werkzeug do. The other problem with using FieldStorage as parser is that it's not WSGI compliant by requiring a size argument on the readline function and that it has a weird API. You can't easily tell it to not accept more than n bytes in memory and to switch between in memory uploading and a temporary file based on the length of the transmitted data. Also cgi.FieldStorage supports nested files which no browser supports and which could cause internal server errors as well because very few developers know that a) nested uploads exist and b) that the field storage object behaves differently if a nested uploaded file is transmitted.
  • Also WebOb barks on invalid cookies and throws away all of them if one is broken. This is especially annoying if you're dealing with cookies outside of your control that use invalid characters (stuff such as advertisement cookies)

Now to the parts where WebOb wins over Django and Werkzeug:

  • Unlike Django and Werkzeug WebOb provides not only a unicode API but also a bytestring based API. This could help existing applications that are not unicode ready yet. Downside is that with the current plans of Graham for WSGI on Python 3 there do not seem to be ways to support it on Python 3.
  • WebOb supports the HTTP range feature.
  • The charset can be switched on the fly in WebOb, in Werkzeug you set the charset for your request/response object and from that point nowards it's used no matter what. In Django the charset is application wide.

An interesting thing is that WebOb uses datetime objects with timezone informations. The tzinfo attribute is set to a tzinfo object with an UTC offset of zero. That's different to Werkzeug and Django which use offset-naive datetime objects. Because Python treats them differently and does not support operations that mix those. Unfortunately the datetime module makes it hard to decide what to do. Personally I decided to use datetime objects that have no tzinfo set and only dates in UTC.


In terms of code base size Werkzeug's next. The problem with Werkzeug certainly is that it does not really know what belongs into it and what not. That situation will slightly improve with the next version of it when some deprecated interfaces go away and when the debugger is moved into a new library together with all sorts of debugging tools such as profilers, leak finders and more (enter flickzeug).

Werkzeug is based on the principle that things should have a nice API but at the same time allow you to use the underlying functions. For example you can easily access request.form to get a dict of uploaded form data, but at the same time you can call werkzeug.parse_form_data to parse the stuff into a multidict. You can even go a layer down and tell Werkzeug to not use the multidict and provide a custom container or a standard dict, list, whatever.

Also Werkzeug has a slightly different goal than WebOb. WebOb focuses on the request and response object only, Werkzeug provides all kind of useful helpers for web applications. The idea is that if there is a function you can use, you are more likely to use it than that you reimplement it. For example many applications take the uploaded file name and just create a file with the same name. This however turns out to be a security problem so Werkzeug gives you a function (werkzeug.secure_filename) you can use to get a secure version of the filename that also is limited to ASCII characters.

So obviously there is a lot of stuff in Werkzeug you probably would not expect there.

So here some of the things I like especially about Werkzeug:

  • The request/response objects. They are designed to be lightweight and can be extended using mixins. Werkzeug also provides full-featured request objects that implement all shipped mixins. Also the request/response objects are not doing any parsing or dumping, that is all available through separate functions as well which makes the code readable and easy to extend.
  • It fixes many problems with the standard library or reimplements broken features. It does not depend on the cgi.FieldStorage since 0.5, allows you to limit the uploaded data before it's consumed. That way an attacker cannot exhaust server resources.
  • The data structures provide handy helpers such as raising key errors that are also bad request exceptions so that if you're not catching them, you are at least not generating internal server errors as long as the base HTTPException is catched.
  • Werkzeug uses a non-data descriptor for the properties on the request and response objects. The first time you access the property code is executed and that is stuffed into the dict. After that there is no runtime penalty when accessing the attributes.

And of course here the list of things that are not that nice:

  • It's too large for a library that only wants to implement request and response objects.
  • There is no support for if-range and friends.
  • The response stream is useless because each write() ends up as a separate “item” in the application iterator. Because each item is followed by a flush it makes the response stream essentially useless.
  • The MultiDict is unordered which means that some information is lost.
  • The response object modifies itself on __call__. This allows some neat things like automatically fixing the location header, but in general that should happen temporarily when called as WSGI application instead of modifying the object.


Now Django isn't exactly a reusable library for WSGI applications but it does have a request and response object with an API, so here my thoughts on it:

  • URL arguments are called request.GET like in WebOb, but files and form data was split up into request.POST and request.FILES.
  • The request object is unicode only and the encoding can be set dynamically.
  • Problem is, they don't work with non-Django WSGI applications.

Chances on a common Request Object?

WebOb and Werkzeug will stick around, and the chances that Django starts depending on external libraries for the Request object are very, very low. However it could be possible to share the implementation of the HTTP parsers etc.

To be humble, I would not want to break Werkzeug into two libraries for utlities and request/response objects and parsers because of the current packaging situation. A lot of small stuff I work on works perfectly fine with nothing but what Werkzeug provides which is pretty handy. So yes, it's selfish to not break it up, but that's how I feel about the situation currently.

This entry was tagged django, python and werkzeug