== Accidentally shooting yourself in the foot in Python

Recently, I stumbled over a small issue in Python's
[[cgi module|http://docs.python.org/lib/module-cgi.html]] that is a
good illustration of how unintended consequences in Python can wind up
shooting you in the foot.

The [[cgi module]]'s main purpose is to create a dictionary-like
object that contains all of the parameters passed to your CGI program
in the _GET_ or _POST_ HTTP command. DWiki uses it roughly like this:

>  form = cgi.FieldStorage()
>  for k in form.keys():
>    ... stuff ...

Then one day a cracker tried an XML-RPC based exploit against DWiki and
this code blew up, getting a _TypeError_ from the _form.keys()_ call.
This is at least reasonable, because an XML-RPC _POST_ is completely
different than a form _POST_ and doesn't actually have any form
parameters. (TypeError is a bit strong, but it did ensure that DWiki
paid attention.)

No problem; I could just guard the _form.keys()_ call with an '_if not
form: return_'. Except that the '_not form_' got the same TypeError.
Which is startling, because you don't normally expect '_not obj_' to
throw an error.

This surprising behavior of the [[cgi module]] happens through three
steps. First, Python decides whether objects are True or False like
this:

* if there is a ((__nonzero__)) method, call that.
* if there is a ((__len__)) method, a zero length is False and otherwise
  you're True (because Python usefully makes collections false if
  they're empty and true if they contain something).
* if there is neither, you're always True.

As a dictionary-like thing, FieldStorage defines a ((__len__)) method
in the obvious way:

>  def __len__(self):
>    return len(self.keys())

Finally, FieldStorage decided to let instances represent several
different things and that calling _.keys()_ on an instance that wasn't
dealing with form parameters should throw TypeError. (This is more
sensible than this description may make it sound.)

Apart from a practical illustration of unintended consequences and
complex interactions, what I've taken away from this is to ~~remember
than [[__len__|]] on objects is used for more than just the [[len()|]]
function~~. (Other special methods also have multiple uses.)

=== Sidebar: so how did I solve this?

My solution was lame:

>  try:
>    form.keys()
>  except TypeError:
>    return

I suspect that the correct solution is to check _form.type_ to make
sure that the _POST_ came in with a Content-Type header of
'application/x-www-form-urlencoded'. (Except I don't know enough to
know if all _POST_s to DWiki will always arrive like that. Ah, HTTP,
we love you so.)