[Web-SIG] An implementation error I just found in PEP 333

Phillip J. Eby pje at telecommunity.com
Wed Oct 6 01:26:46 CEST 2004

Just a quick heads-up...  there's an error in the PEP's CGI implementation, 
so if you are basing a server/gateway implementation on it, you may be 
copying this error into your own code.

Specifically, 'start_response' contains this code:

         elif headers_sent:
             raise AssertionError("Headers already sent!")

It *should* read:

         elif headers_set:
             raise AssertionError("Headers already set!")

This is apparently a typo; it leads to noncompliant behavior (allowing 
set_response() to be called multiple times without error even if exc_info 
isn't supplied).  I discovered it while working on the WSGI reference 
library (wsgiref).  FYI, the ViewCVS for wsgiref is:


And you can also get it via anonymous CVS; see


for instructions, replacing 'co PEAK' with 'co wsgiref'.

At the moment, wsgiref just contains a header manipulation class, a 
FileWrapper class, and a bunch of environment manipulation functions, all 
with extensive automated tests.

I'm in the middle of working on a base class that can be used to implement 
pretty much any kind of WSGI server or gateway, and I noticed that I had 
managed to copy the above error into my new base class.  So I thought I 
should mention it to everybody so they can verify that they didn't make the 
same mistake.  Sorry about the mixup, I'll get it fixed in the next PEP 

In the meantime, you can now feel good about the fact that even *my* PEP 
333 implementation had a compliance bug...  ;)

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