freeze function calls

[Peter Otten]

Santiago Caracol wrote:

>> Python offers an elegant mechanism to calculate values on demand: the
>> generator function:
>>
>> >>> def calculate_answers():
>>
>> ...     for i in range(100):
>> ...             print "calculating answer #%d" % i
>> ...             yield i * i
>> ...
>>
> 
> Thanks for pointing this out. I was aware of the yield statement.
> 
> My problem is this:
> 
> (1) The user submits a query:
> 
> ---------------------------------
> | What is the answer? |
> ---------------------------------
> <Submit>
> 
> (2) The web server gives the user N answers and a button saying "More
> answers":
> 
> . answer 1
> . answer 2
> . answer 3
> 
> <More answers>
> 
> At this stage the function that writes html to the user has been
> called. The call must be terminated, or else, the user doesn't get any
> html. This means that the call of the answer-calculating function,
> whether it uses yield or not, is also terminated. This means, when the
> user presses the <More answers>-button, the calculation has to start
> at the beginning.

Adapted from the wsgiref documentation at

http://docs.python.org/library/wsgiref.html

$ cat wsgi_demo.py
from wsgiref.util import setup_testing_defaults
from wsgiref.simple_server import make_server

from itertools import count, islice

def  answers():
    for i in count():
        yield "Answer #%d\n" % i

gen = answers()

def simple_app(environ, start_response):
    setup_testing_defaults(environ)

    status = '200 OK'
    headers = [('Content-type', 'text/plain')]

    start_response(status, headers)

    return islice(gen, 3)

httpd = make_server('', 8000, simple_app)
print "Serving on port 8000..."
httpd.serve_forever()

Run the above with

$ python wsgi_demo.py
Serving on port 8000...

Now point your favourite browser to http://localhost:8000/ and hit refresh.

Peter