Python + Cherrypy - Why is the variable 'outp' not changing value?

Question

Is the following behaviour a problem intrinsic to python (One cannot change the value of a variable inside a loop if an exception is raised) or is it a problem with cherrypy (One cannot change the value of a variable inside a loop if an HTTPRedirect exception is raised)? I am trying to change the value of 'outp' from 'ORIGINAL' to 'NEWVALUE'.

I am running the following code snippet as a method within a class called "Root(object)" (print statement are highlighted for easy reading):

@cherrypy.expose
def tester(self, cancel=False, submit=False, clear=False, **data):
    outp = "ORIGINAL"        
    if cherrypy.request.method == 'POST':
        print outp                  ############ FIRST PRINT STATEMENT ##########
        if True:
            outp = "NEWVALUE"
            print outp + '1'        ############ SECOND PRINT STATEMENT #########
            raise cherrypy.HTTPRedirect('/tester')
    print outp + '2'                ############ THIRD PRINT STATEMENT ##########

    tmpl = loader.load('tester.html')
    stream = tmpl.generate(outp=outp)
    return stream.render('html', doctype='html')

I receive the following output:

ORIGINAL
NEWVALUE1
127.0.0.1 - - [10/Oct/2012:15:45:33] "POST /tester HTTP/1.1" 303 102  
"http://localhost:8080/tester" "Mozilla/5.0 (X11; Linux x86_64; rv:10.0.7) 
Gecko/20120829 Firefox/10.0.7"
ORIGINAL2

the value of outp only changes inside the if statement. If I comment out the exception raise "raise cherrypy.HTTPRedirect('/tester')":

@cherrypy.expose
def tester(self, cancel=False, submit=False, clear=False, **data):
    outp = "ORIGINAL"        
    if cherrypy.request.method == 'POST':
        print outp                ############ FIRST PRINT STATEMENT ##########
        if True:
            outp = "NEWVALUE"
            print outp + '1'      ############ SECOND PRINT STATEMENT #########
            #raise cherrypy.HTTPRedirect('/tester')
    print outp + '2'              ############ THIRD PRINT STATEMENT ##########

    tmpl = loader.load('tester.html')
    stream = tmpl.generate(outp=outp)
    return stream.render('html', doctype='html')

I receive the following output:

ORIGINAL
NEWVALUE1
NEWVALUE2

I'm not necessarily looking for an in depth answer, just an idea of whether this is Python caused behaviour or Cherrypy caused behaviour. I could not mimic the behaviour by turning the code snippet into its own Python script so indications are that it's a Cherrypy issue.

Answer 1

Python, mostly. The HTTPRedirect, like any Python exception, suspends execution of the function. HTTPRedirect is essentially telling the client to make another HTTP request, which will run the function a second time without continuing from where it left off. Exceptions aren't loops: any variables local to that function will be new each time you run it.