Thanks for the detailed help. I'm dumping sys.stdin into a variable at the very start now, and operating on that variable for everything else, and all is well.
Thanks again, E On Oct 16, 2007, at 12:22 PM, Luke Paireepinart wrote: > Eric Abrahamsen wrote: >> I'm trying to learn the fundamentals of cgi scripting, before >> moving on to actually using the cgi module and, eventually, >> mod_python. I've grasped that, if users submit a form to my cgi >> script, I can read the form contents from os.environ >> ['QUERY_STRING'] if it was stuck in the URL via method=get, and >> from sys.stdin if it was sent via method=post. >> >> The get method works fine, but when it comes to post I can't >> actually read anything off sys.stdin. > Yeah, you can. >> What's particularly odd is that in my test script below, the >> returnform() function should only be called if len(sys.stdin.read >> ())!=0, and yet when I submit a word and the script runs >> returnform(), it tells me that len(sys.stdin.read()) is equal to >> 0. If that's the case, how did returnform() get called in the >> first place? Why didn't it just re-run printform()? >> > Because you're confused about what sys.stdin.read() does. > Imagine stdin is a string: > "Hello, Eric!" > now if I call > sys.stdin.read(5) > it will return > "Hello," > and the contents of sys.stdin.read() will now be > " Eric!" > or, to be more accurate, the current-position pointer will be set > to offset 5, so that future reads will start at that position. > > To understand why they would do it this way, consider you want to > read in 5 characters at a time from a file. > Say the file is 6 GB. If you were to read in the whole file and > loop over it 5 characters at a time, > you'd probably overflow your memory. > However, if you tell the OS you want to open the file for reading, > and you then read 5 characters at a time from the file, > you won't have to load everything into memory. This is > accomplished using pointers inside the file, so you know where you > last read. > > Even though sys.stdin in this case is not a file, it works the same > way, because it's a "File-Like Object." > Python uses Duck Typing, wherein the feature set of a particular > item determines its type, rather than something arbitrary you define. > So if any item has read(), write(), and seek() methods, you can > usually use these in place of file objects. > This is true no matter what your functions actually do. > In other words, your read() function could just count how many > times it's called, but do nothing with the value, or anything else > you may want it to do. >> At first I tried printing each line of sys.stdin to the HTML page, >> so I could see the details of how post works. Nothing was >> printed, and that's when I tried using len() to see whether >> sys.stdin contained anything. Then, thinking that stdin was >> getting reset somehow, > It's not getting reset, really. >> I tried calling returnform() and directly passing in stdin as a >> parameter. That had the same result. > Usually when you get to the point where you're trying random things > hoping something will work, > you've reached the time when you either sleep on it or ask someone > for help. > Even if you reach a solution through exhaustive testing, you still > haven't learned anything, which is pretty useless to you in the > long run. >> Now, I'm thoroughly confused... >> > Don't worry about it. It's good you asked. >> Any help would be much appreciated. >> >> Yours, >> Eric >> >> ************ >> >> #!/Library/Frameworks/Python.framework/Versions/Current/bin/python >> import sys >> import cgitb;cgitb.enable() >> >> def main(): >> if len(sys.stdin.read())!=0: >> > This is your problem right here. > len(sys.stdin.read()) is calling len() on a string returned by > sys.stdin.read() > As mentioned earlier, calling this function sets the current- > location pointer to one after the last-read position. > Since you passed no maximum to read(), the whole stdin contents > were read into this variable, leaving > the pointer at the end of the file. Thus on subsequent calls, > sys.stdin.read() will return nothing. > Try changing this to len(sys.stdin.read(3)) and then pass your > program a string longer than 3 characters. > Your length output in returnform should then be 3 less than your > expected value, or 0, whichever is greater. >> returnform() >> else: >> printform() >> >> def printform(): >> [snip printing form] >> >> def returnform(): >> > This name is a bit confusing, because the function doesn't return > anything. > Perhaps display_form_contents would be a better name? >> print "Content-Type: text/html\n\n" >> > You could include this in the block print statement, if you > wanted. It's fine to keep the header separate, though, > and probably a good idea. >> print """ >> <html> >> <head></head> >> <body> >> <p>Here's what results for standard in:</p>""" >> print "<p>Length of stdin is %s</p>" % len(sys.stdin.read()) >> > So here when you're reading the length of sys.stdin.read() it's > reading in from stdin again, > and since the pointer is at the end of the file, len() is getting > an empty string. > You could reset the pointer to 0 if you wanted, using seek(), but > I'd say just read the data once, > in your main, and pass it as an argument to returnform. > > That makes your returnform function more versatile, as well, > because then if I wanted to have a different form's contents > printed that I had saved in a file, for example, I could just pass > it to your function, without having to jump through the hoops > of redirecting sys.stdin to my file. > > Also I doubt it'd be a problem to have a block-printed format string. > I.E. just do it this way: > > print """ > <html> > <head></head> > <body> > <p>Here's what results for standard in:</p> > <p>Length of stdin is %s</p> > </body> > </html>""" % len(sys.stdin.read()) > > > Perhaps this seems less intuitive to you. It's really just a > preference thing, but it makes for slightly more readable code in > my opinion. >> main() > A neat trick is to do this: > > if __name__ == "__main__": > main() > > The __name__ variable is set to "__main__" only when the script is > executed directly. > So if I call form.py, for example. > However, if I write my own python program, and I want to use your > printform() function, I can do > import form > which will import your functions into my namespace, and I can then do > form.printform() > If you don't have the above 2 lines, then when I import your > form.py module, main() will be run every time, > causing all the main() stuff to be executed even if I really just > wanted to use one of your functions, which in almost every case is > an "unwanted side-effect." > > Hope this has helped in some way, > -Luke > _______________________________________________ Tutor maillist - Tutor@python.org http://mail.python.org/mailman/listinfo/tutor
