I am trying to use the twisted reactor as a basis for a Python server for a Flex app, using pyAMF. My problem is that one of the server methods requires me to run a series of command-line tools to complete parsing data from a file into an XML object, with lots of Python processing on the results of one command-line tool to provide the input to the next one. I have been using subprocess.call to invoke the command-line tools, but these intermittently - and frequently - fail with an error "IOError: [Errno 4] Interrupted system call". I do know that this was supposed to be fixed in twisted for Python 2.6, but it doesn't seem to be. I have python 2.6.3 under Mac OSX 10.5.8, and twisted 8.2.0. Is there a work-around for this problem, or a better way to do this with twisted? I do know that I can make a command-line call with reactor.callLater, and that I can return a deferred from my server method, but I don't see how to easily accomplish the chain of calls that I need, short of breaking up my serverMethod1 into the several sequential parts, and calling each from the result handler of the previous one. I'd rather not go that route, as the logic of which command-line call gets made when is actually quite complicated. What I have now looks like (enormously simplified): class MyPythonServer: def serverMethod1(self, dummyPost, inputFilePath): # Open inputFilePath, do stuff, save results to file2 subprocess.call(["command1", "file2", "file3"]) # Open file3, do stuff to the data, save results to file4 subprocess.call(["command2", "file4", "file5"]) # Open file5, convert data to a large string containing XML. return xmlString - Read Roberts