The equivalent os.spawn calls achieve the same effect, with a slightly more complex call signature that provides more control over the way the program is launched: > result = os.popen('python makewords.py').read( ) In the following interaction, for instance, we start a Python program with a command line in two traditional ways (the second also reads its output): In basic operation, they are similar to the fork/ exec call combination on Unix and can be used as alternatives to the system and popen calls we've already learned. The os.spawn family of calls execute a program named by a command line in a new process, on both Windows and Unix-like systems. We won't talk about the PyWin32 extensions package in this chapter, but the other tools available in the standard library merit a quick look here. Other tools such as the commands and subprocess modules provide additional options in this domain.

Tools in the Python PyWin32 extensions package provide other, less standardized ways to start programs (e.g., the WinExec call). os.startfile makes this even simpler in recent Python releases. The os.system call can be used on Windows to launch a DOS start command, which opens (i.e., runs) a file independently based on its Windows filename associations, as though it were clicked. Today, these calls work on both Windows and Unix-like systems, and additional variants have been added to parrot os.exec.

The os.spawnv and os.spawnve calls were originally introduced to launch programs on Windows, much like a fork/ exec call combination on Unix-like platforms. Luckily, there are other ways to start programs in the Python standard library, some of which are more platform neutral than others: Moreover, for reasons we'll explore later, the os.popen call is prone to blocking (pausing) its caller in some scenarios and requires a potentially platform-specific command-line string. This constraint may be improved by the time you read this book, but it still is a limitation as I write these words.

Along the way, though, I've also been careful to point out numerous times that the os.fork call doesn't work on Windows today.

We've also learned how to go about starting new, independently running programs, with both the fork/ exec combination and with tools for launching shell commands such as os.popen and os.system. In this chapter, we've seen how to portably spawn threads, but these are simply parallel functions, not external programs. That is, you want to write a general and portable launcher program in Python for starting Books are nice, but it's awfully fun to be able to click on demos right away. Suppose, just for a moment, that you've been asked to write a big Python book and you want to provide a way for readers to easily start the book's examples on just about any platform that Python runs on.

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