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In some situations we already have programs that have the functionality we need but we want to automate their use We can do this by using Python s subprocess module which provides facilities for running other programs, passing any command-line options we want, and if desired, communicating with them using pipes We saw one very simple example of this in 5 when we used the subprocesscall() function to clear the console in a platform-speci c way But we can also use these facilities to create pairs of parent child programs, where the parent program is run by the user and this in turn runs as many instances of the child program as necessary, each with different work to do It is this approach that we will cover in this section In 3 we showed a very simple program, grepwordpy, that searches for a word speci ed on the command line in the les listed after the word In this section we will develop a more sophisticated version that can recurse into subdirectories to nd les to read and that can delegate the work to as many separate child processes as we like The output is just a list of lenames (with paths) for those les that contain the speci ed search word The parent program is grepword-ppy and the child program is grepword-pchildpy The relationship between the two programs when they are being run is shown schematically in Figure 101 The heart of grepword-ppy is encapsulated by its main() function, which we will look at in three parts:
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def main(): child = ospathjoin(ospathdirname(__file__), "grepword-p-childpy") opts, word, args = parse_options() filelist = get_files(args, optsrecurse) files_per_process = len(filelist) // optscount start, end = 0, files_per_process + (len(filelist) % optscount) number = 1
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Using the Multiprocessing Module
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Figure 101 Parent and child programs
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get_ files()
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We begin by getting the name of the child program Then we get the user s command-line options The parse_options() function uses the optparse module It returns the opts named tuple which indicates whether the program should recurse into subdirectories and the count of how many processes to use the default is 7, and the program has an arbitrarily chosen maximum of 20 It also returns the word to search for and the list of names ( lenames and directory names) given on the command line The get_files() function returns a list of les to be read Once we have the information necessary to perform the task we calculate how many les must be given to each process to work on The start and end variables are used to specify the slice of the filelist that will be given to the next child process to work on Usually the number of les won t be an exact multiple of the number of processes, so we increase the number of les the rst process is given by the remainder The number variable is used purely for debugging so that we can see which process produced each line of output
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pipes = [] while start < len(filelist): command = [sysexecutable, child] if optsdebug: commandappend(str(number)) pipe = subprocessPopen(command, stdin=subprocessPIPE) pipesappend(pipe) pipestdinwrite(wordencode("utf8") + b"\n") for filename in filelist[start:end]: pipestdinwrite(filenameencode("utf8") + b"\n") pipestdinclose() number += 1 start, end = end, end + files_per_process
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For each start:end slice of the filelist we create a command list consisting of the Python interpreter (conveniently available in sysexecutable), the child program we want Python to execute, and the command-line options in this case just the child number if we are debugging If the child program has a suitable shebang line or le association we could list it rst and not bother including
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the Python interpreter, but we prefer this approach because it ensures that the child program uses the same Python interpreter as the parent program Once we have the command ready we create a subprocessPopen object, specifying the command to execute (as a list of strings), and in this case requesting to write to the process s standard input (It is also possible to read a process s standard output by setting a similar keyword argument) We then write the search word followed by a newline and then every le in the relevant slice of the le list The subprocess module reads and writes bytes, not strings, but the processes it creates always assume that the bytes received from sysstdin are strings in the local encoding even if the bytes we have sent use a different encoding, such as UTF-8 which we have used here We will see how to get around this annoying problem shortly Once the word and the list of les have been written to the child process, we close its standard input and move on It is not strictly necessary to keep a reference to each process (the pipe variable gets rebound to a new subprocessPopen object each time through the loop), since each process runs independently, but we add each one to a list so that we can make them interruptible Also, we don t gather the results together, but instead we let each process write its results to the console in its own time This means that the output from different processes could be interleaved (You will get the chance to avoid interleaving in the exercises)
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while pipes: pipe = pipespop() pipewait()
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Once all the processes have started we wait for each child process to nish This is not essential, but on Unix-like systems it ensures that we are returned to the console prompt when all the processes are done (otherwise, we must press Enter when they are all nished) Another bene t of waiting is that if we interrupt the program (eg, by pressing Ctrl+C), all the processes that are still running will be interrupted and will terminate with an uncaught KeyboardInterrupt exception if we did not wait the main program would nish (and therefore not be interruptible), and the child processes would continue (unless killed by a kill program or a task manager) Apart from the comments and imports, here is the complete grepword-pchildpy program We will look at the program in two parts with two versions of the rst part, the rst for any Python 3x version and the second for Python 31 or later versions:
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BLOCK_SIZE = 8000 number = "{0}: "format(sysargv[1]) if len(sysargv) == 2 else "" stdin = sysstdinbufferread() lines = stdindecode("utf8", "ignore")splitlines() word = lines[0]rstrip()
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