Running Python script from Cocoa application using GCD

Question

I\'m trying to run a Python script from a Cocoa app. It\'s working just fine on the main thread, but I\'d like to have it running in the background, on a concurrent GCD queue.

Answer 1

From this page, it looks like there are some some pretty complex threading concerns specific to embedding python. Is there a reason you couldn't just run these scripts in a separate process? For instance, the following -runBunchOfScripts method would run the script ten times (by calling -runPythonScript) on a parallel background queue, collecting the resulting outputs into an array of strings, and then calling your object back on the main thread once all the scripts has completed:

- (NSString*)runPythonScript
{
    NSTask* task = [[[NSTask alloc] init] autorelease];
    task.launchPath = @"/usr/bin/python";  
    NSString *scriptPath = [[NSBundle mainBundle] pathForResource:@"MyScript" ofType:@"py"];
    task.arguments = [NSArray arrayWithObjects: scriptPath, nil];

    // NSLog breaks if we don't do this...
    [task setStandardInput: [NSPipe pipe]];

    NSPipe *stdOutPipe = nil;
    stdOutPipe = [NSPipe pipe];
    [task setStandardOutput:stdOutPipe];

    NSPipe* stdErrPipe = nil;
    stdErrPipe = [NSPipe pipe];
    [task setStandardError: stdErrPipe];

    [task launch];        

    NSData* data = [[stdOutPipe fileHandleForReading] readDataToEndOfFile];

    [task waitUntilExit];

    NSInteger exitCode = task.terminationStatus;

    if (exitCode != 0)
    {
        NSLog(@"Error!");
        return nil;
    }

    return [[[NSString alloc] initWithBytes: data.bytes length:data.length encoding: NSUTF8StringEncoding] autorelease];
}

- (void)runBunchOfScripts
{
    dispatch_group_t group = dispatch_group_create();
    NSMutableArray* results = [[NSMutableArray alloc] init];
    for (NSUInteger i = 0; i < 10; i++)
    {
        dispatch_group_async(group, dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0), ^{
            NSString* result = [self runPythonScript];
            @synchronized(results)
            {
                [results addObject: result];
            }
        });
    }

    dispatch_group_notify(group, dispatch_get_main_queue(), ^{
        [self scriptsDidFinishWithResults: results];
        dispatch_release(group);
        [results release];
    });
}

- (void)scriptsDidFinishWithResults: (NSArray*)results
{
    NSLog(@"Do something with the results...");
}

Naturally the approach of using separate processes has it's limitations, not the least of which being the hard limit on the number of processes you can launch, but it seems a lot less fraught with peril than embedding the entire interpreter. I would say that unless you need to interact chattily between the scripts and the hosting environment, this would be a better approach.