Symfony\Component\Console\Application::getNamespaces PHP Method

    public function getNamespaces()
    {
        $namespaces = array();
        foreach ($this->all() as $command) {
            $namespaces = array_merge($namespaces, $this->extractAllNamespaces($command->getName()));
            foreach ($command->getAliases() as $alias) {
                $namespaces = array_merge($namespaces, $this->extractAllNamespaces($alias));
            }
        }
        return array_values(array_unique(array_filter($namespaces)));
    }

 public function __construct(Application $application, CompletionContext $context = null)
 {
     $this->application = $application;
     $this->context = $context;
     $this->addHandler(new Completion('help', 'command_name', Completion::TYPE_ARGUMENT, array_keys($application->all())));
     $this->addHandler(new Completion('list', 'namespace', Completion::TYPE_ARGUMENT, $application->getNamespaces()));
 }