<SYSTEM_TASK:>
Solve the following problem using Python, implementing the functions described below, one line at a time
<END_TASK>
<USER_TASK:>
Description:
 def _get_mount_methods(self, disk_type):
 """Finds which mount methods are suitable for the specified disk type. Returns a list of all suitable mount methods. """

        if self.disk_mounter == 'auto':
            methods = []

            def add_method_if_exists(method):
                if (method == 'avfs' and _util.command_exists('avfsd')) or \
                        _util.command_exists(method):
                    methods.append(method)

            if self.read_write:
                add_method_if_exists('xmount')
            else:
                if disk_type == 'encase':
                    add_method_if_exists('ewfmount')
                elif disk_type == 'vmdk':
                    add_method_if_exists('vmware-mount')
                    add_method_if_exists('affuse')
                elif disk_type == 'dd':
                    add_method_if_exists('affuse')
                elif disk_type == 'compressed':
                    add_method_if_exists('avfs')
                elif disk_type == 'qcow2':
                    add_method_if_exists('qemu-nbd')
                elif disk_type == 'vdi':
                    add_method_if_exists('qemu-nbd')
                add_method_if_exists('xmount')
        else:
            methods = [self.disk_mounter]
        return methods