Python Block Begin TOP node

Starts a Python service block

On this page	Top_attributes Parameters Node Files Schedulers
Since	18.0

This node creates a work items that start a python service block, which is terminated using a corresponding Service Block End node.

Note that this node uses python not hython, so the hou module is not available and no license is required. If you want to use a hython instead, see the Houdini Block Begin node.

The work items in this node will start the Python service if it isn’t already running
Each work item in this Begin node will lock one of the service worker processes, until the corresponding end block work item finishes cooking.
You can use Service Block Send nodes within the block to communicate with the service. The work items within the block execute serially (instead of in parallel), one after another. Alternatively, you can use any node that is able to cook using the Python Script service, such as the Python Script TOP.
(Optional) You can schedule multiple parallel loops using the same node. See the Session Count from Upstream Items and Number of Sessions parameters.

For more information see service blocks.

TOP Attributes ¶

`sharedserver`	str	The name of the shared server instance associated with the work item. In the case of the begin item, it’s the name of the server that the work item eventually create. This attribute is deprecated, and is only added to work items when the Server Type parameter is configured to use the old Shared Server service type.
`loopiter`	[int]	This attribute is inherited from the Block Begin Feedback node. The loop iteration number within the set of work items associated with the loop. Since the iteration number at each level is preserved, this attribute can be an array of values when using nested feedback loops. The loop iteration value for the outer most loop is stored in `loopiter[0]`, the next level is stored in `loopiter[1]`, and so on.
`loopnum`	int	This attribute is inherited from the Block Begin Feedback node. Tracks which loop the work item is associated with. This attribute is relevant when generating multiple independent loops in the same Block Begin Feedback node, like when driving the Block Begin Feedback node with a Wedge node.
`loopsize`	int	This attribute is inherited from the Block Begin Feedback node. The total number of iterations in the loop.

Parameters ¶

Node ¶

Work Item Generation

Generate When

Determines when this node will generate work items. You should generally leave this set to “Automatic” unless you know the node requires a specific generation mode, or that the work items need to be generated dynamically.

All Upstream Items are Generated

This node will generate work items once all of the input nodes have generated their work items.

All Upstream Items are Cooked

This node will generate work items once all of the input nodes have cooked their work items.

Each Upstream Item is Cooked

This node will generate work items each time a work item in an input node is cooked.

Automatic

The generation mode is selected based on the generation mode of the input nodes. If any of the input nodes are generating work items when their inputs cook, this node will be set to Each Upstream Item is Cooked. Otherwise, it will be set to All Upstream Items are Generated.

Session Count from Upstream Items

When on, the node creates a single loop with a service session for each upstream work item. Otherwise, a separate loop is created for for each upstream work item and the size of the loop is determined by the Number of Sessions parameter.

Number of Sessions

Specifies the number of sessions to create in each loop. Each session work item cooks in order and in serial with other work items in the same loop.

The chain of work items starting from this node down to the Service Block End node cooks to completion before starting the next session.

This parameter is only available when Session Count from Upstream Items is off.

Copy Inputs For

Determines how input files are copied onto loop items. By default, upstream files are copied onto all input files. However, it is also possible to only copy input files onto the first iteration or none of the loop iterations.

No Iterations

Upstream input files are not copied to the outputs of any loop iteration items.

First Iteration

Upstream input files are copied to the output file list for only the first loop iteration

All Iterations

Upstream input files are copied to the output file list of all iterations.

Feedback Attributes

When on, the specified attributes are copied from the end of each iteration onto the corresponding work item at the beginning of the next iteration. This occurs immediately before the starting work item for the next iteration cooks.

Tip

The attribute(s) to feedback can be specified as a space-separated list or by using the attribute pattern syntax. For more information on how to write attribute patterns, see Attribute Pattern Syntax.

Feedback Output Files

When on, the output files from each iteration are copied onto the corresponding work item at the beginning of the next loop iteration. The files are added as outputs of that work item, which makes them available as inputs to work items inside the loop.

Server Configuration

Server Type

Determines whether this node should create a block that uses Services or a Shared Server. Note that Shared Servers are deprecated in Houdini 20.0.

Service Name

When Server Type is set to Service, this parameter determines the name of the service to use for work items in the block

Reset Service

Determines whether or not service clients should be reset before executing work items. If this node is a service block, the reset operation is only performed once per block loop. Otherwise, the reset operation occurs for each work item in this node.

None

The service client is used as-is. This means that any imported modules, global variables, or scene state are preserved between work item cooks.

Reset Client

The service client is sent the “reset” command, which clears state but does not restart the service process.

Restart Client

The service client process is completely restarted before running a work item.

Connect To Existing Server

When on, the work item connects to an existing server on a host address, rather than starting up a new server process. This can be useful when you want to connect to a central server such as an asset management system. However, you must make sure to serialize access to the server. This parameter is only available when Server Type is set to Shared Server.

Server Address

When Connect to existing server is on, the host address of the server to connect to. Set the Server port parameter to the port used by the existing server. This parameter is only available when Server Type is set to Shared Server.

Server Port

When Connect To Existing Server is on, specifies the port to use to connect to an existing server. When Connect To Existing Server is off, specifies the TCP port number the server should bind to. This parameter is only available when Server Type is set to Shared Server.

The default value 0 tells the system to dynamically select an unused port. This is the behavior you usually want. If you want to keep the ports in a certain range (and can guarantee the port numbers will be available), you can also use an expression like 9000 + @pdg_index. For information on how to pass this value to the server startup script, see the Server Command parameter.

Note

On UNIX systems, ports 0-1023 can only be bound by a superuser. As such, you most likely need to choose a port above 1023.

Load Timeout

Specifies the timeout used when performing an initial verification that the shared server instance can be reached. When this timeout passes without a successful communication, the work item for that server is marked as failed. This parameter is only available when Server Type is set to Shared Server.

Server Command

Specifies the command to run to start the server. This command line should work when running on a remote server. As such, you should put the start script in a known location on your shared network file system. This parameter is only available when Server Type is set to Shared Server.

Loop Attribute Names

These parameters customize the names of the work item attributes created by this node.

Files ¶

File Dependencies

A list of files that should be copied to the PDG working directory before the first work item in this node is executed. This can be used to ensure that supporting files like digital assets and custom scripts are available for the work item job.

The specified paths can be absolute or relative to HOUDINI_PATH.

Schedulers ¶

TOP Scheduler Override

This parameter overrides the TOP scheduler for this node.

Schedule When

When enabled, this parameter can be used to specify an expression that determines which work items from the node should be scheduled. If the expression returns zero for a given work item, that work item will immediately be marked as cooked instead of being queued with a scheduler. If the expression returns a non-zero value, the work item is scheduled normally.

Work Item Label

Determines how the node should label its work items. This parameter allows you to assign non-unique label strings to your work items which are then used to identify the work items in the attribute panel, task bar, and scheduler job names.

Use Default Label

The work items in this node will use the default label from the TOP network, or have no label if the default is unset.

Inherit From Upstream Item

The work items inherit their labels from their parent work items.

Custom Expression

The work item label is set to the Label Expression custom expression which is evaluated for each item.

Node Defines Label

The work item label is defined in the node’s internal logic.

Label Expression

When on, this parameter specifies a custom label for work items created by this node. The parameter can be an expression that includes references to work item attributes or built-in properties. For example, $OS: @pdg_frame will set the label of each work item based on its frame value.

Work Item Priority

This parameter determines how the current scheduler prioritizes the work items in this node.

Inherit From Upstream Item

The work items inherit their priority from their parent items. If a work item has no parent, its priority is set to 0.

Custom Expression

The work item priority is set to the value of Priority Expression.

Node Defines Priority

The work item priority is set based on the node’s own internal priority calculations.

This option is only available on the Python Processor TOP, ROP Fetch TOP, and ROP Output TOP nodes. These nodes define their own prioritization schemes that are implemented in their node logic.

Priority Expression

This parameter specifies an expression for work item priority. The expression is evaluated for each work item in the node.

This parameter is only available when Work Item Priority is set to Custom Expression.