List Handling in VisTrails

In VisTrails you can pass typed and untyped lists between modules. Ports on modules have a depth parameter specifying the list depth it supports. 0 means no list, 1 is a list, 2 is a list of lists etc. Port depth can be specified either by module developers or by using a PythonSource or similar module.

It is important to know how connections of mismatching list depths are handled:

  • List wrapping - If the destination port has a larger list depth, the source will be wrapped in lists until the list depth is matched.
  • Iterating over lists - If the source port has a larger list depth, the destination module will be executed once for each element in the list.

The List type

The List type represents an untyped list and can contain a list of any type. The vistrails Variant type matches any type and a List is equal to a Variant of list depth 1. List depth specified on List types does not include the List itself: A List type with list depth 1 are considered a Variant with list depth 2.

There is one important exception: Variant connects directly to List. This is because Variant ports are allowed to contain lists.

Iterating over lists

Passing a list to a module that does not support lists will cause that module to be executed once for each element in the list. When passing lists on multiple input ports, the inputs will be combined. The default combination is cartesian product, where each element in a list is combined with each element in another list. This combination can be changed by selecting “Looping Options” in the module menu. The options are Cartesian, Pairwise (where the elements are combined pairwise), and Custom. Custom gives you complete control over how inputs are combined and allows you to use both pairwise/cartesian combiners as well as reordering them. The output of an iterated module will be an ordered list with the individual results of the module execution. This will cause modules downstream to also be iterated over, unless they accept a list as input. Iterated modules will have duplicated connections to show that they are being iterated over. A list of lists will have the connection triplicated etc.

Try it Now!

Lets create a simple example showing how to combine strings. First we will create a module that generates lists of strings. Create a new workflow and add a PythonSource module. Give it three output ports named s1, s2, s3 of type String and set their list depth to 1. Enter this code:

s1 = ['A', 'B', 'C']
s2 = ['+', '-', '*']
s3 = ['1', '2', '3']

Next Step!

Add a ConcatenateString module, connect s1->str1, s2->str2, s3->str3. Notice how the connections going into ConcatenateString are duplicated. This indicates that ConcatenateString will iterate over the list inputs. Add a StandardOutput module and connect ConcatenateString.value to StandardOutput.value. This connection will be duplicated in both ends, indicating they will both be iterated over. Your workflow should now look like Figure The complete workflow.


The complete workflow

Next Step!

By default ConcatenateString will combine the inputs using cartesian product ["A+1", "A+2",  "A+3", "A-1",  ...]. Lets change this. Go to Module Menu->Looping Options and click custom. Right click in the port list and add a pairwise product. Rearrange the ports so that it looks like Figure A custom list combiner.


A custom list combiner


str2 and str3 will now be combined pairwise and then combined with str1 using cartesian product. Open the vistrails console and execute the workflow. You should see this output: (Open result)