How to write a while loop?

A while loop consists of a branch, and a condition task inside this branch, which determines whether one revisits the branch again.

One needs to define the branch with loop=True, and then make sure that there if one _if decorator, with a jump back to the branch itself.

We begin from empty repo.

$ tb init
Created repository using module "taskblaster.repository" in "/home/myuser/tmprepo".

Here is the simplest example workflow. Create a following workflow.py

import taskblaster as tb


@tb.workflow
class WhileLoopWorkflow:
    number = tb.var()

    @tb.branch('entry', loop=True)
    @tb.task
    def iterate(self):
        return tb.node(
            'iterate_task',
            number=self.Phi(default=self.number, entry=self.iterate),
        )

    @tb.branch('entry', loop=True)
    @tb._if(false='entry')
    @tb.task
    def converged(self):
        return tb.node('converged_task', number=self.iterate)


def workflow(rn):
    rn.run_workflow(WhileLoopWorkflow(number=3))

and a following tasks.py

def converged_task(number):
    return number < 1


def iterate_task(number):
    return number - 1

This is the absolutely simplest example of workflow. The Phi-operator argument default refers to initialization of the while loop, i.e. on the first iteration, we get the input to the iterate task from the workflow input self.number. On the next step, we are coming from entry-branch, and thus we will refer to the previous result of the iterate.

We can iterate the workflow few times and observe the results

$ tb workflow workflow.py && tb run . >/dev/null && tb ls --sort=topo -c sirITfo
entry:                    add  new      0/0        tree/iterate-001 
if:                       add  new      0/1        tree/converged-001 T=None F=entry 
state    info       tags        worker         time     folder                        output
──────── ────────── ─────────── ─────────── ─────────── ───────────────────────────── ────────────────────────
done     0/0                    N/A-0/1        00:00:00 tree/iterate-001              2
done     1/1                    N/A-0/1        00:00:00 tree/converged-001            False

$ tb workflow workflow.py && tb run . >/dev/null && tb ls --sort=topo -c sirITfo
entry:                   have  done     0/0        tree/iterate-001 
if:                      have  done     1/1        tree/converged-001 T=None F=entry jump: entry
entry:                    add  new      2/2        tree/iterate-002 
if:                       add  new      1/2        tree/converged-002 T=None F=entry 
state    info       tags        worker         time     folder                        output
──────── ────────── ─────────── ─────────── ─────────── ───────────────────────────── ────────────────────────
done     0/0                    N/A-0/1        00:00:00 tree/iterate-001              2
done     1/1                    N/A-0/1        00:00:00 tree/converged-001            False
done     2/2                    N/A-0/1        00:00:00 tree/iterate-002              1
done     2/2                    N/A-0/1        00:00:00 tree/converged-002            False

$ tb workflow workflow.py && tb run . >/dev/null && tb ls --sort=topo -c sirITfo
entry:                   have  done     0/0        tree/iterate-001 
if:                      have  done     1/1        tree/converged-001 T=None F=entry jump: entry
entry:                   have  done     2/2        tree/iterate-002 
if:                      have  done     2/2        tree/converged-002 T=None F=entry jump: entry
entry:                    add  new      3/3        tree/iterate-003 
if:                       add  new      2/3        tree/converged-003 T=None F=entry 
state    info       tags        worker         time     folder                        output
──────── ────────── ─────────── ─────────── ─────────── ───────────────────────────── ────────────────────────
done     0/0                    N/A-0/1        00:00:00 tree/iterate-001              2
done     1/1                    N/A-0/1        00:00:00 tree/converged-001            False
done     2/2                    N/A-0/1        00:00:00 tree/iterate-002              1
done     2/2                    N/A-0/1        00:00:00 tree/converged-002            False
done     3/3                    N/A-0/1        00:00:00 tree/iterate-003              0
done     3/3                    N/A-0/1        00:00:00 tree/converged-003            True

To view an experimental automatically generated workflow diagram of the workflow here, click here.

This structure is the simplest for loop. However, to make the initialization and finalization of while loop more clear, and usable, see How to write a practical while loop How to.