added example for missing ref.

examples/missing_reference.py

@@ -0,0 +1,36 @@
+"""A demonstration of sequential jobs with missing references."""
+
+from jobflow import Flow, OnMissing, job, run_locally
+
+
+@job
+def func(fail: bool = False):
+    """Failable function."""
+    if fail:
+        raise ValueError("An error occurred.")
+    return 1
+
+
+@job
+def collect(job_outputs):
+    """Job that allows some parents to fail."""
+    total = 0
+    for jo in job_outputs:
+        if jo is None:
+            continue
+        total += jo
+    if total < 1:
+        raise ValueError("No enough finished parents.")
+    return total
+
+
+job1, job2, job3 = func(), func(), func(fail=True)
+job_outputs = [job1.output, job2.output, job3.output]
+collect_job = collect(job_outputs)
+collect_job.config.on_missing_references = OnMissing.NONE
+flow = Flow([job1, job2, job3, collect_job])
+
+# run the flow, you can
+res = run_locally(flow)
+n_finished = 2
+assert res[collect_job.uuid][1].output == n_finished