Python-action: detect which dependencies changed and which targets are missing

Essentially need to get the same information in a python-action as provided by doit info on the console.

I need this to process only those dependencies of the task which have changed.

What I’ve tried:

def my_action(task):
    print(task.dep_changed)

However this will print all the dependencies, no matter what. I guess it is related to this comment in the code:

        # dep_changed is set on get_status()
        # Some commands (like `clean` also uses expand_args but do not
        # uses get_status, so `changed` is not available.

And need the missing targets as well, those seem to be separate from dep_changed in doit info.

[patrikturi]

@smarie Thank you! This helper is exactly what I needed.

[smarie]

Also concerning the other topic in this ticket: I had the issue today to try to debug “why did a task run again ??? It should not”.

Being able to print to stdout the reason why tasks are run (dependency change or file dep change or target change) using a simple commandline flag (for example a maven-style flag doit -X or a pytest-style doit -v) would really be a killer feature - if it is not already there (in which case it should probably be highlighted in the doc in a better way). Should we open a ticket for this in particular ?

EDIT: for what’s worth, my current workaround for this is to use the following why_am_i_running python action explicitly at the beginning of all my task actions lists:

from os.path import exists

def why_am_i_running(task, changed):
    for t in task.targets:
        if not exists(t):
            print("Running task '%s' because one of its targets does not exist anymore: '%s'" % (task, t))
            return

    print("Running task '%s' because the following changed: '%s'" % (task, changed))

EDIT2: There might be a case missing in this draft: the case where result_dep is the reason for re-running.