Launching ipdb (or pdb) in steps

[otaviog]

Hello,

Is it possible to use ipdb (or pdb) inside steps? On my code, it hangs without showing anything.

I have something like:

class MyTrain(FlowSpec):

    @step
    def start(self):
        print("Start ipdb")
        __import__("pdb").set_trace()

        self.next(self.load_dataset)

And the final output is:

$ python train.py run
...
2020-01-08 16:45:59.297 [1578512759283042/start/1 (pid 22301)] Task is starting.
2020-01-08 16:46:00.773 [1578512759283042/start/1 (pid 22301)] > /home/user521/metaflow-pytorch-example/train.py(97)start()

Thanks

[romain-intel]

Currently, this is not easily supported. The issue is that each step actually runs in a separate process so the import in the start step doesn't actually propagate to the other steps.

[yariv]

Could metaflow be configured to run all the tasks serially in the parent process? It could make debugging much easier with the help of pdb/ipdb.

[romain-intel]

I am looking at a way of making this work at least locally maybe using existing tools.

[multimeric]

This would be super helpful. In Toil (another workflow engine), they implemented a "no forking" mode which made the workflow run in the same interpreter so that PDB could be used: https://github.com/DataBiosphere/toil/pull/1910

[thesillystudent]

I have implemented a sequential runtime for metaflow. It's in the fork of this repo. I needed to use pdb as well as reuse some components of metaflow for Gcloud functions. YOu can check it out.
https://github.com/thesillystudent/metaflow/tree/functions_runtime This branch also has the support for split-or steps.

[yariv]

This is great! Any chance it can be merged to master?

On Sun, Feb 23, 2020 at 10:24 PM Maroof Ahmad notifications@github.com wrote:

I have implemented a sequential runtime for metaflow. It's in the fork of this repo. I needed to use pdb as well as reuse some components of metaflow for Gcloud functions. YOu can check it out. https://github.com/thesillystudent/metaflow/tree/functions_runtime This branch also has the support for split-or steps.

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[romain-intel]

I am in the process of updating the docs to detail how to use PyCharm or VSCode to step through step code in the way suggested here.

On a related note, you can get a "sequential" runtime for Metaflow by specifying --max-workers 1 in the command line. This will restrict Metaflow to executing only one task at a time.

[multimeric]

Does --max-workers 1 prevent any forking, such that you can use a debugger? Or does that just mean that it forks only once (which doesn't really help)?

[romain-intel]

@TMiguelT: yes, after I typed that comment I realized that in light of debugging it doesn't help at all; it will still fork a new process for every task just only allow one fork at a time... so indeed pretty useless for debugging. I should have the doc updated shortly though so hopefully that will help.

[romain-intel]

I added a section in the docs (https://docs.metaflow.org/metaflow/debugging#debugging-your-flow-code) that hopefully will help with this. If this is sufficient, I will close this issue, if you would like more support/information, please chime in again :).

[multimeric]

Oh, does PyCharm let you debug across multiple threads? I had no idea. Not OP but this looks like it solves this issue from my perspective. Great work documenting this!

[otaviog]

@romain-intel nice documentation about how to debug with VS Code and PyCharm. That is working for me. Thanks.

Although, I hope that in the long run this can be supported (maybe it's an issue from (i)pdb?). The team that I work likes ipdb to prototype PyTorch experiments since it is possible to print tensors and evaluate expressions more comfortably than with VS Code or PyCharm. Metaflow+ipdb could make prototyping even more agile.

[romain-intel]

Yes, both PyCharm and VSCode are actually pretty good because they allow you to debug the master process as well as all subprocesses. There is also probably a way to get it to work with remote debugging but for now, hopefully local debugging is sufficient to debug any issue.

[yariv]

I use vim and debug python from the command line using ipdb so these instructions won't work for me, unfortunately. If I could submit a feature request it would be to make it so that --max-workers=1 prevents forking child processes and serializes the execution in the main process.

[ifokeev]

use https://pypi.org/project/web-pdb/, it works fine

[xguse]

Any progress here? Not being able to run a debugger is not a minor annoyance. Its a pretty major adoption blocker in my opinion.

Also IMHO, the docs document (https://docs.metaflow.org/metaflow/debugging#debugging-your-flow-code) is NOT actual debugging any more than slapping a bunch of print statements into code is a substitute for a debugger. Being able to move up and down the call stack and being able to evaluate local variables is a MUCH more productive solution.

[addisonklinke]

Agreed with @xguse print() is not an actual debugger. It would be great for Metaflow to have a serial run option

@romain-intel Yes, both PyCharm and VSCode are actually pretty good because they allow you to debug the master process as well as all subprocesses

As mentioned in several threads, PyCharm has a "Attach to subprocess automatically while debugging" setting. However I have this enabled and the debugger still doesn't pick up breakpoints inside a Metaflow step. I believe the setting only applies to Popen.subprocess() and not whatever forking Metaflow uses

I even tried pausing a step so I could use PyCharm's attach to process option, but this fails