TACTIC Python API Tweak: Hack To Report Copied Byte Amount To Qt Widget

During the development of some Maya Tools that used the Southpaw Tactic Python API I bumped into the following, at first simple, problem: I wanted to give a visual report of the uploading progress process. Each artist had to check-in their work to the asset management system via internet.

The first version of the tool only gave report of the progress by means of a progress bar that visually was enough to notify when the upload had finished. This worked ideally for multiple tiny files. But soon Groom & Hair artists, as well as VFX artist where generating a lot of huge simulating data that needed to be uploaded.

We were working remotely and uploading the artist's work could easily take a couple of hours. The first approach was to use HTTP protocol to transfer those huge amounts of files. There we found a bug in the Python API of Tactic v4.4.04 that limited the file size to 10 MB (10*1024*104 bytes) that forced us to look in the documentation and upgrade to a newer version of Tactic that had this bug fixed. But that's another story.

What interests me here is that the Python Tactic API upload functions dont give any report of the number of bytes uploaded. It only gives a report of when an entire file has been checked-in, this is, by doing a Piecewise Check-In.

So we changed the upload method to use Tactic's handoff dir which consists basically on replacing the HTTP protocol by a protocol like CIFS or NFS where you just perform a copy from your local to the server's directory just like you would between two directories on your local filesystem.

That was the first step.

Now once, definitely using the most powerful transfer method. I only needed to have a look at the API. The "tactic_client_stub.py" module and the "TacticServerStub" class. The Piecewise Check-in works as explained here.

You can see that the API uses the "shutil.copy" and "shutil.move" methods to upload. I cannot tweak the "shutil" module, since it's a built-in one that comes by default with the Maya Python Interpreter. But i can build my own :))!!

My goal is to be able to report the amount of bytes transferred using a Qt Widget so basically i have to simulate a Signal/Slot behaviour from the copy/move methods. It would be nice if i could add a callback inside that method that triggered a Qt Signal, isnt it?!

A LEAST INTRUSIVE SOLUTION

The shutil module uses a lot of different methods to copy files considering the metadata, creation and last modification time, user owner and group owner and the permissions bits, etc. It is explained here.

All of them at last, call the "copyfileobj" method. That's the method i want to tweak.

Now, what kind of function can trigger a Qt Signal?? what are its requisites??

I remembered all Qt Classes inherit from the QObject Class.. A quick look at the PyQt Documentation explains it.

"The central feature in this model is a very powerful mechanism for seamless object communication called signals and slots"

So basically, the only thing i need is to define a class that inherits from QObject, define a custom signal and have the callback method to emit the signal!!. The following is not production code, it is just an example of how it would work.

All that is left is to catch the signal in the proper QWidget, with this information you can compute the time left for the upload to finish and hence give an estimate based on internet speed.

This solution is simple, straightforward and doesnt imply rewriting the TacticServerStub Class. Maybe if i find myself in the need of tweaking again i would consider writing my own TacticServerStub class.

Comments & Critics Welcome!!