Hey, thanks for including robust automation support in Natron! I'm hitting a bit of a snag when trying to use setExpression on various effects, as shown below:

Problem

Expected behavior: The program renders a one-frame video file.

Actual behavior: Program hangs at the point indicated below, and is not responsive to SIGINT or SIGTERM.

Steps to Reproduce

Create repro.py with the following contents:

def createInstance(app, group):
    write = app.createNode("fr.inria.openfx.WriteFFmpeg") # also occurs with WriteOIIO
    write.getParam("filename").set("/tmp/repro.mkv") # path shouldn't matter here; change as needed
    text = app.createNode("net.fxarena.openfx.Text")
    text.getParam("text").setExpression("0", False) # also hangs with ("'0'", False) and ("ret='0'", True)
    write.connectInput(0, text)
    app.render(write, 0, 1)

Then do this:

$ Natron --version
Natron Version 2.3.15
Copyright (C) 2013-2018 INRIA and Alexandre Gauthier-Foichat
>>>Use the --help or -h option to print usage.<<<
Natron version 2.3.15 at commit 513a2491008c152417ae913f8d1ab83e432aa7e3 on
$ Natron -b -t repro.py
Natron Version 2.3.15
Copyright (C) 2013-2018 INRIA and Alexandre Gauthier-Foichat
>>>Use the --help or -h option to print usage.<<<
Note: -t argument given, loading in command-line interpreter mode, only Python commands / scripts are accepted
Restoring the image cache...
Loading plugin cache...
Info: init.py script not loaded (this is not an error)
Loading PyPlugs...
Note: -t argument given, loading in command-line interpreter mode, only Python commands / scripts are accepted
Write1 ==> Rendering started
[it hangs here]

When the first argument to setExpression is invalid Python syntax such as ",,,,", the program does not hang and produces a video file without the text, as expected. When the setExpression line is commented out, it also produces a video file with "Enter text" in the correct position.

I'm able to manually set params to Python expressions and render the results in the GUI without issue; this appears to only happen when I'm running scripts.

Versions

• Natron commit: 513a249; built via natron-bin 2.3.15.1-1 in the AUR: https://aur.archlinux.org/packages/natron-bin/
• OS version: Arch Linux