Unable to interactively see matplotlib plots with `QtAgg` backend

[cactus1549]

Environment data

• debugpy version: 1.5.1
• OS and version: Windows 10 Pro 21H2 (64-bit) 10.0.19044
• Python version (& distribution if applicable, e.g. Anaconda): 3.9.4
• matplotlib

Name                    Version            Build                       Channel
matplotlib                3.4.2            pypi_0                      pypi
matplotlib-base           3.5.1            py39h581301d_0              conda-forge
matplotlib-inline         0.1.3            pyhd8ed1ab_0                conda-forge

The matplotlib settings are:

backend QtAgg        
interactive True     
non interactive False

• Using VS Code or Visual Studio: 1.64.2

Actual behavior

I would like to be able to debug python scripts that use matplotlib interactively. By interactively I mean placing a break point and watching my figure update as I actually step through my code.

Expected behavior

The figure does not update when stepping through code.

Steps to reproduce:

1. Create a python script test.py with the following code:

import matplotlib.pyplot as plt
 

figure = plt.figure()
ax = plt.gca()
ax.plot([1,2,3])
ax.set_title("hello")
plt.show()

2. Place a breakpoint on line 4 and then step through the following lines of code one by one.
3. The figure appears blank and doesn't update.

[fabioz]

As a note, your environment seems pretty strange if you have:

• matplotlib 3.4.2 pypi_0 pypi
• matplotlib-base 3.5.1 py39h581301d_0 conda-forge

It's as if you're mixing things and have one part of the matplotlib package from pypi in a version and another part from conda-forge in another version...

Anyways, I was able to reproduce this with an environment with matplotlib 3.5.1 from conda-forge.

The problem is that the backend being chosen now is QtAgg (I think this is something new as it usually just reported Qt5Agg or Qt4Agg).

I'll provide a fix shortly...

@cactus1549 as a workaround, if you can execute:

import matplotlib
matplotlib.use("Qt5Agg")

At the start of your script, things also work.

[cactus1549]

@fabioz, brilliant this works, thank you!