Memory leak when calling certain python packages from Julia via PythonCall #759

Affects: PythonCall

Describe the bug

Memory is not released when repeatedly creating and closing matplotlib figures via PythonCall. I originally had the issue with an elaborate cartopy plotting loop, but have managed to recreate it in a simple while true loop MWE. Each iteration allocates a new figure/axes via plt.subplots, immediately closes it with plt.close, calls PythonCall.pydel! on both the figure and axes objects, and calls GC.gc(). However, there is still a leak until the process is killed.

This does not occur when repeatedly allocating and deleting a numpy array in an equivalent loop, so the issue has something to do with what kind of python objects are created. I also have similar memory leak issues with scikit-learn.

The issue is reproducable both in the Julia REPL and in the VS Code Julia extension, and when I execute a script containing the below code from the command line.

MWE:
using PythonCall
@py import matplotlib.pyplot as plt

i = 0
while true
global i += 1
fig, ax = plt.subplots(1, 1)
plt.close(fig)
PythonCall.pydel!(ax)
PythonCall.pydel!(fig)
GC.gc()
println("Iteration $i done")
end

My system

OS: macOS 15.7.4 (arm64, Apple Silicon)
Julia: 1.12.6
Python: 3.14.2 (conda-forge, Clang 20.1.8)
PythonCall: 0.9.31
matplotlib: 3.10.8

Thanks for your help!