[Bug]: Saving a figure with indicate_inset_zoom to pdf and then pickling it causes TypeError

[lmarti-dev]

Bug summary

If a fig with an ax using the indicate_inset_zoom is saved to pdf and then pickled, one gets the following error:

TypeError: cannot pickle '_io.BufferedWriter' object

Code for reproduction

import pickle
import matplotlib.pyplot as plt


def plot(inset_zoom: bool, format: str):
    fig, ax = plt.subplots()
    ax.plot(
        [1, 1],
        [2, 2],
    )

    if inset_zoom:
        ax.indicate_inset_zoom(ax)

    if format is not None:
        fig.savefig(f"./test1.{format}", format=f"{format}")

    pickle.dumps(fig)


for format in (None, "png", "pdf"):
    for inset_zoom in (False, True):
        print(f"format: {format} inset_zoom {inset_zoom}")
        plot(inset_zoom, format)
        print("ok")

Actual outcome

(venv) C:\Users\Moi4\Desktop\code\test\pickle_buffwr_err>python error_with_mpl_pickle.py
format: None inset_zoom False
ok
format: None inset_zoom True
ok
format: png inset_zoom False
ok
format: png inset_zoom True
ok
format: pdf inset_zoom False
ok
format: pdf inset_zoom True
Traceback (most recent call last):
  File "C:\Users\Moi4\Desktop\code\test\pickle_buffwr_err\error_with_mpl_pickle.py", line 24, in <module>
    plot(inset_zoom, format)
  File "C:\Users\Moi4\Desktop\code\test\pickle_buffwr_err\error_with_mpl_pickle.py", line 18, in plot
    pickle.dumps(fig)
TypeError: cannot pickle '_io.BufferedWriter' object

Expected outcome

The expected outcome would be no TypeError

Additional information

Saving to pickle before saving the figure is a workaround.

Operating system

Windows 10

Matplotlib Version

3.7.1 and 3.8.3

Matplotlib Backend

TkAgg

Python version

3.10.11

Jupyter version

No response

Installation

pip

[ksunden]

The culprit here appears to be these two lines in ConnectionPatch:

matplotlib/lib/matplotlib/patches.py

Lines 4699 to 4700 in 897dd0a

	if renderer is not None:
	self._renderer = renderer

Namely, that the ConnectionPatch object (which is used to draw the zoom lines in inset_zoom) holds a reference to to the renderer on draw.

This combined with the fact that the PDF backend holds a reference to a BytesIO object (as well as I believe at least one other unpicklable generator object... didn't trace exactly where that came from, but was able to poke it to give me a different error) means that upon drawing with the PDF backend the ConnectionPatch becomes unpicklable.

As far as I can tell, there is no reason to hold a reference here, it is never accessed. It has existed in this class since the beginning (August of 2009).

There are a handful of other places we store a renderer to an instance attribute, as a roundup:

• offsetbox.AnnotationBbox: Added around the same time, similarly does not appear to be accessed
• Various points within backends themselves, unlikely to be problematic
• patheffects.PathEffectRenderer: this is the whole point of the object, and is not on an artist, so likely fine.
• text.Text/text.Annotation: Is handled as a special case in __getstate__/__setstate__, and thus explicitly picklable.
  • I'm guessing that the other two copied Text as a template for writing the draw method, and that the attribute was never actually used for them and thus is likely safe to remove.