[Issue]: Manually placing contour labels using `clabel` not working

[carmaza]

Bug summary

Manually placing contour labels using clabel not working.

Code for reproduction

import matplotlib                                                                                       
import numpy as np                                                                                      
import sys                                                                                              
                                                                                                        
from matplotlib import pyplot as plt                                                                    
                                                                                                        
print("Python version:", sys.version)                                                                   
print("MPL version:", matplotlib.__version__)                                                           
print("NumPy version:", np.__version__)                                                                 
print("MPL backend:", matplotlib.get_backend())                                                         
                                                                                                        
t = np.linspace(0., 1., 100)                                                                            
X, Y = np.meshgrid(t, t)                                                                                
                                                                                                        
contours = plt.contour(X, Y, X*X + Y*Y)                                                                 
plt.clabel(contours, manual=True)                                                             
                                                                                                        
plt.savefig("test.pdf", bbox_inches='tight')

Actual outcome

Running python3 test.py in file location gives the following output:

Python version: 3.8.10 (default, Nov 26 2021, 20:14:08) 
[GCC 9.3.0]
MPL version: 3.5.1
NumPy version: 1.22.2
MPL backend: GTK3Agg
Select label locations manually using first mouse button.
End manual selection with second mouse button.

Then the figure shows, expecting me to click on it to place labels manually. Upon clicking anywhere on the image, I get:

Traceback (most recent call last):
  File "/home/cristobal/.local/lib/python3.8/site-packages/matplotlib/backends/backend_gtk3.py", line 159, in button_press_event
    FigureCanvasBase.button_press_event(
  File "/home/cristobal/.local/lib/python3.8/site-packages/matplotlib/backend_bases.py", line 1877, in button_press_event
    self.callbacks.process(s, mouseevent)
  File "/home/cristobal/.local/lib/python3.8/site-packages/matplotlib/cbook/__init__.py", line 292, in process
    self.exception_handler(exc)
  File "/home/cristobal/.local/lib/python3.8/site-packages/matplotlib/cbook/__init__.py", line 96, in _exception_printer
    raise exc
  File "/home/cristobal/.local/lib/python3.8/site-packages/matplotlib/cbook/__init__.py", line 287, in process
    func(*args, **kwargs)
  File "/home/cristobal/.local/lib/python3.8/site-packages/matplotlib/contour.py", line 70, in _contour_labeler_event_handler
    if event.inaxes == cs.ax:
AttributeError: 'QuadContourSet' object has no attribute 'ax'

and no label is placed in the contours.

I can close the figure normally using third click button, and the figure is saved normally but without any labels on the contours.

Expected outcome

Clicking on canvas should place labels.

Additional information

I am able to place labels as expected using the following versions:

('Python version:', '2.7.18 (default, Mar  8 2021, 13:02:45) \n[GCC 9.3.0]')
('MPL version:', '2.2.5')
('NumPy version:', '1.16.6')
('MPL backend:', 'TkAgg')

(this is the output after running python test.py).

If this is not a bug, I propose to modify the documentation of clabel (https://matplotlib.org/stable/api/contour_api.html#matplotlib.contour.ContourLabeler.clabel) accordingly.

Operating system

Ubuntu 20.04

Matplotlib Version

3.5.1

Matplotlib Backend

GTK3Agg

Python version

3.8.10

Jupyter version

No response

Installation

pip

[oscargus]

I can confirm that this happens (not surprisingly) for QtAgg as well.

Python version: 3.8.12 | packaged by conda-forge | (default, Jan 30 2022, 23:42:07) 
[GCC 9.4.0]
MPL version: 3.5.1
NumPy version: 1.21.2
MPL backend: QtAgg
Select label locations manually using first mouse button.
End manual selection with second mouse button.
Traceback (most recent call last):
  File "/local/data1/miniconda3/lib/python3.8/site-packages/matplotlib/cbook/__init__.py", line 287, in process
    func(*args, **kwargs)
  File "/local/data1/miniconda3/lib/python3.8/site-packages/matplotlib/contour.py", line 70, in _contour_labeler_event_handler
    if event.inaxes == cs.ax:
AttributeError: 'QuadContourSet' object has no attribute 'ax'

(Running it in Spyder/IPython with the inline backend gives

Python version: 3.8.12 | packaged by conda-forge | (default, Jan 30 2022, 23:42:07) 
[GCC 9.4.0]
MPL version: 3.5.1
NumPy version: 1.21.2
MPL backend: module://matplotlib_inline.backend_inline
Select label locations manually using first mouse button.
End manual selection with second mouse button.
/local/data1/quadcountourclabel.py:23: UserWarning: Matplotlib is currently using module://matplotlib_inline.backend_inline, which is a non-GUI backend, so cannot show the figure.
  plt.clabel(contours, manual=True)

where it probably makes more sense that the prompt returns without having to kill it.)

[oscargus]

I've created a fix for it in #22534. (Very simple and probably missed when changing name from ax to axes.)