Bug report

Bug summary

When dealing with callbacks for event that are subclass of LocationEvent (KeyEvent and MouseEvent) in FigureCanvasBase methods (like motion_notify_event), an event object can be passed to callbacks.process for 'axes_enter_event' before being fully initialized. This can cause bugs.

Code for reproduction
I took the first example of the examples page and added theses lines:

def mycallback (event):
  print (type(event), hasattr (event, 'button'))
  print (event) # raise an exception as self.button is needed in MouseEvent.__str__

plt.gca().figure.canvas.mpl_connect ('axes_enter_event', mycallback)

Full code

import numpy as np
import matplotlib.pyplot as plt


N = 5
menMeans = (20, 35, 30, 35, 27)
womenMeans = (25, 32, 34, 20, 25)
menStd = (2, 3, 4, 1, 2)
womenStd = (3, 5, 2, 3, 3)
ind = np.arange(N)    # the x locations for the groups
width = 0.35       # the width of the bars: can also be len(x) sequence

p1 = plt.bar(ind, menMeans, width, yerr=menStd)
p2 = plt.bar(ind, womenMeans, width,
             bottom=menMeans, yerr=womenStd)

plt.ylabel('Scores')
plt.title('Scores by group and gender')
plt.xticks(ind, ('G1', 'G2', 'G3', 'G4', 'G5'))
plt.yticks(np.arange(0, 81, 10))
plt.legend((p1[0], p2[0]), ('Men', 'Women'))

def mycallback (event):
  print (type(event), hasattr (event, 'button'))
  print (event)

plt.gca().figure.canvas.mpl_connect ('axes_enter_event', mycallback)

plt.show()

Actual outcome

When the mouse is moved over the axes, it prints

<class 'matplotlib.backend_bases.MouseEvent'> False
Traceback (most recent call last):
  File "/home/cyril/.local/lib/python3.7/site-packages/matplotlib/cbook/__init__.py", line 216, in process
    func(*args, **kwargs)
  File "bar_stacked.py", line 37, in mycallback
    print (event)
  File "/home/cyril/.local/lib/python3.7/site-packages/matplotlib/backend_bases.py", line 1445, in __str__
    return (f"{self.name}: "
AttributeError: 'MouseEvent' object has no attribute 'button'

The MouseEvent object is not fully initialized (or else it would have a 'button' attribute).
The problem is due to the following calls:

• when a event is captured, the method motion_notify_event of FigureCanvasBase is called
• it creates an MouseEvent object, so MouseEvent.__init__ is called
• the first thing the initializer does is calling LocationEvent.__init__ (initializer of the parent class) [ before setting self.button ]
• the last thing LocationEvent.__init__ does is calling self._update_enter_leave
• In the method self._update_enter_leave (of the class LocationEvent), callbacks.process is called for axes_enter_event (if mouse is in the right location) with self (a MouseEvent object) as argument for the callbacks function. The problem is here, the argument should be of LocationEvent type (according to the doc). And self cannot be use here as it is not finished being initialized.

A similar problem occurs for KeyEvent (a subclass of LocationEvent) with the attributes 'key'.

Expected outcome

According to the doc, the type of event for axes_enter_event shoud be LocationEvent not MouseEvent

Matplotlib version

• Operating system: Linux
• Matplotlib version: 3.1
• Matplotlib backend (print(matplotlib.get_backend())): Qt5Agg
• Python version: 3.7