Cursor and event handling in Matplotlib - Step-by-Step Execution

Concept Flow - Cursor and event handling

Start plot display

↓

User moves mouse

↓

Event triggered: mouse move

↓

Cursor position updated

↓

Display updated with cursor info

↓

User clicks or closes plot

↓

Event triggered: click or close

↓

Handle event or exit

The flow shows how matplotlib listens for mouse events, updates cursor info on move, and handles clicks or closing events.

Execution Sample

Matplotlib

import matplotlib.pyplot as plt
from matplotlib.widgets import Cursor

fig, ax = plt.subplots()

cursor = Cursor(ax, useblit=True, color='red', linewidth=1)

plt.show()

This code creates a plot with a red cursor line that follows the mouse pointer.

Execution Table

Step	Event	Cursor Position	Action	Output
1	Plot shown	None	Wait for mouse events	Plot window appears
2	Mouse move at (0.5, 0.5)	(0.5, 0.5)	Update cursor lines	Cursor lines drawn at x=0.5 and y=0.5
3	Mouse move at (0.7, 0.3)	(0.7, 0.3)	Update cursor lines	Cursor lines move to x=0.7 and y=0.3
4	Mouse click at (0.7, 0.3)	(0.7, 0.3)	Trigger click event handler	Print or handle click info (if defined)
5	Plot window closed	None	Trigger close event handler	Program ends
6	No more events	None	Exit	Plot closed, program stops

💡 Plot window closed or user exits, stopping event listening

Variable Tracker

Variable	Start	After Step 2	After Step 3	After Step 4	Final
cursor position	None	(0.5, 0.5)	(0.7, 0.3)	(0.7, 0.3)	None (plot closed)
event type	None	mouse move	mouse move	mouse click	close

Key Moments - 3 Insights

Why does the cursor position update only when the mouse moves?

Because the cursor update is tied to the mouse move event (see execution_table steps 2 and 3). Without mouse movement, no event triggers an update.

What happens if no event handler is defined for mouse clicks?

The click event occurs (step 4), but no action happens unless you define a handler. The cursor still shows position but no extra output.

Why does the program stop after closing the plot window?

Closing the plot triggers the close event (step 5), which ends the event loop and stops the program (step 6).

Visual Quiz - 3 Questions

Look at the execution table, what is the cursor position at step 3?

A (0.7, 0.3)

B None

C (0.5, 0.5)

D (0.3, 0.7)

💡 Hint

Check the 'Cursor Position' column at step 3 in the execution_table.

At which step does the program stop listening for events?

A Step 4

B Step 5

C Step 6

D Step 2

💡 Hint

Look at the 'exit_note' and the last rows in the execution_table.

If the mouse never moves, what will the cursor position be?

A Always (0,0)

B None

C Last clicked position

D Random position

💡 Hint

Refer to variable_tracker for 'cursor position' start value and step 2.

Concept Snapshot

matplotlib cursor and event handling:
- Use Cursor(ax) to show crosshair following mouse
- Connect event handlers with fig.canvas.mpl_connect(event, handler)
- Events: 'motion_notify_event' for mouse move, 'button_press_event' for clicks
- Cursor updates only on mouse move events
- Closing plot stops event listening and ends program

Full Transcript

This visual execution shows how matplotlib handles cursor and mouse events. When the plot window opens, it waits for mouse events. Moving the mouse triggers 'motion_notify_event', updating the cursor position and redrawing crosshair lines at the mouse location. Clicking triggers 'button_press_event', which can be handled to perform actions. Closing the plot triggers a close event that stops the program. Variables like cursor position update only on mouse moves. This step-by-step trace helps beginners see how event-driven cursor updates work in matplotlib.

Practice

1. What is the main purpose of using mpl_connect in matplotlib?

easy

A. To create a new figure window

B. To connect an event like mouse click to a custom function

C. To save the current plot as an image file

D. To change the color of the plot lines

Solution

1. Step 1: Understand what mpl_connect does

   mpl_connect links events such as mouse clicks or key presses to functions you define.

2. Step 2: Identify the correct purpose

   It does not save images, create figures, or change colors directly. It connects events to functions.

3. Final Answer:

   To connect an event like mouse click to a custom function -> Option B

4. Quick Check:

   Event connection = C [OK]

Hint: Remember: mpl_connect links events to your functions [OK]

Common Mistakes:

• Thinking mpl_connect saves or modifies plots directly
• Confusing mpl_connect with figure creation
• Assuming mpl_connect changes plot styles

2. Which of the following is the correct syntax to connect a mouse click event to a function named on_click using matplotlib?

easy

A. fig.connect_event('button_press', on_click)

B. fig.mpl_connect('click', on_click)

C. fig.canvas.mpl_connect('button_press_event', on_click)

D. fig.connect('mouse_click', on_click)

Solution

1. Step 1: Recall the correct event name for mouse clicks

   The correct event name in matplotlib for mouse button press is 'button_press_event'.

2. Step 2: Check the syntax of mpl_connect

   The method is called on the figure's canvas as fig.canvas.mpl_connect(event_name, function).

3. Final Answer:

   fig.canvas.mpl_connect('button_press_event', on_click) -> Option C

4. Quick Check:

   Correct event name and syntax = A [OK]

Hint: Use 'button_press_event' for mouse clicks with mpl_connect [OK]

Common Mistakes:

• Using wrong event names like 'click' or 'mouse_click'
• Using non-existent methods like connect_event
• Mixing up method names and event strings

3. Consider the code below:

import matplotlib.pyplot as plt

fig, ax = plt.subplots()

def on_move(event):
    print(f"Mouse at: {event.xdata}, {event.ydata}")

cid = fig.canvas.mpl_connect('motion_notify_event', on_move)
plt.show()

What will happen when you move the mouse over the plot area?

medium

A. The coordinates of the mouse pointer inside the plot will be printed continuously

B. Nothing will happen because the event is not connected properly

C. The plot will close immediately

D. An error will occur because event.xdata is undefined

Solution

1. Step 1: Understand the event type 'motion_notify_event'

   This event triggers whenever the mouse moves over the figure canvas.

2. Step 2: Analyze the function on_move

   The function prints the mouse coordinates inside the plot area using event.xdata and event.ydata.

3. Final Answer:

   The coordinates of the mouse pointer inside the plot will be printed continuously -> Option A

4. Quick Check:

   Mouse move event prints coords = B [OK]

Hint: motion_notify_event tracks mouse movement over plot [OK]

Common Mistakes:

• Assuming no output because event is not connected
• Thinking event.xdata is always None or undefined
• Expecting plot to close on mouse move

4. The following code is intended to print the mouse button pressed, but it raises an error:

import matplotlib.pyplot as plt

fig, ax = plt.subplots()

def on_click(event):
    print(f"Button pressed: {event.button}")

fig.canvas.mpl_connect('button_press_event', on_click())
plt.show()

What is the error and how to fix it?

medium

A. The function is called immediately; remove parentheses in mpl_connect

B. The event name is wrong; use 'mouse_press' instead

C. event.button does not exist; use event.key instead

D. mpl_connect should be called on ax, not fig

Solution

1. Step 1: Identify the error in function connection

   Using on_click() calls the function immediately instead of passing it as a reference.

2. Step 2: Correct the function reference in mpl_connect

   Remove parentheses to pass the function itself: fig.canvas.mpl_connect('button_press_event', on_click).

3. Final Answer:

   The function is called immediately; remove parentheses in mpl_connect -> Option A

4. Quick Check:

   Pass function, not call it = D [OK]

Hint: Pass function name without () to mpl_connect [OK]

Common Mistakes:

• Calling the function instead of passing it
• Using wrong event names
• Trying to access wrong event attributes

5. You want to create an interactive plot where clicking inside the plot area prints the nearest data point's coordinates from a scatter plot. Which approach correctly combines cursor event handling and data lookup?

hard

A. Use plt.scatter() with a parameter to automatically print nearest point on click

B. Use 'motion_notify_event' to print coordinates continuously without checking points

C. Connect 'key_press_event' to print data points when any key is pressed

D. Connect 'button_press_event' to a function that calculates distances from click to all points and prints nearest

Solution

1. Step 1: Identify the correct event for mouse clicks

   Use 'button_press_event' to detect mouse clicks inside the plot.

2. Step 2: Implement logic to find nearest data point

   Calculate distances from click position to all scatter points, then print the closest one.

3. Step 3: Verify other options

   'motion_notify_event' prints continuously, 'key_press_event' is unrelated, and plt.scatter has no auto-print feature.

4. Final Answer:

   Connect 'button_press_event' to a function that calculates distances from click to all points and prints nearest -> Option D

5. Quick Check:

   Click event + nearest point logic = A [OK]

Hint: Use click event plus distance check to find nearest point [OK]

Common Mistakes:

• Using motion event instead of click for selection
• Expecting built-in auto-print in scatter
• Confusing key press with mouse click events