Animation update function in Matplotlib - Step-by-Step Execution

Concept Flow - Animation update function

Start Animation

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Call update(frame)

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Update plot elements

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Return updated elements

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Render frame

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Next frame or End

The animation calls the update function for each frame, which changes plot elements and returns them for rendering.

Execution Sample

Matplotlib

import matplotlib.pyplot as plt
from matplotlib.animation import FuncAnimation

fig, ax = plt.subplots()
line, = ax.plot([], [])

def update(frame):
    x = list(range(frame))
    y = [i**2 for i in x]
    line.set_data(x, y)
    return line,

This code defines an update function that changes the line data for each animation frame.

Execution Table

Frame	x values	y values	Action	Returned Elements
1	[0]	[0]	Set line data to x=[0], y=[0]	line
2	[0, 1]	[0, 1]	Set line data to x=[0,1], y=[0,1]	line
3	[0, 1, 2]	[0, 1, 4]	Set line data to x=[0,1,2], y=[0,1,4]	line
4	[0, 1, 2, 3]	[0, 1, 4, 9]	Set line data to x=[0,1,2,3], y=[0,1,4,9]	line
5	[0, 1, 2, 3, 4]	[0, 1, 4, 9, 16]	Set line data to x=[0,1,2,3,4], y=[0,1,4,9,16]	line
6	[0, 1, 2, 3, 4, 5]	[0, 1, 4, 9, 16, 25]	Set line data to x=[0,1,2,3,4,5], y=[0,1,4,9,16,25]	line
7	[0, 1, 2, 3, 4, 5, 6]	[0, 1, 4, 9, 16, 25, 36]	Set line data to x=[0,1,2,3,4,5,6], y=[0,1,4,9,16,25,36]	line
8	[0, 1, 2, 3, 4, 5, 6, 7]	[0, 1, 4, 9, 16, 25, 36, 49]	Set line data to x=[0,1,2,3,4,5,6,7], y=[0,1,4,9,16,25,36,49]	line
9	[0, 1, 2, 3, 4, 5, 6, 7, 8]	[0, 1, 4, 9, 16, 25, 36, 49, 64]	Set line data to x=[0,1,2,3,4,5,6,7,8], y=[0,1,4,9,16,25,36,49,64]	line
10	[0, 1, 2, 3, 4, 5, 6, 7, 8, 9]	[0, 1, 4, 9, 16, 25, 36, 49, 64, 81]	Set line data to x=[0,1,2,3,4,5,6,7,8,9], y=[0,1,4,9,16,25,36,49,64,81]	line
End	-	-	Reached last frame, animation stops	-

💡 Animation stops after frame 10, no more frames to update.

Variable Tracker

Variable	Start	After frame 1	After frame 2	After frame 3	After frame 4	After frame 5	After frame 6	After frame 7	After frame 8	After frame 9	After frame 10
frame	0	1	2	3	4	5	6	7	8	9	10
x	[]	[0]	[0,1]	[0,1,2]	[0,1,2,3]	[0,1,2,3,4]	[0,1,2,3,4,5]	[0,1,2,3,4,5,6]	[0,1,2,3,4,5,6,7]	[0,1,2,3,4,5,6,7,8]	[0,1,2,3,4,5,6,7,8,9]
y	[]	[0]	[0,1]	[0,1,4]	[0,1,4,9]	[0,1,4,9,16]	[0,1,4,9,16,25]	[0,1,4,9,16,25,36]	[0,1,4,9,16,25,36,49]	[0,1,4,9,16,25,36,49,64]	[0,1,4,9,16,25,36,49,64,81]

Key Moments - 3 Insights

Why does the update function return 'line,' with a comma?

The comma makes it a tuple, which FuncAnimation expects to know which artists to redraw. See execution_table rows where 'Returned Elements' is 'line'.

What happens if the update function does not return anything?

The animation may not update properly because FuncAnimation uses the returned artists to redraw frames. This is shown in the 'Returned Elements' column in execution_table.

Why do x and y lists grow with each frame?

Because the update function uses 'range(frame)', so each frame adds one more point. See variable_tracker for x and y values increasing.

Visual Quiz - 3 Questions

Look at the execution_table at frame 4, what are the y values set in the update function?

A [1, 4, 9, 16]

B [0, 1, 2, 3]

C [0, 1, 4, 9]

D [0, 1, 4]

💡 Hint

Check the 'y values' column in execution_table row for frame 4.

At which frame does the x list first contain 5 elements?

A Frame 5

B Frame 3

C Frame 4

D Frame 6

💡 Hint

Look at variable_tracker row for x and count elements after each frame.

If the update function returned nothing, how would the 'Returned Elements' column in execution_table change?

A It would show 'line' as usual

B It would be empty or missing

C It would show an error message

D It would show 'None' explicitly

💡 Hint

Returned Elements shows what update returns; no return means empty.

Concept Snapshot

Animation update function:
- Called each frame with frame number
- Updates plot elements (e.g., line data)
- Returns updated artists as tuple
- FuncAnimation uses returned artists to redraw
- Controls animation visuals frame-by-frame

Full Transcript

The animation update function is called by FuncAnimation for each frame number. It updates plot elements like line data using the frame number to generate new x and y values. The function returns the updated plot elements as a tuple, which tells FuncAnimation what to redraw. This process repeats for each frame until the animation ends. The update function is essential for changing the plot dynamically over time.