Saving figures to files in Matplotlib - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

What file is created by this code?

Consider this Python code using matplotlib to save a plot. What is the name and format of the saved file?

Matplotlib

import matplotlib.pyplot as plt
plt.plot([1, 2, 3], [4, 5, 6])
plt.savefig('myplot.png')

A A file named 'myplot.png' is created in the current directory.

B A file named 'myplot.pdf' is created in the current directory.

C A file named 'myplot.jpg' is created in the current directory.

D No file is created because plt.show() is missing.

💡 Hint

The filename extension in savefig determines the file format.

The plt.savefig() function saves the current figure to the filename given. Here, 'myplot.png' means a PNG image file is created.

❓ Predict Output

intermediate

What happens if you save a figure twice with different names?

What files are created after running this code?

Matplotlib

import matplotlib.pyplot as plt
plt.plot([1, 2], [3, 4])
plt.savefig('first.png')
plt.savefig('second.png')

A An error occurs because savefig is called twice.

B Only 'second.png' is created, overwriting 'first.png'.

C No files are created because plt.close() is missing.

D Both 'first.png' and 'second.png' files are created separately.

💡 Hint

Each savefig call writes the current figure to the given filename.

Calling savefig twice with different filenames creates two separate files with the same figure content.

🔧 Debug

advanced

Why does this code save a blank image?

This code saves a blank image file. What is the reason?

Matplotlib

import matplotlib.pyplot as plt
plt.savefig('blank.png')
plt.plot([1, 2, 3], [4, 5, 6])
plt.show()

A The plot is created after savefig, so the saved image is blank.

B plt.show() clears the figure before saving.

C The filename extension 'png' is unsupported.

D The plot command is missing plt.draw() before savefig.

💡 Hint

Think about the order of commands and when the figure is saved.

savefig saves the current figure state. Since no plot was drawn before savefig, the image is blank.

❓ data_output

advanced

What is the size of the saved figure in pixels?

Given this code, what is the pixel size of the saved image 'plot.png'?

Matplotlib

import matplotlib.pyplot as plt
fig = plt.figure(figsize=(4, 3), dpi=100)
plt.plot([0, 1], [0, 1])
fig.savefig('plot.png')

A 100 pixels wide by 100 pixels tall

B 4 pixels wide by 3 pixels tall

C 400 pixels wide by 300 pixels tall

D Cannot determine without plt.show()

💡 Hint

Image size in pixels = figsize (in inches) × dpi.

The figure size is 4×3 inches and dpi is 100, so pixels = 4*100 by 3*100 = 400×300.

🚀 Application

expert

How to save a figure with transparent background?

Which code snippet correctly saves a matplotlib figure with a transparent background?

Matplotlib

import matplotlib.pyplot as plt
plt.plot([1, 2, 3], [4, 5, 6])

A plt.savefig('transparent.png', transparent=False)

B plt.savefig('transparent.png', transparent=True)

C plt.savefig('transparent.png', alpha=0.5)

D plt.savefig('transparent.png', bg='transparent')

💡 Hint

Check the savefig parameter that controls background transparency.

The 'transparent=True' argument saves the figure with a transparent background.