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.

Practice

1. What does the plt.savefig() function do in matplotlib?

easy

A. It saves the current figure to a file on your computer.

B. It displays the figure on the screen.

C. It clears the current figure.

D. It creates a new figure window.

Solution

1. Step 1: Understand the purpose of plt.savefig()

   This function is used to save the current plot or figure to a file, such as PNG or PDF.

2. Step 2: Differentiate from display functions

   Functions like plt.show() display the figure, but plt.savefig() writes it to disk.

3. Final Answer:

   It saves the current figure to a file on your computer. -> Option A

4. Quick Check:

   plt.savefig() saves figure [OK]

Hint: Remember: savefig saves, show displays [OK]

Common Mistakes:

• Confusing plt.savefig() with plt.show()
• Thinking savefig displays the plot
• Assuming savefig clears the figure

2. Which of the following is the correct syntax to save a figure as a PNG file named 'plot.png'?

easy

A. plt.save('plot.png')

B. plt.savefig('plot.png')

C. plt.savefig(plot.png)

D. plt.save_figure('plot.png')

Solution

1. Step 1: Recall the correct function name

   The function to save figures is plt.savefig(), not plt.save() or others.

2. Step 2: Check the argument format

   The filename must be a string, so it should be in quotes: 'plot.png'.

3. Final Answer:

   plt.savefig('plot.png') -> Option B

4. Quick Check:

   Correct function and string filename [OK]

Hint: Use plt.savefig('filename.ext') with quotes [OK]

Common Mistakes:

• Missing quotes around filename
• Using incorrect function name
• Passing filename without quotes

3. What will happen if you run this code?

import matplotlib.pyplot as plt
plt.plot([1, 2, 3], [4, 5, 6])
plt.savefig('myplot.pdf', dpi=300)
plt.close()

medium

A. The plot is shown on screen but not saved.

B. An error occurs because dpi is not allowed with PDF files.

C. A PDF file named 'myplot.pdf' is saved with high resolution.

D. The file 'myplot.pdf' is saved but with default dpi 72.

Solution

1. Step 1: Understand plt.savefig() with dpi parameter

   Setting dpi=300 increases the resolution of the saved image, including PDF files.

2. Step 2: Confirm file saving and closing

   The code saves the plot as 'myplot.pdf' with dpi 300, then closes the figure to free memory.

3. Final Answer:

   A PDF file named 'myplot.pdf' is saved with high resolution. -> Option C

4. Quick Check:

   savefig with dpi=300 saves high-res PDF [OK]

Hint: dpi controls resolution; works with PDF too [OK]

Common Mistakes:

• Thinking dpi is ignored for PDF
• Expecting plot to display automatically
• Confusing plt.close() with plt.show()

4. Identify the error in this code snippet that tries to save a figure:

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

medium

A. The file extension '.png' is invalid.

B. plt.show() must come before plt.savefig().

C. plt.plot() is missing a figure creation step.

D. The filename is not enclosed in quotes.

Solution

1. Step 1: Check the filename argument in plt.savefig()

   The filename must be a string, so it needs quotes: 'myplot.png'.

2. Step 2: Verify order of plt.show() and plt.savefig()

   Calling plt.savefig() before plt.show() is correct and common practice.

3. Final Answer:

   The filename is not enclosed in quotes. -> Option D

4. Quick Check:

   Filename must be a string in quotes [OK]

Hint: Always put filename in quotes in savefig() [OK]

Common Mistakes:

• Omitting quotes around filename
• Thinking plt.show() must come first
• Assuming file extension is wrong

5. You want to save a plot tightly cropped around the content to avoid extra white space. Which plt.savefig() parameter helps achieve this?

hard

A. bbox_inches='tight'

B. format='png'

C. transparent=True

D. dpi=300

Solution

1. Step 1: Understand the role of bbox_inches

   The parameter bbox_inches='tight' trims the saved figure to fit the content closely, removing extra margins.

2. Step 2: Differentiate from other parameters

   dpi controls resolution, transparent controls background, and format sets file type, none affect cropping.

3. Final Answer:

   bbox_inches='tight' -> Option A

4. Quick Check:

   Use bbox_inches='tight' to crop figure [OK]

Hint: Use bbox_inches='tight' to crop saved figure [OK]

Common Mistakes:

• Confusing dpi with cropping
• Using transparent instead of bbox_inches
• Setting format does not crop figure