Saving figures in Data Analysis Python - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

What is the output file format?

Consider this Python code that saves a plot:

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

What is the format of the saved file?

Data Analysis Python

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

A PNG image file

B JPEG image file

C SVG vector graphic

D PDF document

💡 Hint

Look at the file extension in the filename.

The file extension '.png' tells matplotlib to save the figure as a PNG image file.

❓ data_output

intermediate

How many files are saved?

This code saves multiple figures:

import matplotlib.pyplot as plt
for i in range(3):
    plt.figure()
    plt.plot([1, 2], [i, i+1])
    plt.savefig(f'plot_{i}.png')

How many image files are created after running this code?

Data Analysis Python

import matplotlib.pyplot as plt
for i in range(3):
    plt.figure()
    plt.plot([1, 2], [i, i+1])
    plt.savefig(f'plot_{i}.png')

A 0 files

B 1 file

C 3 files

D 2 files

💡 Hint

Count how many times savefig is called.

The loop runs 3 times, saving one file each time, so 3 files are created.

🔧 Debug

advanced

Why is the saved figure blank?

Look at this code:

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

After running it, the saved file 'figure.png' is blank. Why?

Data Analysis Python

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

A The savefig filename is invalid

B The figure is saved after showing, so it is blank

C The plot command is missing

D The figure is saved before showing, so it is blank

💡 Hint

Think about when the figure content is finalized.

Calling plt.show() clears the figure. Saving before showing saves the correct image. But if the figure is blank, it means the figure was saved after it was cleared by show.

❓ visualization

advanced

Which option saves a figure with transparent background?

You want to save a plot with a transparent background. Which code does this?

Data Analysis Python

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

A plt.savefig('plot.png', transparent=True)

B plt.savefig('plot.png', bg='transparent')

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

D plt.savefig('plot.png', transparent=False)

💡 Hint

Check the savefig parameters for transparency.

The transparent=True argument saves the figure background as transparent.

🚀 Application

expert

How to save a high-resolution figure?

You want to save a figure with higher quality for printing. Which option correctly sets the resolution to 300 dots per inch (DPI)?

Data Analysis Python

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

A plt.savefig('highres.png', dpi='300dpi')

B plt.savefig('highres.png', resolution=300)

C plt.savefig('highres.png', quality=300)

D plt.savefig('highres.png', dpi=300)

💡 Hint

Look for the correct parameter name and type for DPI.

The dpi parameter sets the resolution as an integer. Other parameters like resolution or quality are invalid.

Practice

1. What is the main function used in Python's matplotlib to save a plot as an image file?

easy

A. plt.show()

B. plt.savefig()

C. plt.plot()

D. plt.save()

Solution

1. Step 1: Identify the function for saving plots

   The function plt.savefig() is designed to save the current figure to a file.

2. Step 2: Differentiate from other functions

   plt.show() displays the plot, plt.plot() creates the plot, and plt.save() does not exist.

3. Final Answer:

   plt.savefig() -> Option B

4. Quick Check:

   Save plot = plt.savefig() [OK]

Hint: Remember: save figure uses 'savefig' word [OK]

Common Mistakes:

• Confusing plt.show() with saving
• Using plt.save() which is not a valid function
• Thinking plt.plot() saves the figure

2. Which of the following is the correct syntax to save a plot as a PNG file with matplotlib?

easy

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

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

C. plt.savefig(plot.png)

D. plt.savefig('plot')

Solution

1. Step 1: Check the function name and argument type

   The correct function is plt.savefig() and it requires the filename as a string.

2. Step 2: Verify the filename format

   The filename must be a string with the extension, e.g., 'plot.png'. plt.savefig('plot.png') matches this exactly.

3. Final Answer:

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

4. Quick Check:

   Save with filename string = plt.savefig('plot.png') [OK]

Hint: Use quotes around filename with extension [OK]

Common Mistakes:

• Omitting quotes around filename
• Using plt.save() instead of plt.savefig()
• Missing file extension in filename

3. What will be the effect of this code snippet?

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

medium

A. Saves a PNG file named 'myplot.pdf' ignoring dpi and bbox_inches

B. Displays the plot on screen with 300 dpi resolution

C. Raises an error because dpi cannot be set for PDF files

D. Saves a PDF file named 'myplot.pdf' with high resolution and tight layout

Solution

1. Step 1: Understand plt.savefig parameters

   The code saves the plot as 'myplot.pdf' with dpi=300 for high resolution and bbox_inches='tight' to reduce whitespace.

2. Step 2: Confirm file format and parameters compatibility

   PDF supports dpi and bbox_inches parameters, so no error occurs and the file is saved correctly.

3. Final Answer:

   Saves a PDF file named 'myplot.pdf' with high resolution and tight layout -> Option D

4. Quick Check:

   Save PDF with dpi and tight bbox = correct [OK]

Hint: File extension sets format; dpi and bbox_inches improve quality [OK]

Common Mistakes:

• Thinking dpi is ignored for PDF
• Confusing save with display (plt.show())
• Assuming dpi causes error for PDF

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

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

medium

A. File extension '.png' is invalid

B. plt.plot() must be called after plt.savefig()

C. Missing quotes around filename 'myplot.png'

D. dpi parameter is required in plt.savefig()

Solution

1. Step 1: Check the argument to plt.savefig()

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

2. Step 2: Verify other parts of the code

   Calling plt.plot() before plt.savefig() is correct, dpi is optional, and '.png' is a valid extension.

3. Final Answer:

   Missing quotes around filename 'myplot.png' -> Option C

4. Quick Check:

   Filename must be string with quotes [OK]

Hint: Always put filename in quotes when saving [OK]

Common Mistakes:

• Forgetting quotes around filename
• Thinking dpi is mandatory
• Misordering plot and savefig calls

5. You want to save a plot as a high-quality PNG file with minimal white space around it. Which of the following code snippets achieves this correctly?

hard

A. plt.savefig('figure.png', dpi=600, bbox_inches='tight')

B. plt.savefig('figure.png', dpi=72, bbox_inches='standard')

C. plt.savefig('figure.png', dpi=600, bbox_inches='loose')

D. plt.savefig('figure.png', dpi='high', bbox_inches='tight')

Solution

1. Step 1: Understand dpi and bbox_inches parameters

   High dpi (e.g., 600) means better image quality. bbox_inches='tight' reduces extra white space.

2. Step 2: Evaluate each option

   plt.savefig('figure.png', dpi=600, bbox_inches='tight')

   uses dpi=600 and bbox_inches='tight' correctly.

   plt.savefig('figure.png', dpi=72, bbox_inches='standard')

   uses low dpi and invalid bbox_inches.

   plt.savefig('figure.png', dpi=600, bbox_inches='loose')

   uses bbox_inches='loose' which adds space.

   plt.savefig('figure.png', dpi='high', bbox_inches='tight')

   uses invalid dpi value 'high'.

3. Final Answer:

   plt.savefig('figure.png', dpi=600, bbox_inches='tight') -> Option A

4. Quick Check:

   High dpi + tight bbox = best quality [OK]

Hint: Use dpi=600 and bbox_inches='tight' for quality and layout [OK]

Common Mistakes:

• Using invalid dpi values like strings
• Choosing bbox_inches='loose' which adds whitespace
• Ignoring dpi leading to low-quality images