Saving figures in Data Analysis Python - Cheat Sheet & Quick Revision

Recall & Review

beginner

What function is used in Python's matplotlib to save a figure to a file?

The function savefig() is used to save the current figure to a file in matplotlib.

beginner

Which file formats can you save a matplotlib figure as?

You can save figures in formats like PNG, JPG, PDF, SVG, and EPS by specifying the file extension in savefig().

intermediate

How do you save a figure with a transparent background in matplotlib?

Use plt.savefig('filename.png', transparent=True) to save the figure with a transparent background.

intermediate

Why might you want to call plt.close() after saving a figure?

Calling plt.close() frees up memory by closing the figure, especially useful when saving many figures in a loop.

beginner

What does the dpi parameter control in savefig()?

The dpi parameter controls the resolution (dots per inch) of the saved image, affecting its quality and file size.

Which command saves the current matplotlib figure as a PNG file named 'plot.png'?

A plt.savefig('plot.png')

B plt.save('plot.png')

C plt.savefig('plot')

D plt.export('plot.png')

The correct function to save a figure is savefig(), and the filename must include the extension.

What happens if you save a figure without specifying the file extension?

A Matplotlib raises an error

B Matplotlib saves it but the format is unknown

C Matplotlib saves it as a PNG by default

D Matplotlib saves it as a PDF by default

Matplotlib saves it as a PNG by default if no file extension is provided.

How can you save a figure with higher resolution?

A Save as SVG format only

B Increase the figure size only

C Use <code>transparent=True</code>

D Use <code>dpi</code> parameter with a higher value in <code>savefig()</code>

Increasing the dpi value in savefig() increases the image resolution.

Which of these is NOT a valid file format for saving matplotlib figures?

A JPG

B DOCX

C PNG

D SVG

DOCX is a document format, not an image format supported by matplotlib.

Why is it good practice to call plt.close() after saving a figure in a loop?

A To clear the figure and free memory

B To save the figure again

C To display the figure on screen

D To rename the figure

Closing the figure frees memory and prevents too many open figures when saving multiple plots.

Explain how to save a matplotlib figure as a high-quality PNG file with a transparent background.

Think about the parameters you pass to savefig.

Describe why managing figures with plt.close() is important when saving many plots in a script.

Consider what happens if you don't close figures.

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