Saving figures to files in Matplotlib - Cheat Sheet & Quick Revision

Recall & Review

beginner

What function in matplotlib do you use to save a figure to a file?

You use plt.savefig() to save the current figure to a file.

beginner

How do you save a figure as a PNG file named 'plot.png'?

Use plt.savefig('plot.png') to save the figure as a PNG file named 'plot.png'.

intermediate

What parameter do you use in savefig() to set the resolution of the saved image?

Use the dpi parameter to set the resolution, for example plt.savefig('plot.png', dpi=300).

beginner

True or False: You must call plt.show() before plt.savefig() to save the figure.

False. You should call plt.savefig() before plt.show() because plt.show() can clear the figure.

intermediate

How can you save a figure in vector format using matplotlib?

Save the figure as a PDF or SVG by using the file extension in savefig(), for example plt.savefig('plot.pdf') or plt.savefig('plot.svg').

Which command saves the current matplotlib figure as a JPEG file named 'image.jpg'?

A plt.save('image.jpg')

B plt.savefig('image.jpg')

C plt.savefig('image.png')

D plt.export('image.jpg')

plt.savefig() is the correct function to save figures. The filename extension determines the format.

What does the dpi parameter control in plt.savefig()?

A The resolution (dots per inch) of the saved image

B The size of the figure in inches

C The color of the figure background

D The file format of the saved image

dpi sets the resolution, higher dpi means better image quality.

If you want to save a figure as a vector graphic, which file extension should you use?

A .png

B .jpg

C .bmp

D .svg

SVG is a vector graphic format. PDF also works for vector graphics.

When should you call plt.savefig() in your code?

A Before <code>plt.show()</code>

B Only if you want to display the figure

C It does not matter

D After <code>plt.show()</code>

Call plt.savefig() before plt.show() because plt.show() can clear the figure.

Which of these is NOT a valid file format for plt.savefig()?

A .png

B .pdf

C .exe

D .svg

.exe is an executable file, not an image format.

Explain how to save a matplotlib figure to a file with high resolution.

Think about the function name and the dpi setting.

Describe the difference between saving a figure as PNG and SVG in matplotlib.

Consider image quality and scaling.

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