Fig, ax = plt.subplots pattern in Matplotlib - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

What does this code output?

Consider the following code using plt.subplots(). What will be the type of ax?

Matplotlib

import matplotlib.pyplot as plt
fig, ax = plt.subplots()
print(type(ax))

A <class 'tuple'>

B <class 'matplotlib.axes._subplots.AxesSubplot'>

C <class 'list'>

D <class 'matplotlib.figure.Figure'>

💡 Hint

Remember that plt.subplots() returns a figure and axes object.

The plt.subplots() function returns a tuple: the first element is a Figure object, and the second is an AxesSubplot object representing the plot area.

❓ data_output

intermediate

How many axes are created?

What is the shape and type of ax after running this code?

Matplotlib

import matplotlib.pyplot as plt
fig, ax = plt.subplots(2, 3)
print(type(ax))
print(ax.shape)

A numpy.ndarray with shape (2, 3)

B list with length 6

C tuple with 2 elements

D matplotlib.axes._subplots.AxesSubplot with no shape

💡 Hint

When multiple subplots are created, ax is an array of axes.

With plt.subplots(2, 3), ax is a 2x3 numpy array of AxesSubplot objects.

❓ visualization

advanced

Identify the correct subplot layout

Which option shows the correct subplot layout for fig, ax = plt.subplots(1, 4)?

Matplotlib

import matplotlib.pyplot as plt
fig, ax = plt.subplots(1, 4)
for i, axis in enumerate(ax):
    axis.plot([0, 1], [i, i+1])
plt.show()

A Four plots arranged vertically in one column

B Two rows and two columns of plots

C Four plots arranged horizontally in one row

D Single plot only

💡 Hint

The arguments (1, 4) mean 1 row and 4 columns.

The plt.subplots(1, 4) creates one row with four columns of subplots arranged horizontally.

🔧 Debug

advanced

Why does this code raise an error?

What error does this code raise and why?

import matplotlib.pyplot as plt
fig, ax = plt.subplots()
ax[0].plot([1, 2, 3])

Matplotlib

import matplotlib.pyplot as plt
fig, ax = plt.subplots()
ax[0].plot([1, 2, 3])

A TypeError: 'AxesSubplot' object is not subscriptable

B IndexError: list index out of range

C AttributeError: 'Figure' object has no attribute 'plot'

D No error, plots successfully

💡 Hint

Check the type of ax when only one subplot is created.

When only one subplot is created, ax is a single AxesSubplot object, not a list or array, so indexing it causes a TypeError.

🚀 Application

expert

How to flatten axes array for iteration?

Given fig, ax = plt.subplots(3, 2), which option correctly iterates over all axes to plot a line on each?

Matplotlib

import matplotlib.pyplot as plt
fig, ax = plt.subplots(3, 2)
# Fill in the blank to plot on each axis
for axis in _____:
    axis.plot([0, 1], [1, 2])
plt.show()

A list(ax)

B ax.tolist()

C ax

D ax.flatten()

💡 Hint

The axes array is 2D; flatten it to 1D for simple iteration.

The ax is a 2D numpy array of axes. Using ax.flatten() converts it to a 1D array for easy iteration.

Practice

1. What does the variable fig represent when you write fig, ax = plt.subplots() in matplotlib?

easy

A. The specific plot area where data is drawn

B. The whole figure container that holds all plot elements

C. A list of all plots in the figure

D. The title of the plot

Solution

1. Step 1: Understand the role of fig in plt.subplots()

   The fig variable holds the entire figure container, which can include multiple plots, titles, legends, and other elements.

2. Step 2: Differentiate fig from ax

   The ax variable is the specific plot area inside the figure where data is drawn, not the whole container.

3. Final Answer:

   The whole figure container that holds all plot elements -> Option B

4. Quick Check:

   fig = whole figure container [OK]

Hint: Remember: fig = full figure, ax = plot area [OK]

Common Mistakes:

• Confusing fig with ax as the plot area
• Thinking fig is just the plot title
• Assuming fig is a list of plots

2. Which of the following is the correct syntax to create a figure and axes using matplotlib's plt.subplots()?

easy

A. plt.subplots = fig, ax

B. fig = plt.subplots(ax)

C. ax, fig = plt.subplots()

D. fig, ax = plt.subplots()

Solution

1. Step 1: Recall the correct unpacking syntax

   The function plt.subplots() returns two objects: the figure and the axes. The correct way to capture them is fig, ax = plt.subplots().

2. Step 2: Check each option for syntax correctness

   fig = plt.subplots(ax) tries to pass ax as an argument incorrectly. ax, fig = plt.subplots() reverses the order, which is wrong. plt.subplots = fig, ax tries to assign incorrectly.

3. Final Answer:

   fig, ax = plt.subplots() -> Option D

4. Quick Check:

   Correct unpacking = fig, ax = plt.subplots() [OK]

Hint: Remember: fig first, then ax from plt.subplots() [OK]

Common Mistakes:

• Swapping fig and ax order
• Passing ax as argument to plt.subplots()
• Incorrect assignment syntax

3. What will be the output type of ax after running this code?

import matplotlib.pyplot as plt
fig, ax = plt.subplots()

medium

A.

B.

C.

D.

Solution

1. Step 1: Understand what plt.subplots() returns

   The function returns a figure object and an axes object. The axes object is usually an instance of AxesSubplot class.

2. Step 2: Identify the type of ax

   After unpacking, ax is the axes object, which is of type matplotlib.axes._subplots.AxesSubplot.

3. Final Answer:

   <class 'matplotlib.axes._subplots.AxesSubplot'> -> Option A

4. Quick Check:

   ax type = AxesSubplot [OK]

Hint: ax is the plot area object, not the figure [OK]

Common Mistakes:

• Confusing ax type with fig type
• Thinking ax is a list or tuple
• Assuming ax is a string or number

4. Identify the error in this code snippet:

import matplotlib.pyplot as plt
fig, ax = plt.subplots
ax.plot([1, 2, 3], [4, 5, 6])
plt.show()

medium

A. fig and ax are reversed in assignment

B. ax.plot() arguments are incorrect

C. Missing parentheses after plt.subplots

D. plt.show() is called before plotting

Solution

1. Step 1: Check the call to plt.subplots

   The code uses plt.subplots without parentheses, so it assigns the function itself, not its result.

2. Step 2: Understand the impact on ax.plot

   Since fig, ax are assigned incorrectly, ax is a function, not an axes object, so ax.plot() will cause an error.

3. Final Answer:

   Missing parentheses after plt.subplots -> Option C

4. Quick Check:

   Call plt.subplots() with () [OK]

Hint: Always add () to call plt.subplots function [OK]

Common Mistakes:

• Forgetting parentheses on plt.subplots
• Mixing up fig and ax variables
• Calling plt.show() too early

5. You want to create a figure with 2 rows and 1 column of plots using plt.subplots(). Which code correctly creates the figure and axes, and plots a line on the second subplot?

hard

A. fig, ax = plt.subplots(2, 1)
ax[1].plot([1, 2, 3], [3, 2, 1])

B. fig, ax = plt.subplots(1, 2)
ax[2].plot([1, 2, 3], [3, 2, 1])

C. fig, ax = plt.subplots(2, 1)
ax.plot([1, 2, 3], [3, 2, 1])

D. fig, ax = plt.subplots(2)
ax[0].plot([1, 2, 3], [3, 2, 1])

Solution

1. Step 1: Create figure with 2 rows and 1 column

   Use plt.subplots(2, 1) to create 2 vertical subplots. This returns ax as an array of axes.

2. Step 2: Plot on the second subplot

   Access the second subplot with ax[1] (indexing starts at 0) and call plot() on it.

3. Final Answer:

   fig, ax = plt.subplots(2, 1) ax[1].plot([1, 2, 3], [3, 2, 1]) -> Option A

4. Quick Check:

   2 rows, 1 col = plt.subplots(2,1), plot on ax[1] [OK]

Hint: Use ax[index] for multiple subplots, index starts at 0 [OK]

Common Mistakes:

• Using wrong subplot shape (1,2) instead of (2,1)
• Trying to plot on ax without indexing when multiple axes exist
• Using invalid index like ax[2] when only 2 subplots exist