Can I see all attributes of a pyplot without showing the graph?

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I am working on developing homework as a TA for a course at my university.

We are using Otter Grader (an extension of OKPy) to grade student submissions of guided homework we provide through Jupyter Notebooks.

Students are being asked to plot horizontal lines on their plots using matplotlib.pyplot.axhline(), and I am hoping to use an assert call to determine whether they added the horizontal line to their plots.

Is there a way to see all attributes that have been added to a pyplot in matplotlib?

jupyter jupyter-lab matplotlib python-3.x
1 Answers

I don't believe there is a way to see if the axhline attribute has been used or not, but there is a way to see if the lines are horizontal by accessing all the line2D objects using the lines attribute.

import matplotlib.pyplot as plt
import numpy as np


def is_horizontal(line2d):
    x, y = line2d.get_data()
    y = np.array(y)  # The axhline method does not return data as a numpy array
    y_bool = y == y[0]  # Returns a boolean array of True or False if the first number equals all the other numbers
    return np.all(y_bool)


t = np.linspace(-10, 10, 1000)

plt.plot(t, t**2)
plt.plot(t, t)
plt.axhline(y=5, xmin=-10, xmax=10)

ax = plt.gca()

assert any(map(is_horizontal, ax.lines)), 'There are no horizontal lines on the plot.'

plt.show()

This code will raise the error if there is not at least one line2D object that contains data in which all the y values are the same.

Note that in order for the above to work, the axhline attribute has to be used instead of the hlines method. The hlines method does not add the line2D object to the axes object.

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