pyplot.contourf() returns error when specified levels argument

Question

EDIT: The issue is most likely about the version. The levels argument takes an integer argument in version 3.0.0, while this issue has occurred while using version 2.2.2.

UPDATE: The issue didn't occur after installing the version >=3.0.0.

I'm trying to make a contour plot in Python using the matplotlib.pyplot.contourf() function, and it works perfectly like this:

plt.contourf(x, y, z)

but when I try to specify an integer for the levels argument, like this:

plt.contourf(x, y, z, levels=100)

it always returns the error: TypeError: len() of unsized object

In the documentation, it says that the argument levels can be either int or array_like so I don't know why it would even call the len() function

Any ideas why this happens and any suggestion on how to fix it?

Answer 1

Sorry, this happens to you. The documentation changed in version 2.2.3 without this feature being fully implemented. So depending on the version of matplotlib the levels argument is interpreted differently.

matplotlib < 3.0.0

levels is interpreted as a list of levels where to draw contours. An integer is interpreted as a single level. For a contourf (a filled contour) plot you need at least two levels. Use the previously known way to specfify the number of levels as the second or fourth unnamed argument

plt.contourf(z, 100)
plt.contourf(x, y, z, 100)

matplotlib >= 3.0.0

levels can take either a list or an integer. When an integer, it signifies the (approximate [ * ]) number of levels. The relevant PR is this .

plt.contourf(z, levels=100)
plt.contourf(x, y, z, levels=100)