AttributeError while adding colorbar in matplotlib

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(This is a very old question I know) The reason you are seeing this issue is because you have mixed the use of the state machine (matplotlib.pyplot) with the OO approach of adding images to an axes.

The plt.imshow function differs from the ax.imshow method in just one subtly different way.
The method ax.imshow:

  • creates and returns an Image which has been added to the axes

The function plt.imshow:

  • creates and returns an Image which has been added to the current axes, and sets the image to be the “current” image/mappable (which can then be automatically picked up by the plt.colorbar function).

If you want to be able to use the plt.colorbar (which in all but the most extreme cases, you do) with the ax.imshow method, you will need to pass the returned image (which is an instance of a ScalarMappable) to plt.colorbar as the first argument:

plt.imshow(image_file)
plt.colorbar()

is equivalent (without using the state machine) to:

img = ax.imshow(image_file)
plt.colorbar(img, ax=ax)

If ax is the current axes in pyplot, then the kwarg ax=ax is not needed.

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