How do I change matplotlib's subplot projection of an existing axis?

You can't change the projection of an existing axes, the reason is given below. However the solution to your underlying problem is simply to use the subplot_kw argument to plt.subplots() described in the matplotlib documentation here. For example, if you wanted all your subplots to have the cartopy.crs.PlateCarree projection you could do

import matplotlib.pyplot as pltimport cartopy.crs as ccrs# Create a grid of plotsfig, (ax1, ax2) = plt.subplots(ncols=2, subplot_kw={'projection': ccrs.PlateCarree()})

Regarding the actual question, specifying a projection when you create an axes set determines the axes class you get, which is different for each projection type. For example

import matplotlib.pyplot as pltimport cartopy.crs as ccrsax1 = plt.subplot(311)ax2 = plt.subplot(312, projection='polar')ax3 = plt.subplot(313, projection=ccrs.PlateCarree())print(type(ax1))print(type(ax2))print(type(ax3))

This code will print the following

<class 'matplotlib.axes._subplots.AxesSubplot'><class 'matplotlib.axes._subplots.PolarAxesSubplot'><class 'cartopy.mpl.geoaxes.GeoAxesSubplot'>

Notice how each axes is actually an instance of a different class.

Assuming there are multiple axes being used for 2D plotting, like...

fig = matplotlib.pyplot.Figure()axs = fig.subplots(3, 4) # prepare for multiple subplots# (some plotting here)axs[0,0].plot([1,2,3])

... one can simply destroy one of them and replace it with a new one having the 3D projection:

axs[2,3].remove()ax = fig.add_subplot(3, 4, 12, projection='3d')ax.plot_surface(...)

Just note that unlike rest of Python, the add_subplot uses row-column indexing starting from 1 (not from 0).

EDIT: Changed my typo about indexing.

following the answer to this question :

In python, how can I inherit and override a method on a class instance, assigning this new version to the same name as the old one?

I found a hack to change the projection of an axe after creating it which seems to work at least in the simple example below, but I have no idea if this solution is the best way

from matplotlib.axes import Axesfrom matplotlib.projections import register_projectionclass CustomAxe(Axes):    name = 'customaxe'    def plotko(self, x):        self.plot(x, 'ko')        self.set_title('CustomAxe')register_projection(CustomAxe)if __name__ == '__main__':    import matplotlib.pyplot as plt    fig = plt.figure()    ## use this syntax to create a customaxe directly    # ax = fig.add_subplot(111, projection="customaxe")    ## change the projection after creation    ax = plt.gca()    ax.__class__ = CustomAxe    ax.plotko(range(10))        plt.show()