matplotlib contour plot: warning about `no contour levels were found`

Question

I was working through an example on a blog post about plotting animations with matplotlib. One example involved a contour plot--the code is below. However when I run this code, I get a UserWarning

 UserWarning: No contour levels were found within the data range.
 warnings.warn("No contour levels were found")

The code for the plot is below.

x = np.linspace(-3,3,91)
t = np.linspace(0,25,30)
y = np.linspace(-3,3,91)
X3, Y3, T3 = np.meshgrid(x,y, t)

sinT3 = np.sin(2*np.pi*T3/T3.max(axis=2)[...,np.newaxis])
G = (X3**2 + Y3**2)*sinT3
contour_opts = {'levels': np.linspace(-9, 9, 10),
                'cmap':'RdBu', 'linewidths': 2}
cax = ax.contour(x, y, G[..., 0], **contour_opts)

def animate(i):
    ax.collections = []
    ax.contour(x, y, G[..., i], **contour_opts)
anim = FuncAnimation(fig, animate, interval=100, frames = len(t)-1)

HTML(anim.to_html5_video())

The plot still works, but I keep getting that user warning.

I checked the matplotlib documentation and found that the levels parameter is still the correct name. So not sure what is causing this error.

Answer 1

The sinus of 0 is 0. Hence you get one complete slice of G, namely the first one G[:,:,0], which has all zeros in it. Zero is not one of the levels, but even if it was, drawing a contour of a constant array is not defined (should the complete surface be the line of contour?)

Therefore matplotlib correctly warns you that there is no contour to be drawn in this frame.