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or maybe this !

var('x,y') f(x,y) = sqrt(x^2 + y^2) C = contour_plot(f ,(-0.2,0.2), (-0.2,0.2), axes = True, cmap = "Spectral", labels = True, label_fmt=lambda x: "$%s$"%round(x,2), label_colors="black", label_inline=True, label_fontsize=16, gridlines=True, axes_labels=["$x$","$y$"],

        #ticks = [[-2/10,-1/10,0,1/10,2/10],[-2/10,-1/10,0,1/10,2/10]],
        ticks = [[-0.2,-0.1,0,0.1,0.2],[-0.2,-0.1,0,0.1,0.2]],
        fontsize= 16)

or maybe this ! , just remove tickformatter!

var('x,y')
f(x,y) = sqrt(x^2 + y^2)
C = contour_plot(f ,(-0.2,0.2), (-0.2,0.2), axes = True, cmap = "Spectral", 
            labels = True,  label_fmt=lambda x: "$%s$"%round(x,2), label_colors="black", 
            label_inline=True,
            label_fontsize=16,
            gridlines=True, axes_labels=["$x$","$y$"],
axes_labels=["$x$","$y$"],
  
         #ticks = [[-2/10,-1/10,0,1/10,2/10],[-2/10,-1/10,0,1/10,2/10]],
         ticks = [[-0.2,-0.1,0,0.1,0.2],[-0.2,-0.1,0,0.1,0.2]],
         fontsize= 16)