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# https://matplotlib.org/3.1.1/gallery/color/colormap_reference.html
import numpy as np
import matplotlib.pyplot as plt

cm = plt.get_cmap('Reds')
print(cm.N)  # how many color levels in this cm

# cm takes list or single value in [0,N-1] or [0.0, 1.0], depending on the value type
# value lower or bigger than boundary are default to boundry color

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# 自动处理除零错误
# numpy >= 1.7
a = np.array([-1, 0, 1, 2, 3], dtype=float)
b = np.array([ 0, 0, 0, 2, 2], dtype=float)

# If you don't pass `out` the indices where (b == 0) will be uninitialized!
c = np.divide(a, b, out=np.zeros_like(a), where=b!=0)
print(c)
# [ 0.   0.   0.   1.   1.5]

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