python传统艺能，看个更离谱的numpy.r_：

If slice notation is used, the syntax start:stop:step is equivalent to np.arange(start, stop, step) inside of the brackets. However, if step is an imaginary number (i.e. 100j) then its integer portion is interpreted as a number-of-points desired and the start and stop are inclusive. In other words start:stop:stepj is interpreted as np.linspace(start, stop, step, endpoint=1) inside of the brackets. After expansion of slice notation, all comma separated sequences are concatenated together.

第三个参数step可以是实数也可以是虚数：实数表示步长，虚数的时候去掉j表示总共要多少步。
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