How do I do conditional array arithmetic on a numpy array?

In [45]: a = np.array([1,2,3,-1,-2,-3])

In [46]: a[a<0]+=1

In [47]: a
Out[47]: array([ 1,  2,  3,  0, -1, -2])

To mutate it:

a[a<0] += 1

To leave the original array alone:

a+[a<0]

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