A python function handles both a single number and a numpy array

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As the title says, suppose I want to write a sign function (let's forget sign(0) for now), obviously we expect sign(2) = 1 and sign(array([-2,-2,2])) = array([-1,-1,1]). The following function won't work however, because it can't handle numpy arrays.

def sign(x):
    if x>0: return 1
    else: return -1

The next function won't work either since x doesn't have a shape member if it's just a single number. Even if some trick like y = x*0 + 1 is used, y won't have a [] method.

def sign(x):
    y = ones(x.shape)
    y[x<0] = -1
    return y

Even with the idea from another question(how can I make a numpy function that accepts a numpy array, an iterable, or a scalar?), the next function won't work when x is a single number because in this case x.shape and y.shape are just () and indexing y is illegal.

def sign(x):
    x = asarray(x)
    y = ones(x.shape)
    y[x<0] = -1
    return y

The only solution seems to be that first decide if x is an array or a number, but I want to know if there is something better. Writing branchy code would be cumbersome if you have lots of small functions like this.

asked 4 mins ago

Taozi

19

A python function handles both a single number and a numpy array