Mysterious in-place function call

import numpy

def main():
    xs0 = [1, 2, 3]
    xs1 = numpy.array([1, 2, 3, 4, 5])
    f = lambda x: x**2
    #print(f(xs0))  # invalid of course
    print(f(xs1))  # why does it work?

if __name__ == '__main__':
    main()