python 函数参数不指定如何进行分配

# _*_ coding: utf-8 _*_
__author__ = 'pythonwu'
__date__ = "2018/5/21 0:07"

def interval(start,stop=None,step=1):
    if stop is None:
        start,stop=0,start    #如果没有stop值，进行指定
    result = []
    i = start
    print(i)
    while i < stop:
        result.append(i)
        i += step
    return result


print(interval(10))
print(interval(1,10))

print(interval(1,10,2))