join和+的区别

连接字符串的时候可以用join也可以用+，但这两者有没有区别呢？

我们先来看一下用join和+连接字符串的例子

str1 = " ".join(["hello", "world"])
str2 = "hello " + "world"
print(str1)  # 输出 “hello world"
print(str2)  # 输出 “hello world"

两者的结果是一样，那么考虑这样一个问题，这两者在性能上有区别吗？

我们来做个实验，比较下join和+的性能

import timeit,time
def test1(strlist):
    return "".join(strlist)

def test2(strlist):
    result = ""
    for v in strlist:
        result = result+v
    return result

if __name__ == "__main__":
    strlist = ["a very very very very very very very long string" for n in range(1000)]
    timer1 = timeit.Timer("test1(strlist)", "from __main__ import strlist, test1")
    timer2 = timeit.Timer("test2(strlist)", "from __main__ import strlist, test2")
    time1 = timer1.timeit(number=100)
    time2 = timer2.timeit(number=100)
    print("join: %f, plus: %f" % (time1, time2))

    strlist1 = ["very very very long long" ,"very long long long","very long long long","very long long long","very long long long"]
    time1 = time.time()
    for i in range(100000):
        test1(strlist1)
    time2 = time.time()
    time3 = time.time()
    for i in range(100000):
        test2(strlist1)
    time4 = time.time()
    print ("join:%s" %(time2-time1))
    print("+ :%s" % (time4-time3))

输出：

join: 0.003507, plus: 0.083788
join:0.18189620971679688
+ :0.3727850914001465

　　

可以看到，join的性能明显好于+。这是为什么呢？

原因是这样的，字符串是不可变对象，当用操作符+连接字符串的时候，每执行一次+都会申请一块新的内存，因此用+连接字符串的时候会涉及好几次内存申请和复制。而join在连接字符串的时候，会先计算需要多大的内存存放结果，然后一次性申请所需内存并将字符串复制过去，这是为什么join的性能优于+的原因。所以在连接字符串数组的时候，我们应考虑优先使用join