>>> names=['zhangsan','lisi','wangwu']
>>> print('I have '+len(names)+' friends.')
Traceback (most recent call last):
File "<pyshell#6>", line 1, in <module>
print('I have '+len(names)+' friends.')
TypeError: can only concatenate str (not "int") to str

len(names)改为str(len(names))

用len求出的是int啊，
改print('I have ', len(names), ' friends.')
或print('I have '+str(len(names))+' friends.')