为什么内置的连接对我的代码没有影响？

Question

我有一个错误,我减少到这个:

a = ['a','b','c']
print( "Before", a )
" ".join(a)
print( "After", a )

哪个输出:

runfile('C:/program.py', wdir=r'C:/')

Before ['a', 'b', 'c']
After ['a', 'b', 'c']

这里发生了什么？

Answer 1

str.join因为字符串对象在Python中是不可变的,所以不会就地操作.相反,它返回一个全新的字符串对象.

如果要a引用此新对象,则需要显式重新分配:

a = " ".join(a)

演示:

>>> a = ['a','b','c']
>>> print "Before", a
Before ['a', 'b', 'c']
>>> a = " ".join(a)
>>> print "After", a
After a b c
>>>