为什么内置的联接对我的代码没有影响? [英] Why is the join built-in having no influence on my code?

问题描述

我有一个错误可以归结为:

I had a bug that I reduced down to this:

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']

这是怎么回事?

推荐答案

str.join 不能就地操作，因为字符串对象在Python中是不可变的.而是返回一个全新的字符串对象.

str.join does not operate in-place because string objects are immutable in Python. Instead, it returns an entirely new string object.

如果要a引用此新对象，则需要显式重新分配它:

If you want a to reference this new object, you need to explicitly reassign it:

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
>>>

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