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问题描述
我有一个错误,我减少到这个:
a = ['a','b','c']打印(之前",一个)" ".join(a)打印(之后",一个)输出这个:
runfile('C:/program.py', wdir=r'C:/')在 ['a', 'b', 'c'] 之前['a', 'b', 'c'] 之后这是怎么回事?
解决方案str.join 不能就地操作,因为字符串对象在 Python 中是不可变的.相反,它返回一个全新的字符串对象.
如果你想让 a 引用这个新对象,你需要显式地重新赋值:
a = " ".join(a)演示:
>>>a = ['a','b','c']>>>打印之前",一个在 ['a', 'b', 'c'] 之前>>>a = " ".join(a)>>>打印之后",一个在 a b c 之后>>>I had a bug that I reduced down to this:
a = ['a','b','c'] print( "Before", a ) " ".join(a) print( "After", a )Which outputs this:
runfile('C:/program.py', wdir=r'C:/') Before ['a', 'b', 'c'] After ['a', 'b', 'c']What's going on here?
解决方案str.join does not operate in-place because string objects are immutable in Python. Instead, it returns an entirely new string object.
If you want a to reference this new object, you need to explicitly reassign it:
a = " ".join(a)Demo:
>>> 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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为什么内置连接对我的代码没有影响?
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