为什么除非您分配其输出，否则调用 Python 字符串方法不会执行任何操作? [英] Why doesn't calling a Python string method do anything unless you assign its output?

问题描述

我尝试做一个简单的字符串替换，但我不知道为什么它似乎不起作用:

I try to do a simple string replacement, but I don't know why it doesn't seem to work:

X = "hello world"
X.replace("hello", "goodbye")

我想把hello这个词改成goodbye，所以要把字符串"hello world"改成"goodbye>世界".但 X 仍然是 "hello world".为什么我的代码不起作用?

I want to change the word hello to goodbye, thus it should change the string "hello world" to "goodbye world". But X just remains "hello world". Why is my code not working?

推荐答案

这是因为字符串在 Python 中是不可变的.

这意味着 X.replace("hello","goodbye") 返回 X 的副本，并进行了替换.因此，您需要替换此行:

Which means that X.replace("hello","goodbye") returns a copy of X with replacements made. Because of that you need replace this line:

X.replace("hello", "goodbye")

用这一行:

X = X.replace("hello", "goodbye")

更广泛地说，这适用于所有就地"更改字符串内容的 Python 字符串方法，例如replace,strip,translate,lower/upper,join,...

More broadly, this is true for all Python string methods that change a string's content "in-place", e.g. replace,strip,translate,lower/upper,join,...

如果你想使用它，你必须将它们的输出分配给某些东西而不是扔掉它，例如

X  = X.strip(' \t')
X2 = X.translate(...)
Y  = X.lower()
Z  = X.upper()
A  = X.join(':')
B  = X.capitalize()
C  = X.casefold()

等等.

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