“为什么"? python数据类型是不可变的 [英] "Why" python data types are immutable

问题描述

为什么(不是怎么做)像int和string这样的python基本数据类型是不可变的.那是因为脚本语言的实现限制.

Why (not how) python primitive data types like int and string are immutable. Is that because of a implementation limitation of scripting language.

作为示例

a = 5;
a = 6; 

在第二行(a = 6;)中

而不是创建新的存储位置，为什么不能将第一个存储位置更改为6

in second line(a = 6;) instead of creating a new memory location, why cant it change the first memory location to 6

推荐答案

某些Python数据类型是不可变的，因为Python使用引用/指针语义.

Some Python data types are immutable because Python uses reference/pointer semantics.

这意味着，每当将表达式分配给变量时，实际上并没有将值复制到该变量表示的存储位置，而只是给实际存在该值的存储位置命名.

This means that whenever you assign an expression to a variable, you're not actually copying the value into a memory location denoted by that variable, but you're merely giving a name to the memory location where the value actually exists.

现在，例如，如果字符串是可变的，则会发生这种情况:

Now, if for example strings were mutable, this would happen:

a = "test"
b = a
b[2] = "o"

# Now a would be "tost", oops.

这种行为被认为是不直观的，因此字符串变得不可变.

This behaviour was considered unintuitive, so strings were made immutable.

与整数类似，如果分配新值将更改原始位置，则会发生以下情况:

Similarly for integers, if assigning a new value would change the original location, the following would happen:

a = 5
b = a
b += 5

# a is now 10 :(

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