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EX.Mutating the immutable

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暮晨 2018-11-17 18:16:21 +08:00 committed by 暮晨
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@ -43,7 +43,7 @@ So, here we go...
- [> What's wrong with booleans?/布尔你咋了?](#-whats-wrong-with-booleans布尔你咋了) - [> What's wrong with booleans?/布尔你咋了?](#-whats-wrong-with-booleans布尔你咋了)
- [> Class attributes and instance attributes/类属性和实例属性](#-class-attributes-and-instance-attributes类属性和实例属性) - [> Class attributes and instance attributes/类属性和实例属性](#-class-attributes-and-instance-attributes类属性和实例属性)
- [> yielding None/生成 None](#-yielding-none生成-none) - [> yielding None/生成 None](#-yielding-none生成-none)
- [> Mutating the immutable!](#-mutating-the-immutable) - [> Mutating the immutable!/强人所难](#-mutating-the-immutable强人所难)
- [> The disappearing variable from outer scope](#-the-disappearing-variable-from-outer-scope) - [> The disappearing variable from outer scope](#-the-disappearing-variable-from-outer-scope)
- [> When True is actually False](#-when-true-is-actually-false) - [> When True is actually False](#-when-true-is-actually-false)
- [> From filled to None in one instruction...](#-from-filled-to-none-in-one-instruction) - [> From filled to None in one instruction...](#-from-filled-to-none-in-one-instruction)
@ -931,7 +931,7 @@ def some_func(val):
--- ---
### > Mutating the immutable! ### > Mutating the immutable!/强人所难
```py ```py
some_tuple = ("A", "tuple", "with", "values") some_tuple = ("A", "tuple", "with", "values")
@ -942,7 +942,7 @@ another_tuple = ([1, 2], [3, 4], [5, 6])
```py ```py
>>> some_tuple[2] = "change this" >>> some_tuple[2] = "change this"
TypeError: 'tuple' object does not support item assignment TypeError: 'tuple' object does not support item assignment
>>> another_tuple[2].append(1000) #This throws no error >>> another_tuple[2].append(1000) # 这里不出现错误
>>> another_tuple >>> another_tuple
([1, 2], [3, 4], [5, 6, 1000]) ([1, 2], [3, 4], [5, 6, 1000])
>>> another_tuple[2] += [99, 999] >>> another_tuple[2] += [99, 999]
@ -951,16 +951,18 @@ TypeError: 'tuple' object does not support item assignment
([1, 2], [3, 4], [5, 6, 1000, 99, 999]) ([1, 2], [3, 4], [5, 6, 1000, 99, 999])
``` ```
But I thought tuples were immutable... 我还以为元组是不可变的呢...
#### 💡 Explanation: #### 💡 说明:
* Quoting from https://docs.python.org/2/reference/datamodel.html * 引用 https://docs.python.org/2/reference/datamodel.html
> Immutable sequences > 不可变序列
An object of an immutable sequence type cannot change once it is created. (If the object contains references to other objects, these other objects may be mutable and may be modified; however, the collection of objects directly referenced by an immutable object cannot change.) 不可变序列的对象一旦创建就不能再改变. (如果对象包含对其他对象的引用，则这些其他对象可能是可变的并且可能会被修改; 但是，由不可变对象直接引用的对象集合不能更改.)
* `+=` operator changes the list in-place. The item assignment doesn't work, but when the exception occurs, the item has already been changed in place. * `+=` 操作符在原地修改了列表. 元素赋值操作并不工作, 但是当异常抛出时, 元素已经在原地被修改了.
(译: 对于不可变对象, 这里指tuple, `+=` 并不是原子操作, 而是 `extend` 和 `=` 两个动作, 这里 `=` 操作虽然会抛出异常, 但 `extend` 操作已经修改成功了. 详细解释可以看[这里](https://segmentfault.com/a/1190000010767068))
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