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182 lines
2.0 KiB
Markdown
182 lines
2.0 KiB
Markdown
# 可变和不可变类型
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## 列表是可变的（Mutable）
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In [1]:
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```py
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a = [1,2,3,4]
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a
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```
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Out[1]:
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```py
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[1, 2, 3, 4]
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```
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通过索引改变：
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In [2]:
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```py
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a[0] = 100
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a
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```
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Out[2]:
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```py
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[100, 2, 3, 4]
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```
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通过方法改变：
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In [3]:
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```py
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a.insert(3, 200)
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a
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```
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Out[3]:
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```py
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[100, 2, 3, 200, 4]
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```
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In [4]:
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```py
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a.sort()
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a
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```
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Out[4]:
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```py
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[2, 3, 4, 100, 200]
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```
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## 字符串是不可变的（Immutable）
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In [5]:
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```py
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s = "hello world"
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s
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```
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Out[5]:
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```py
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'hello world'
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```
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通过索引改变会报错：
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In [6]:
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```py
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s[0] = 'z'
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```
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```py
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---------------------------------------------------------------------------
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TypeError Traceback (most recent call last)
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<ipython-input-6-83b06971f05e> in <module>()
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----> 1 s[0] = 'z'
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TypeError: 'str' object does not support item assignment
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```
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字符串方法只是返回一个新字符串，并不改变原来的值：
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In [7]:
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```py
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print s.replace('world', 'Mars')
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print s
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```
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```py
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hello Mars
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hello world
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```
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如果想改变字符串的值，可以用重新赋值的方法：
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In [8]:
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```py
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s = "hello world"
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s = s.replace('world', 'Mars')
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print s
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```
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```py
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hello Mars
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```
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或者用 `bytearray` 代替字符串：
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In [9]:
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```py
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s = bytearray('abcde')
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s[1:3] = '12'
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s
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```
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Out[9]:
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```py
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bytearray(b'a12de')
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```
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数据类型分类：
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| 可变数据类型 | 不可变数据类型 |
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| --- | --- |
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| `list`, `dictionary`, `set`, `numpy array`, `user defined objects` | `integer`, `float`, `long`, `complex`, `string`, `tuple`, `frozenset` |
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## 字符串不可变的原因
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其一，列表可以通过以下的方法改变，而字符串不支持这样的变化。
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In [10]:
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```py
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a = [1, 2, 3, 4]
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b = a
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```
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此时， `a` 和 `b` 指向同一块区域，改变 `b` 的值， `a` 也会同时改变：
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In [11]:
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```py
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b[0] = 100
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a
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```
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Out[11]:
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```py
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[100, 2, 3, 4]
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```
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其二，是字符串与整数浮点数一样被认为是基本类型，而基本类型在Python中是不可变的。 |