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153 lines
1.6 KiB
Markdown
153 lines
1.6 KiB
Markdown
# 元组
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## 基本操作
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与列表相似，元组`Tuple`也是个有序序列，但是元组是不可变的，用`()`生成。
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In [1]:
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```py
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t = (10, 11, 12, 13, 14)
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t
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```
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Out[1]:
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```py
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(10, 11, 12, 13, 14)
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```
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可以索引，切片：
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In [2]:
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```py
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t[0]
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```
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Out[2]:
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```py
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10
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```
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In [3]:
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```py
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t[1:3]
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```
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Out[3]:
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```py
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(11, 12)
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```
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但是元组是不可变的：
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In [4]:
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```py
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# 会报错
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t[0] = 1
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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-4-da6c1cabf0b0> in <module>()
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1 # 会报错
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----> 2 t[0] = 1
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TypeError: 'tuple' object does not support item assignment
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```
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## 单个元素的元组生成
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由于`()`在表达式中被应用，只含有单个元素的元组容易和表达式混淆，所以采用下列方式定义只有一个元素的元组：
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In [5]:
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```py
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a = (10,)
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print a
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print type(a)
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```
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```py
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(10,)
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<type 'tuple'>
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```
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In [6]:
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```py
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a = (10)
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print type(a)
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```
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```py
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<type 'int'>
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```
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将列表转换为元组：
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In [7]:
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```py
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a = [10, 11, 12, 13, 14]
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tuple(a)
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```
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Out[7]:
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```py
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(10, 11, 12, 13, 14)
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```
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## 元组方法
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由于元组是不可变的，所以只能有一些不可变的方法，例如计算元素个数 `count` 和元素位置 `index` ，用法与列表一样。
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In [8]:
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```py
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a.count(10)
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```
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Out[8]:
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```py
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1
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```
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In [9]:
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```py
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a.index(12)
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```
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Out[9]:
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```py
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2
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```
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## 为什么需要元组
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旧式字符串格式化中参数要用元组；
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在字典中当作键值；
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数据库的返回值…… |