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537 lines
9.1 KiB
Markdown
# Python 字符串
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> 原文： [https://thepythonguru.com/python-strings/](https://thepythonguru.com/python-strings/)
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* * *
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于 2020 年 1 月 10 日更新
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* * *
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python 中的字符串是由单引号或双引号分隔的连续字符系列。 Python 没有任何单独的字符数据类型，因此它们表示为单个字符串。
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## 创建字符串
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* * *
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```py
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>>> name = "tom" # a string
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>>> mychar = 'a' # a character
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```
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您还可以使用以下语法创建字符串。
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```py
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>>> name1 = str() # this will create empty string object
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>>> name2 = str("newstring") # string object containing 'newstring'
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```
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```py
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name = "tom" # a string
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mychar = 'a' # a character
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print(name)
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print(mychar)
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name1 = str() # this will create empty string object
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name2 = str("newstring") # string object containing 'newstring'
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print(name1)
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print(name2)
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```
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## Python 中的字符串是不可变的
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* * *
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这对您而言意味着，一旦创建了字符串，便无法对其进行修改。 让我们以一个例子来说明这一点。
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```py
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>>> str1 = "welcome"
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>>> str2 = "welcome"
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```
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这里`str1`和`str2`指的是存储在内存中某个位置的相同字符串对象“`welcome`”。 您可以使用[`id()`](/python-builtin-functions/id/)函数测试`str1`是否与`str2`引用相同的对象。
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什么是身份证？
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python 中的每个对象都存储在内存中的某个位置。 我们可以使用`id()`获得该内存地址。
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```py
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>>> id(str1)
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78965411
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>>> id(str2)
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78965411
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```
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由于`str1`和`str2`都指向相同的存储位置，因此它们都指向同一对象。
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让我们尝试通过向其添加新字符串来修改`str1`对象。
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```py
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>>> str1 += " mike"
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>>> str1
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welcome mike
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>>> id(str1)
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>>> 78965579
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```
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如您现在所见，`str1`指向完全不同的内存位置，这证明了并置不会修改原始字符串对象而是创建一个新的字符串对象这一点。 同样，数字（即`int`类型）也是不可变的。
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试试看：
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```py
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str1 = "welcome"
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str2 = "welcome"
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print(id(str1), id(str2))
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str1 += " mike"
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print(str1)
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print(id(str1))
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```
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## 字符串操作
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* * *
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字符串索引从`0`开始，因此要访问字符串类型中的第一个字符：
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```py
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>>> name[0] #
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t
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```
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试一试：
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```py
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name = "tom"
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print(name[0])
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print(name[1])
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```
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`+`运算符用于连接字符串，而`*`运算符是字符串的重复运算符。
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```py
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>>> s = "tom and " + "jerry"
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>>> print(s)
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tom and jerry
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```
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```py
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>>> s = "spamming is bad " * 3
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>>> print(s)
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'spamming is bad spamming is bad spamming is bad '
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```
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试一试：
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```py
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s = "tom and " + "jerry"
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print(s)
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s = "spamming is bad " * 3
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print(s)
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```
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## 字符串切片
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* * *
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您可以使用`[]`运算符（也称为切片运算符）从原始字符串中提取字符串的子集。
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**语法**：`s[start:end]`
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这将从索引`start`到索引`end - 1`返回字符串的一部分。
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让我们举一些例子。
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```py
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>>> s = "Welcome"
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>>> s[1:3]
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el
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```
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一些更多的例子。
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```py
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>>> s = "Welcome"
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>>>
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>>> s[:6]
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'Welcom'
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>>>
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>>> s[4:]
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'ome'
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>>>
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>>> s[1:-1]
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'elcom'
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```
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试一试：
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```py
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s = "Welcome"
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print(s[1:3])
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print(s[:6])
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print(s[4:])
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print(s[1:-1])
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```
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**注意**：
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`start`索引和`end`索引是可选的。 如果省略，则`start`索引的默认值为`0`，而`end`的默认值为字符串的最后一个索引。
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## `ord()`和`chr()`函数
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* * *
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`ord()`-函数返回字符的 ASCII 码。
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`chr()`-函数返回由 ASCII 数字表示的字符。
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```py
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>>> ch = 'b'
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>>> ord(ch)
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98
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>>> chr(97)
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'a'
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>>> ord('A')
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65
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```
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试一试：
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```py
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ch = 'b'
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print(ord(ch))
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print(chr(97))
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print(ord('A'))
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```
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## Python 中的字符串函数
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* * *
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| 函数名称 | 函数说明 |
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| --- | --- |
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| `len ()` | 返回字符串的长度 |
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| `max()` | 返回具有最高 ASCII 值的字符 |
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| `min()` | 返回具有最低 ASCII 值的字符 |
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```py
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>>> len("hello")
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5
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>>> max("abc")
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'c'
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>>> min("abc")
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'a'
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```
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试一试：
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```py
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print(len("hello"))
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print(max("abc"))
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print(min("abc"))
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```
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## `in`和`not in`运算符
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* * *
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您可以使用`in`和`not in`运算符检查另一个字符串中是否存在一个字符串。 他们也被称为会员运营商。
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```py
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>>> s1 = "Welcome"
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>>> "come" in s1
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True
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>>> "come" not in s1
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False
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>>>
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```
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试一试：
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```py
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s1 = "Welcome"
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print("come" in s1)
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print("come" not in s1)
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```
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## 字符串比较
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* * *
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您可以使用[`>`，`<`，`<=`，`<=`，`==`，`!=`）比较两个字符串。 Python 按字典顺序比较字符串，即使用字符的 ASCII 值。
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假设您将`str1`设置为`"Mary"`并将`str2`设置为`"Mac"`。 比较`str1`和`str2`的前两个字符（`M`和`M`）。 由于它们相等，因此比较后两个字符。 因为它们也相等，所以比较了前两个字符（`r`和`c`）。 并且因为`r`具有比`c`更大的 ASCII 值，所以`str1`大于`str2`。
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这里还有更多示例：
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```py
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>>> "tim" == "tie"
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False
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>>> "free" != "freedom"
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True
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>>> "arrow" > "aron"
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True
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>>> "right" >= "left"
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True
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>>> "teeth" < "tee"
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False
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>>> "yellow" <= "fellow"
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False
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>>> "abc" > ""
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True
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>>>
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```
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试一试：
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```py
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print("tim" == "tie")
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print("free" != "freedom")
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print("arrow" > "aron")
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print("right" >= "left")
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print("teeth" < "tee")
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print("yellow" <= "fellow")
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print("abc" > "")
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```
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## 使用`for`循环迭代字符串
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* * *
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字符串是一种序列类型，也可以使用`for`循环进行迭代（要了解有关`for`循环的更多信息，[请单击此处](/python-loops/)）。
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```py
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>>> s = "hello"
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>>> for i in s:
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... print(i, end="")
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hello
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```
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**注意**：
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默认情况下，`print()`函数用换行符打印字符串，我们通过传递名为`end`的命名关键字参数来更改此行为，如下所示。
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```py
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print("my string", end="\n") # this is default behavior
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print("my string", end="") # print string without a newline
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print("my string", end="foo") # now print() will print foo after every string
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```
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试一试：
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```py
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s = "hello"
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for i in s:
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print(i, end="")
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```
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## 测试字符串
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* * *
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python 中的字符串类具有各种内置方法，可用于检查不同类型的字符串。
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| 方法名称 | 方法说明 |
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| --- | --- |
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| `isalnum()` | 如果字符串是字母数字，则返回`True` |
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| `isalpha()` | 如果字符串仅包含字母，则返回`True` |
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| `isdigit()` | 如果字符串仅包含数字，则返回`True` |
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| `isidentifier()` | 返回`True`是字符串是有效的标识符 |
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| `islower()` | 如果字符串为小写，则返回`True` |
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| `isupper()` | 如果字符串为大写则返回`True` |
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| `isspace()` | 如果字符串仅包含空格，则返回`True` |
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```py
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>>> s = "welcome to python"
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>>> s.isalnum()
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False
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>>> "Welcome".isalpha()
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True
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>>> "2012".isdigit()
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True
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>>> "first Number".isidentifier()
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False
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>>> s.islower()
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True
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>>> "WELCOME".isupper()
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True
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>>> " \t".isspace()
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True
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```
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试一试：
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```py
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s = "welcome to python"
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print(s.isalnum())
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print("Welcome".isalpha())
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print("2012".isdigit())
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print("first Number".isidentifier())
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print(s.islower())
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print("WELCOME".isupper())
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print(" \t".isspace())
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```
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## 搜索子串
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* * *
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| 方法名称 | 方法说明 |
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| --- | --- |
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| `endwith(s1: str): bool` | 如果字符串以子字符串`s1`结尾，则返回`True` |
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| `startswith(s1: str): bool` | 如果字符串以子字符串`s1`开头，则返回`True` |
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| `count(s: str): int` | 返回字符串中子字符串出现的次数 |
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| `find(s1): int` | 返回字符串中`s1`起始处的最低索引，如果找不到字符串则返回`-1` |
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| `rfind(s1): int` | 从字符串中`s1`的起始位置返回最高索引，如果找不到字符串则返回`-1` |
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```py
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>>> s = "welcome to python"
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>>> s.endswith("thon")
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True
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>>> s.startswith("good")
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False
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>>> s.find("come")
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3
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>>> s.find("become")
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-1
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>>> s.rfind("o")
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15
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>>> s.count("o")
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3
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>>>
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```
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试一试：
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```py
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s = "welcome to python"
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print(s.endswith("thon"))
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print(s.startswith("good"))
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print(s.find("come"))
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print(s.find("become"))
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print(s.rfind("o"))
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print(s.count("o"))
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```
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## 转换字符串
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* * *
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| 方法名称 | 方法说明 |
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| --- | --- |
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| `capitalize(): str` | 返回此字符串的副本，仅第一个字符大写。 |
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| `lower(): str` | 通过将每个字符转换为小写来返回字符串 |
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| `upper(): str` | 通过将每个字符转换为大写来返回字符串 |
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| `title(): str` | 此函数通过大写字符串中每个单词的首字母来返回字符串 |
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| `swapcase(): str` | 返回一个字符串，其中小写字母转换为大写，大写字母转换为小写 |
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| `replace(old, new): str` | 此函数通过用新字符串替换旧字符串的出现来返回新字符串 |
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```py
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s = "string in python"
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>>>
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>>> s1 = s.capitalize()
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>>> s1
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'String in python'
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>>>
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>>> s2 = s.title()
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>>> s2
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'String In Python'
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>>>
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>>> s = "This Is Test"
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>>> s3 = s.lower()
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>>> s3
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'this is test'
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>>>
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>>> s4 = s.upper()
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>>> s4
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'THIS IS TEST'
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>>>
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>>> s5 = s.swapcase()
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>>> s5
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'tHIS iS tEST'
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>>>
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>>> s6 = s.replace("Is", "Was")
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>>> s6
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'This Was Test'
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>>>
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>>> s
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'This Is Test'
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>>>
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```
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试一试：
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```py
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s = "string in python"
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s1 = s.capitalize()
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print(s1)
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s2 = s.title()
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print(s2)
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s = "This Is Test"
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s3 = s.lower()
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print(s3)
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s4 = s.upper()
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print(s4)
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s5 = s.swapcase()
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print(s5)
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s6 = s.replace("Is", "Was")
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print(s6)
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print(s)
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```
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