Python 简明教程

Python - Strings

Python 中, stringUnicode characters 的不可变序列。每个字符都具有一个根据 UNICODE 标准的唯一 numeric value 。但是,作为一个整体的序列并没有任何数字值,即使所有字符都是数字。为了将字符串与数字和其他标识符区分开来,字符序列在它的文字表示中包含在单引号、双引号或三引号内。因此,1234 是一个数字(整数),但“1234”是一个字符串。

In Python, a string is an immutable sequence of Unicode characters. Each character has a unique numeric value as per the UNICODE standard. But, the sequence as a whole, doesn’t have any numeric value even if all the characters are digits. To differentiate the string from numbers and other identifiers, the sequence of characters is included within single, double or triple quotes in its literal representation. Hence, 1234 is a number (integer) but '1234' is a string.

Creating Python Strings

只要相同字符序列被括起来, singledoubletriple 引号无关紧要。因此,以下字符串表示是等效的。

As long as the same sequence of characters is enclosed, single or double or triple quotes don’t matter. Hence, following string representations are equivalent.

Example

>>> 'Welcome To TutorialsPoint'
'Welcome To TutorialsPoint'
>>> "Welcome To TutorialsPoint"
'Welcome To TutorialsPoint'
>>> '''Welcome To TutorialsPoint'''
'Welcome To TutorialsPoint'
>>> """Welcome To TutorialsPoint"""
'Welcome To TutorialsPoint'

查看以上语句,很明显,Python 在内部以包含在单引号中的方式存储字符串。

Looking at the above statements, it is clear that, internally Python stores strings as included in single quotes.

Accessing Values in Strings

Python 并不支持字符类型;这些被视为长度为一的字符串,因此也被视为子字符串。

Python does not support a character type; these are treated as strings of length one, thus also considered a substring.

要访问子字符串,请使用方括号与索引来分片以获取你的子字符串。例如 −

To access substrings, use the square brackets for slicing along with the index or indices to obtain your substring. For example −

var1 = 'Hello World!'
var2 = "Python Programming"

print ("var1[0]: ", var1[0])
print ("var2[1:5]: ", var2[1:5])

执行上述代码后,将生成以下结果 −

When the above code is executed, it produces the following result −

var1[0]:  H
var2[1:5]:  ytho

Updating Strings

你可以通过将变量(重新)分配给另一个字符串来“更新”一个现有字符串。新值可以与它的前一个值相关,或者也可以与一个完全不同的字符串相关。例如 −

You can "update" an existing string by (re)assigning a variable to another string. The new value can be related to its previous value or to a completely different string altogether. For example −

var1 = 'Hello World!'
print ("Updated String :- ", var1[:6] + 'Python')

执行上述代码后,将生成以下结果 −

When the above code is executed, it produces the following result −

Updated String :-  Hello Python

访问我们的 Python - Modify Strings 教程以了解有关更新/修改字符串的更多信息。

Visit our Python - Modify Strings tutorial to know more about updating/modifying strings.

Escape Characters

下表是可以用反斜杠标记表示的转义符或不可打印字符的列表。

Following table is a list of escape or non-printable characters that can be represented with backslash notation.

escape character 得到解释;在单引号和双引号字符串中。

An escape character gets interpreted; in a single quoted as well as double quoted strings.

Backslash notation

Hexadecimal character

Description

\a

0x07

Bell or alert

\b

0x08

Backspace

\cx

Control-x

\C-x

Control-x

\e

0x1b

Escape

\f

0x0c

Formfeed

\M-\C-x

Meta-Control-x

\n

0x0a

Newline

\nnn

Octal notation, where n is in the range 0.7

\r

0x0d

Carriage return

\s

0x20

Space

\t

0x09

Tab

\v

0x0b

Vertical tab

\x

Character x

\xnn

Hexadecimal notation, where n is in the range 0.9, a.f, or A.F

String Special Operators

假设字符串变量 a 存储“Hello”,变量 b 存储“Python”,则 −

Assume string variable a holds 'Hello' and variable b holds 'Python', then −

Operator

Description

Example

+

Concatenation - Adds values on either side of the operator

a + b will give HelloPython

*

Repetition - Creates new strings, concatenating multiple copies of the same string

a*2 will give -HelloHello

[]

Slice - Gives the character from the given index

a[1] will give e

[ : ]

Range Slice - Gives the characters from the given range

a[1:4] will give ell

in

Membership - Returns true if a character exists in the given string

H in a will give 1

not in

Membership - Returns true if a character does not exist in the given string

M not in a will give 1

r/R

Raw String - Suppresses actual meaning of Escape characters. The syntax for raw strings is exactly the same as for normal strings with the exception of the raw string operator, the letter "r," which precedes the quotation marks. The "r" can be lowercase (r) or uppercase ® and must be placed immediately preceding the first quote mark.

print r'\n' prints \n and print R'\n’prints \n

%

Format - Performs String formatting

See at next section

String Formatting Operator

Python 最酷的功能之一是字符串格式运算符 %。此运算符对于字符串是唯一的,并且弥补了 C 的 printf() 系列中没有函数的缺陷。以下是一个简单的示例:

One of Python’s coolest features is the string format operator %. This operator is unique to strings and makes up for the pack of having functions from C’s printf() family. Following is a simple example −

print ("My name is %s and weight is %d kg!" % ('Zara', 21))

执行上述代码后,将生成以下结果 −

When the above code is executed, it produces the following result −

My name is Zara and weight is 21 kg!

以下是可与 % 一起使用的完整符号集列表:

Here is the list of complete set of symbols which can be used along with % −

Sr.No.

Format Symbol & Conversion

1

%c character

2

%s string conversion via str() prior to formatting

3

%i signed decimal integer

4

%d signed decimal integer

5

%u unsigned decimal integer

6

%o octal integer

7

%x hexadecimal integer (lowercase letters)

8

%X hexadecimal integer (UPPERcase letters)

9

%e exponential notation (with lowercase 'e')

10

%E exponential notation (with UPPERcase 'E')

11

%f floating point real number

12

%g the shorter of %f and %e

13

%G the shorter of %f and %E

支持的其他符号和功能列在以下表中:

Other supported symbols and functionality are listed in the following table −

Sr.No.

Symbol & Functionality

1

* argument specifies width or precision

2

- left justification

3

+ display the sign

4

<sp> leave a blank space before a positive number

5

# add the octal leading zero ( '0' ) or hexadecimal leading '0x' or '0X', depending on whether 'x' or 'X' were used.

6

0 pad from left with zeros (instead of spaces)

7

% '%%' leaves you with a single literal '%'

8

(var) mapping variable (dictionary arguments)

9

m.n. m is the minimum total width and n is the number of digits to display after the decimal point (if appl.)

访问我们的 Python - String Formatting 教程,了解格式化字符串的各种方法。

Visit our Python - String Formatting tutorial to learn about various ways to format strings.

Double Quotes in Python Strings

您想将一些文本作为字符串的一部分嵌入到双引号中,那么字符串本身应该放在单引号中。要嵌入单引号文本,则应在双引号中编写字符串。

You want to embed some text in double quotes as a part of string, the string itself should be put in single quotes. To embed a single quoted text, string should be written in double quotes.

Example

var = 'Welcome to "Python Tutorial" from TutorialsPoint'
print ("var:", var)

var = "Welcome to 'Python Tutorial' from TutorialsPoint"
print ("var:", var)

它将生成以下 output

It will produce the following output

var: Welcome to "Python Tutorial" from TutorialsPoint
var: Welcome to 'Python Tutorial' from TutorialsPoint

Triple Quotes

要使用三引号来形成一个字符串,你可以使用三个单引号或三个双引号 − 两个版本是类似的。

To form a string with triple quotes, you may use triple single quotes, or triple double quotes − both versions are similar.

Example

var = '''Welcome to TutorialsPoint'''
print ("var:", var)

var = """Welcome to TutorialsPoint"""
print ("var:", var)

它将生成以下 output

It will produce the following output

var: Welcome to TutorialsPoint
var: Welcome to TutorialsPoint

Python Multiline Strings

三引号字符串对形成多行字符串很有用。

Triple quoted string is useful to form a multi-line string.

Example

var = '''
Welcome To
Python Tutorial
from TutorialsPoint
'''
print ("var:", var)

它将生成以下 output

It will produce the following output

var:
Welcome To
Python Tutorial
from TutorialsPoint

Arithmetic Operators with Strings

字符串是一个非数字数据类型。显然,我们不能对字符串操作数使用算术运算符。在这种情况下,Python 会引发 TypeError。

A string is a non-numeric data type. Obviously, we cannot use arithmetic operators with string operands. Python raises TypeError in such a case.

print ("Hello"-"World")

执行以上程序会产生如下错误 -

On executing the above program it will generate the following error −

>>> "Hello"-"World"
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
TypeError: unsupported operand type(s) for -: 'str' and 'str'

Getting Type of Python Strings

Python 中的字符串是 str 类的一个对象。它可以用 type() 函数验证。

A string in Python is an object of str class. It can be verified with type() function.

Example

var = "Welcome To TutorialsPoint"
print (type(var))

它将生成以下 output

It will produce the following output

<class 'str'>

Built-in String Methods

Python 包含以下内置方法来操作字符串 -

Python includes the following built-in methods to manipulate strings −

Sr.No.

Methods with Description

1

capitalize()Capitalizes first letter of string.

2

casefold()Converts all uppercase letters in string to lowercase. Similar to lower(), but works on UNICODE characters alos.

3

center(width, fillchar)Returns a space-padded string with the original string centered to a total of width columns.

4

../python/string_count.html[count(str, beg=0, end=len(string))]Counts how many times str occurs in string or in a substring of string if starting index beg and ending index end are given.

5

../python/string_decode.html[decode(encoding='UTF-8', ), errors=strict]Decodes the string using the codec registered for encoding. encoding defaults to the default string encoding.

6

../python/string_encode.html[encode(encoding='UTF-8', ), errors=strict]Returns encoded string version of string; on error, default is to raise a ValueError unless errors is given with 'ignore' or 'replace'.

7

../python/string_endswith.html[endswith(suffix, beg=0, end=len(string))]Determines if string or a substring of string (if starting index beg and ending index end are given) ends with suffix; returns true if so and false otherwise.

8

expandtabs(tabsize=8)Expands tabs in string to multiple spaces; defaults to 8 spaces per tab if tabsize not provided.

9

../python/string_find.html[find(str, beg=0 end=len(string))]Determine if str occurs in string or in a substring of string if starting index beg and ending index end are given returns index if found and -1 otherwise.

10

format(*args, **kwargs)This method is used to format the current string value.

11

format_map(mapping)This method is also use to format the current string the only difference is it uses a mapping object.

12

../python/string_index.html[index(str, beg=0, end=len(string))]Same as find(), but raises an exception if str not found.

13

isalnum()Returns true if string has at least 1 character and all characters are alphanumeric and false otherwise.

14

isalpha()Returns true if string has at least 1 character and all characters are alphabetic and false otherwise.

15

isascii()Returns True is all the characters in the string are from the ASCII character set.

16

isdecimal()Returns true if a unicode string contains only decimal characters and false otherwise.

17

isdigit()Returns true if string contains only digits and false otherwise.

18

isidentifier()Checks whether the string is a valid Python identifier.

19

islower()Returns true if string has at least 1 cased character and all cased characters are in lowercase and false otherwise.

20

isnumeric()Returns true if a unicode string contains only numeric characters and false otherwise.

21

isprintable()Checks whether all the characters in the string are printable.

22

isspace()Returns true if string contains only whitespace characters and false otherwise.

23

istitle()Returns true if string is properly "titlecased" and false otherwise.

24

isupper()Returns true if string has at least one cased character and all cased characters are in uppercase and false otherwise.

25

join(seq)Merges (concatenates) the string representations of elements in sequence seq into a string, with separator string.

26

ljust(width[, fillchar])Returns a space-padded string with the original string left-justified to a total of width columns.

27

lower()Converts all uppercase letters in string to lowercase.

28

lstrip()Removes all leading white space in string.

29

maketrans()Returns a translation table to be used in translate function.

30

partition()Splits the string in three string tuple at the first occurrence of separator.

31

removeprefix()Returns a string after removing the prefix string.

32

removesuffix()Returns a string after removing the suffix string.

33

replace(old, new [, max])Replaces all occurrences of old in string with new or at most max occurrences if max given.

34

../python/string_rfind.html[rfind(str, beg=0, end=len(string))]Same as find(), but search backwards in string.

35

../python/string_rindex.html[rindex( str, beg=0, end=len(string))]Same as index(), but search backwards in string.

36

rjust(width,[, fillchar])Returns a space-padded string with the original string right-justified to a total of width columns.

37

rpartition()Splits the string in three string tuple at the ladt occurrence of separator.

38

rsplit()Splits the string from the end and returns a list of substrings.

39

rstrip()Removes all trailing whitespace of string.

40

../python/string_split.html[split(str="", num=string.count(str))]Splits string according to delimiter str (space if not provided) and returns list of substrings; split into at most num substrings if given.

41

splitlines( num=string.count('\n'))Splits string at all (or num) NEWLINEs and returns a list of each line with NEWLINEs removed.

42

../python/string_startswith.html[startswith(str, beg=0, end=len(string))]Determines if string or a substring of string (if starting index beg and ending index end are given) starts with substring str; returns true if so and false otherwise.

43

strip([chars])Performs both lstrip() and rstrip() on string.

44

swapcase()Inverts case for all letters in string.

45

title()Returns "titlecased" version of string, that is, all words begin with uppercase and the rest are lowercase.

46

../python/string_translate.html[translate(table, ), deletechars=]Translates string according to translation table str(256 chars), removing those in the del string.

47

upper()Converts lowercase letters in string to uppercase.

48

zfill (width)Returns original string leftpadded with zeros to a total of width characters; intended for numbers, zfill() retains any sign given (less one zero).

Built-in Functions with Strings

以下是我们能用于字符串的内置函数 −

Following are the built-in functions we can use with strings −

Sr.No.

Function with Description

1

len(list)Returns the length of the string.

2

max(list)Returns the max alphabetical character from the string str.

3

min(list)Returns the min alphabetical character from the string str.