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Python - Strings
  • 时间：2024-11-05

Python - Strings

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Strings are amongst the most popular types in Python. We can create them simply by enclosing characters in quotes. Python treats single quotes the same as double quotes. Creating strings is as simple as assigning a value to a variable. For example −

var1 =  Hello World! 
var2 = "Python Programming"

Accessing Values in Strings

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

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

#!/usr/bin/python

var1 =  Hello World! 
var2 = "Python Programming"

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

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

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

Updating Strings

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 −

#!/usr/bin/python

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

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

Updated String :-  Hello Python

Escape Characters

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

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
 0x08 Backspace
cx   Control-x
C-x   Control-x
e 0x1b Escape
f 0x0c Formfeed
M-C-x   Meta-Control-x
0x0a Newpne
nn   Octal notation, where n is in the range 0.7
0x0d Carriage return
s 0x20 Space
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

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
[] Spce - Gives the character from the given index a[1] will give e
[ : ] Range Spce - 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 (R) and must be placed immediately preceding the first quote mark. print r prints and print R prints
% Format - Performs String formatting See at next section

String Formatting Operator

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 −

#!/usr/bin/python

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

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

My name is Zara and weight is 21 kg!

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

Format Symbol Conversion
%c character
%s string conversion via str() prior to formatting
%i signed decimal integer
%d signed decimal integer
%u unsigned decimal integer
%o octal integer
%x hexadecimal integer (lowercase letters)
%X hexadecimal integer (UPPERcase letters)
%e exponential notation (with lowercase e )
%E exponential notation (with UPPERcase E )
%f floating point real number
%g the shorter of %f and %e
%G the shorter of %f and %E

Other supported symbols and functionapty are psted in the following table −

Symbol Functionapty
* argument specifies width or precision
- left justification
+ display the sign
<sp> leave a blank space before a positive number
# add the octal leading zero ( 0 ) or hexadecimal leading 0x or 0X , depending on whether x or X were used.
0 pad from left with zeros (instead of spaces)
% %% leaves you with a single pteral %
(var) mapping variable (dictionary arguments)
m.n. m is the minimum total width and n is the number of digits to display after the decimal point (if appl.)

Triple Quotes

Python s triple quotes comes to the rescue by allowing strings to span multiple pnes, including verbatim NEWLINEs, TABs, and any other special characters.

The syntax for triple quotes consists of three consecutive single or double quotes.

#!/usr/bin/python

para_str = """this is a long string that is made up of
several pnes and non-printable characters such as
TAB ( 	 ) and they will show up that way when displayed.
NEWLINEs within the string, whether exppcitly given pke
this within the brackets [ 
 ], or just a NEWLINE within
the variable assignment will also show up.
"""
print para_str

When the above code is executed, it produces the following result. Note how every single special character has been converted to its printed form, right down to the last NEWLINE at the end of the string between the "up." and closing triple quotes. Also note that NEWLINEs occur either with an exppcit carriage return at the end of a pne or its escape code ( ) −

this is a long string that is made up of
several pnes and non-printable characters such as
TAB (    ) and they will show up that way when displayed.
NEWLINEs within the string, whether exppcitly given pke
this within the brackets [
 ], or just a NEWLINE within
the variable assignment will also show up.

Raw strings do not treat the backslash as a special character at all. Every character you put into a raw string stays the way you wrote it −

#!/usr/bin/python

print  C:\nowhere

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

C:
owhere

Now let s make use of raw string. We would put expression in r expression as follows −

#!/usr/bin/python

print r C:\nowhere

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

C:\nowhere

Unicode String

Normal strings in Python are stored internally as 8-bit ASCII, while Unicode strings are stored as 16-bit Unicode. This allows for a more varied set of characters, including special characters from most languages in the world. I ll restrict my treatment of Unicode strings to the following −

#!/usr/bin/python

print u Hello, world!

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

Hello, world!

As you can see, Unicode strings use the prefix u, just as raw strings use the prefix r.

Built-in String Methods

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

Sr.No. Methods with Description
1 capitapze()

Capitapzes first letter of string
2 center(width, fillchar)

Returns a space-padded string with the original string centered to a total of width columns.
3 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.
4 decode(encoding= UTF-8 ,errors= strict )

Decodes the string using the codec registered for encoding. encoding defaults to the default string encoding.
5 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 .
6 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.
7 expandtabs(tabsize=8)

Expands tabs in string to multiple spaces; defaults to 8 spaces per tab if tabsize not provided.
8 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.
9 index(str, beg=0, end=len(string))

Same as find(), but raises an exception if str not found.
10 isalnum()

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

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

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

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

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

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

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

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

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

Returns the length of the string
20 ljust(width[, fillchar])

Returns a space-padded string with the original string left-justified to a total of width columns.
21 lower()

Converts all uppercase letters in string to lowercase.
22 lstrip()

Removes all leading whitespace in string.
23 maketrans()

Returns a translation table to be used in translate function.
24 max(str)

Returns the max alphabetical character from the string str.
25 min(str)

Returns the min alphabetical character from the string str.
26 replace(old, new [, max])

Replaces all occurrences of old in string with new or at most max occurrences if max given.
27 rfind(str, beg=0,end=len(string))

Same as find(), but search backwards in string.
28 rindex( str, beg=0, end=len(string))

Same as index(), but search backwards in string.
29 rjust(width,[, fillchar])

Returns a space-padded string with the original string right-justified to a total of width columns.
30 rstrip()

Removes all traipng whitespace of string.
31 sppt(str="", num=string.count(str))

Sppts string according to depmiter str (space if not provided) and returns pst of substrings; sppt into at most num substrings if given.
32 spptpnes( num=string.count( ))

Sppts string at all (or num) NEWLINEs and returns a pst of each pne with NEWLINEs removed.
33 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.
34 strip([chars])

Performs both lstrip() and rstrip() on string.
35 swapcase()

Inverts case for all letters in string.
36 title()

Returns "titlecased" version of string, that is, all words begin with uppercase and the rest are lowercase.
37 translate(table, deletechars="")

Translates string according to translation table str(256 chars), removing those in the del string.
38 upper()

Converts lowercase letters in string to uppercase.
39 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).
40 isdecimal()

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