A string is a built-in sequence data type. A string object is an ordered collection of Unicode characters put between single, double or triple quotes.
There are different ways of literal representation of string as follows:
'Hello Python' Out[1]: 'Hello Python' In [2]: #using double quotes "Hello Python" Out[2]: 'Hello Python' In [3]: #using triple quotes ''' Hello World Welcome to Python ''' Out[3]: '\nHello World\nWelcome to Python\n'
Note that even though double or triple quotes are used, Python uses single quotes for internal representation. Triple quotes are helpful in forming a string of more than one lines. Triple quotes may be of triple single quotes ('''…''') or triple double quotes ("""…"""). For each physical line the string includes a newline character \n. When the string object is displayed using print() statement, the effect newline characters is visible.
In [4]: #using triple quotes aString=''' Hello World Welcome to Python ''' print (aString) Out[4]: Hello World Welcome to Python
The newline character \n is one of the escape sequences identified by Python. Escape sequence invokes an alternative implementation character subsequence to \. In Python \ is used as escape character. Following table shows list of escape sequences.
| Escape sequence | Description | Example | Result |
|---|---|---|---|
| \a | Bell or alert | "\a" | Bell sound |
| \b | Backspace | "ab\bc" | ac |
| \f | Formfeed | "hello\fworld" | hello world |
| \n | Newline | "hello\nworld" | hello world |
| \nnn | Octal notation, where n is in the range 0-7 | '\101' | A |
| \t | Tab | 'Hello\tPython' | HelloPython |
| \xnn | Hexadecimal notation, where n is in the range 0-9, a-f, or A-F | '\x41' | A |
String is also built using Python’s built-in str() function.
In [5]: str(10) Out[5]: '10'
All strings are objects of built-in str class. The str class has a number of methods to perform various operations on string. Let us discuss string methods and their uses.
Following group of string methods deal with rearrangement of casing of alphabets in a string.
capitalize() | Changes first letter of string to capital |
lower() | All uppercase letters in string are converted to lowercase. |
swapcase() | Changes the case of letter from upper to lower and vice versa. |
title() | First letter of all words changes to uppercase and the rest are lowercase. |
upper() | All lowercase letters in string changed to uppercase. |
In [6]: word='hello' word.capitalize() Out[6]: 'Hello' In [7]: line='HELLO WORLD' line.lower() Out[7]: 'hello world' In [8]: line='Hello World' line.swapcase() Out[8]: 'hELLO wORLD' In [9]: line='python is beautiful' line.title() Out[9]: 'Python Is Beautiful' In [10]: line='hello world' line.upper() Out[10]: 'HELLO WORLD'
Following group of methods are Boolean in nature. They return either True or False.
endswith() | Determines if string ends with given substring. |
startswith() | Determines if string starts with given substring. |
isalnum() | Returns true if all characters are alphanumeric and false otherwise. |
isalpha() | Returns true if all characters are alphabetic and false otherwise. |
isdigit() | Returns true if string contains only digits. |
islower() | Returns true if string has all cased characters in lowercase and false otherwise. |
istitle() | Returns true if each word in string starts with uppercase character and others in lowercase. |
isupper() | Returns true if all characters in string are in uppercase and false otherwise. |
In [11]: line="How are you?" line.endswith("?") Out[11]: True In [12]: line.startswith("How") Out[12]: True In [13]: string="AMD64" string.isalnum() Out[13]: True In [14]: string.isalpha() Out[14]: False In [15]: string="102.556" # . is not a digit string.isdigit() Out[15]: False In [16]: line="How are you" line.islower() Out[16]: False In [17]: line.istitle() Out[17]: False In [18]: line.isupper() Out[18]: False
Following set of methods deal with find/replace operations within a string.
find() | Determine if a substring occurs in given string. Returns index if found and -1 otherwise |
index() | Same as find(), but raises an exception if substring not found. |
rfind() | Same as find(), but search string in reverse direction. |
rindex() | Same as index(), but search string in reverse direction. |
replace() | Replaces all occurrences of a given substring and replace with another. |
In [19]: string="Simple is Better Than Complex" string.find("an") Out[19]: 19 In [20]: string.find("be") Out[20]: -1 In [21]: string.index("is") Out[21]: 7 In [22]: string.index('Is') --------------------------------------------------------------------------- ValueError Traceback (most recent call last) <ipython-input-22-2f723114bb0b> in <module>() ----> 1 string.index('Is') ValueError: substring not found In [23]: string="Simple is Better Than Complex" string.replace("is","was") Out[23]: 'Simple was Better Than Complex'
Following methods are defined for the purpose of alignment of characters in string.
| center() | Returns a string padded with given character so that original string is centered to a specified width. |
| expandtabs() | replaces tab escape sequence in string with multiple spaces. default tab size is 8. |
| ljust() | string is padded with spaces on left and left-justified to a specified width. |
| lstrip() | Removes leading whitespaces in string. |
rjust() | string is padded with spaces on right and right-justified to a specified width. |
| rstrip() | Removes trailing whitespaces of string. |
| split() | Splits given string according to delimiter character or substring (default is space) and returns list of substrings. |
| strip() | Performs both lstrip() and rstrip() on string. |
In [24]: string="Computer" string.center(30, "*") Out[24]: '***********Computer***********' In [25]: string.ljust(30,"*") Out[25]: 'Computer**********************' In [26]: string.rjust(30,"*") Out[26]: '**********************Computer' In [27]: string = "Hello\tWorld" string.expandtabs() Out[27]: 'Hello World' In [28]: string=' computer ' string.lstrip() Out[28]: 'computer ' In [29]: string.rstrip() Out[29]: ' computer' In [30]: string='192.168.001.001' string.split('.') Out[30]: ['192', '168', '001', '001']
Python uses C style format specification symbols (%d, %f, %s etc) to construct a string by substituting these symbols by Python objects.
In the example below, the symbols %s and %d in the string are substituted by values of objects in tuple outside the string, prefixed by % symbol
In [31]: name="Kiran" marks=95 "Hi my name is %s and I have secured %d percent marks in my B.E exam" %(name, marks) Out[31]: 'Hi my name is Kiran and I have secured 95 percent marks in my B.E exam'
The formatting symbols which can be used in Python string are listed in table below:
| Symbol | Purpose |
|---|---|
| %c | character |
| %s | string |
| %i | signed decimal integer |
| %d | signed decimal integer |
| %u | unsigned decimal integer |
| %o | octal integer |
| %x / %X | hexadecimal integer |
| %e / %E | exponential notation |
| %f | floating point real number |
In numeric formatting symbols, width before and/or after decimal point can be specified.
In [32]: x=10 y=1001.21 "x=%5d y=%10.3f" %(x,y) Out[32]: 'x= 10 y= 1001.210'
By default string is aligned to left. To make it right aligned prefix – symbol to width in %s symbol.
In [33]: string='computer' "%30s"%(string,) Out[33]: ' computer' In [34]: "%-30s"%(string,) Out[34]: 'computer
Python 3.x has a format() method which is more efficient and elegant as far as formatting with variable substitution is concerned.
Instead of C like % formatting operator, {} symbols are used as place holders.
In [35]: name="Kiran" marks=95 "Hi my name is {} and I have secured {} percent marks in my B.E exam".format(name, marks) Out[35]: 'Hi my name is Kiran and I have secured 95 percent marks in my B.E exam'
Format specification characters are used with : instead of %. It means to insert a string in place holder use {:s} and for integer use {:d}. Width of numeric and string variables is specified as before.
In [36]: x=10 y=1001.21 "x={:5d} y={:10.3f}".format(x,y) Out[36]: 'x= 10 y= 1001.210'
Alignment of strings is formatted by <, > and ^ symbols in place holder. They make the substituted string left aligned, right aligned or center aligned respectively. Default is < for left alignment.
In [37]: string='computer' "{:<30s}".format(string,) Out[37]: 'computer ' In [38]: "{:30s}".format(string,) #default is < - left alignment Out[38]: 'computer ' In [39]: "{:^30s}".format(string,) Out[39]: ' computer '
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