Python - Strings

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.

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.

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.

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.

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.

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']

String formatting

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:

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       '