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Chapters


Python 3 - Introduction

What Is New In Python 3
Python 3 - Environment Setup
Python 3 - Basic Syntax
Python 3 - Variable Types
Python 3 - Basic Operators
Python 3 - Decision Making
Python 3 - Loops
Python 3 - Numbers
Python 3 - Strings
Python 3 - Lists
Python 3 - Tuples
Python 3 - Dictionary
Python 3 - Date & Time
Python 3 - Functions
Python 3 - Modules
Python 3 - Files I/O
Python 3 - Exceptions Handling
Python 3 - Object Oriented
Python 3 - Regular Expressions
Python 3 - MySQL Database Access
Python 3 - Network Programming
Python 3 - Sending Email Using SMTP
Python 3 - Multithreaded Programming
Python 3 - XML Processing
Python 3 - GUI Programming (Tkinter)
Python 3 - Extension Programming With C


Outlines


Accessing Values In Strings

Updating Strings
Escape Characters
String Special Operators 
String Formatting Operator 
Unicode String
Built-in String Methods 

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Python 3 - 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 slicing along with the index or indices to obtain your substring. For example −

#!/usr/bin/python3

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/python3

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 list 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
\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 

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 (R) 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 

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/python3

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 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

 

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.)

 

Triple Quotes

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

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

#!/usr/bin/python3

para_str = """this is a long string that is made up of
several lines and non-printable characters such as
TAB ( \t ) and they will show up that way when displayed.
NEWLINEs within the string, whether explicitly given like
this within the brackets [ \n ], 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 explicit carriage return at the end of a line or its escape code (\n)  

this is a long string that is made up of
several lines and non-printable characters such as
TAB (    ) and they will show up that way when displayed.
NEWLINEs within the string, whether explicitly given like
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/python3

print ('C:\\nowhere')

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

C:\nowhere

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

#!/usr/bin/python3

print (r'C:\\nowhere')

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

C:\\nowhere

Unicode String

In Python 3, all strings are represented in Unicode.In Python 2 are stored internally as 8-bit ASCII, hence it is required to attach 'u' to make it Unicode. It is no longer necessary now.

Built-in String Methods 

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

Sr.No. Methods & Description
1 capitalize()

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

Returns a string padded with fillchar 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 trailing whitespace of string.
31 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.
32 splitlines( num=string.count('\n'))

Splits string at all (or num) NEWLINEs and returns a list of each line 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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