Python Programming Essentials - M7 - Strings

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Strings
Pavan Verma
Python Programming Essentials
@YinYangPavan

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What is a String
 A string is a sequence of characters
message = "Welcome to Python"
2

Access a character in string
3
message = 'Welcome to Python'
print message[0]
print message[1]
print message[2]
print message[3]
print message[4]
Output:
W
e
l
c
o
str[index]

Negative Indexes
4
print message[-1]
print message[-2]
print message[-3]
print message[-4]
Output:
n
o
h
t
str[-index]

Length of a string
5
print len(message)
Output:
17
len(string)

Single quotes Vs Double quotes
 You can use either single quotes or double
quotes for string literals – they are the same
6
>>> text = 'some text'
>>> text = "some text"

Single quotes Vs Double quotes (2)
 Need to escape double quotes in double quoted
strings
• Use single quotes for strings that contain double quotes
>>> s = "He said "Hello""
>>> s
'He said "Hello"'
 Need to escape single quotes in single quoted strings
• Use double quotes for strings that contain single quotes
>>> s = 'You've got an error!'
>>> s
"You've got an error!"
7

Triple quoted strings
 Python also has triple quoted strings available
 In some cases, when you need to include really long
string using triple quoted strings is useful
>>> message = """
This is a multi line message
use triple quotes if the text is too long
"""
8

Triple quoted strings (2)
 You can also uses triple single quotes, there is no
difference between single triple quoted strings and
double triple quoted strings
>>> message = '''
This is a multi line message
use triple quotes if the text is too long
'''
9
Triple quoted strings are also used as Docstrings which will be
covered in functions

Note for C/C++ Programmers
 There is no separate char data type in Python
 In Python, a character is just a string of length 1
eg: text ='f'
10

Note for Perl/PHP Programmers
 Remember that single-quoted strings and double-
quoted strings are the same – they do not differ in
any significant way
11

String Concatenation
12
>>> 'foo' + 'bar'
'foobar'
>>> 'foo' + 'bar' + '123'
'foobar123'
>>> name = 'Monty'
>>> last_name = 'Python'
>>> name + last_name
'MontyPython'
+ operator is used to
concatenate strings

String Concatenation (2)
13
>>> 'foo' + 'bar' + 123
TypeError: cannot concatenate 'str' and
'int' objects
string concatenation does not works with other types

String Concatenation (2)
14
>>> 'foo' + 'bar' + str(123)
'foobar123'
Use built in str() function to convert to a string

Strings are immutable
15
>>> message = 'Python is awesome'
>>> message[0] = 'j'
TypeError: 'str' object does not support
item assignment
>>> del message[0]
TypeError: 'str' object does not support
item deletion.
Python strings
cannot be changed

Strings are immutable (2)
16
Strings are immutable but that does not mean the variable
cannot change, variable can point to anything
>>> message
'Python is awesome'
>>> message = 'Python is dynamicaly
typed'
>>> message
'Python is dynamicaly typed'

What is Slicing
17
slicing in Python is powerful way of extracting sub-parts of
a string, lists, tuples
Use Case:
You can use slicing to extract sub-string out of a
string

Slicing
18
message = 'Python is awesome'
print message[0:5]
print message[7:10]
print message[10:17]
print message[:]
print message[5:]
print message[:6]
Outputs:
Pytho
is
awesome
Python is awesome
n is awesome
Python
str[start:end]
start: substring starts from this element
end: end of substring excluding the element at this index

Slicing (2)
19
str[start:end]
1. Slicing always returns a new string. Remember strings are
immutable
2. If you don’t provide start the substring starts from the beginning
of the string. eg: message[:5]
3. If end is not provided the substring runs till the end of the
string
4. If both start and end are missing the entire string is returned

in operator
in is a Boolean operator which takes two strings
and returns True if the first string is a sub-string
of the second string, False otherwise
't' in 'Welcome to Python'
True
'Python' in 'Welcome to Python'
True
'Python' in 'Welcome to Python'
True
20

Summary
 What is a string
 Access characters in a string
 Negative indexes
 Length of string
 Single quotes Vs Double quotes
 Triple quoted strings
 String concatenation
 Strings are Immutable
 in operator
21

Resources
 Tutorial on python strings
http://www.tutorialspoint.com/Python/Python_strings.htm
 Single vs double strings
http://docs.ckan.org/en/latest/Python-coding-standards.html
22

Python Programming Essentials - M7 - Strings

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