Slides from the training on the topic "Python Programming Essentials"

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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"
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Access a character in string
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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]

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Negative Indexes
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message = 'Welcome to Python'
print message[-1]
print message[-2]
print message[-3]
print message[-4]
Output:
n
o
h
t
str[-index]

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Length of a string
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message = 'Welcome to Python'
print len(message)
Output:
17
len(string)

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

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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!"
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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
"""
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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
'''
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Triple quoted strings are also used as Docstrings which will be
covered in functions

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

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

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

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

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

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What is Slicing
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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

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Slicing
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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

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Slicing (2)
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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

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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
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21. 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
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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
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