String

String Creation

String is one of the most common used data type in Python. You create a string by enclosing some characters or numbers in single (') or double (") quotes.

# using double quote
>>> a = "hello"

# using single quote             
>>> b = 'world'         

# concatenates string    
>>> a + b                  
'helloworld'

# query for data type
>>> type(a)                 
<type 'str'>

# mixing single and double quotes
>>> c = "How're you doing?" 
>>> c
"How're you doing?"

>>> d = """ A long string
... that spans a few
... lines can be enclosed in
... triple quotes.
... """

>>> d
' A long string \nthat spans a few\nlines can be enclosed in \ntriple quotes.\n'             
						

Accessing Values in a String

A string comprises of a sequence of characters, and quite a few methods/functions that work on list is applicable to String as well.

>>> a = "hello"

# get first character (same as list, zero indexed)
>>> a[0]                
'h'

# get get last character
>>> a[-1]               
'o'

# slicing works on string as well
>>> a[0:2]              
'he'

# get number of characters in the string
>>> len(a)              
5

# get the number of occurence of a particular character
>>> a.count('l')        
2      
						

Modifying a String

String is immutable, and therefore its value cannot be modified directly. However, you can "modify" a string by re-assigning the value to another string.

>>> a = "hello"

# Not allow, produce error
>>> a[0] = 'a'          
Traceback (most recent call last):
  File "<interactive input>", line 1, in <module>
TypeError: 'str' object does not support item assignment

# replace the first character with 'a', and reassign the result to a new string
>>> b = 'a' + a[1:]     
>>> b
'aello'
						

Built-in String Methods

Python provides quite a few very handy string methods. Some commonly used ones are shown below.

>>> a = "hello"
>>> a.capitalize()
'Hello'
>>> a.upper()
'HELLO'
>>> a.lower()
'hello'
>>> a.replace("e", "a")
'hallo'               
1
						

Methods for Finding Substrings

>>> a = "Hello World"
>>> a.startswith("H")
True
>>> a.endswith("D")
False
>>> a.find("World")
6
>>> a.index("World")
6

# 'find' returns -1 if no substring is found.
>>> a.find("world") 
-1

# 'index' returns a error
>>> a.index("world") 
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
ValueError: substring not found
						

Code Snippets

This section will contain some code snippets to show some practical use of the topic that you have learnt.

# Doing a Word Count
# ----------------------------------
sentence = "This sentence contains some words"

# Use split() function to breakdown the sentence into a word list
wordList = sentence.split()

# Use len() function to count number of items in the list
wordCount = len(wordList)

print "Number of words: ", wordCount

# output
Number of words:  5

# Checking for 4 digit number
# ----------------------------------
def isFourDigitNumber(x):
    if len(x) == 4 and x.isdigit():
        return True
    else:
        return False

print isFourDigitNumber("2345") # True
print isFourDigitNumber("23456") # False
print isFourDigitNumber("123a") # True