Python data handling notes

Python List Tuple & dictionary
Prof. K. Adisesha| 1
Python Lists
Python Collections (Arrays)
There are four collection data types in the Python programming language:
 List is a collection, which is ordered and changeable. Allows duplicate members.
 Tuple is a collection, which is ordered and unchangeable. Allows duplicate members.
 Set is a collection, which is unordered and unindexed. No duplicate members.
 Dictionary is a collection, which is unordered, changeable and indexed. No duplicate
members.
When choosing a collection type, it is useful to understand the properties of that type. Choosing
the right type for a particular data set could mean retention of meaning, and, it could mean an
increase in efficiency or security.
List: A list is a collection, which is ordered and changeable. In Python, lists are written with
square brackets.
Example
Create a List:
StuList = ["Prajwal", "Sunny", "Rekha"]
print(StuList)
Output: ['Prajwal', 'Sunny', 'Rekha']
Access Items: You access the list items by referring to the index number:
Example
Print the second item of the list:
print(StuList[1])
Output: Sunny
Negative Indexing: Negative indexing means beginning from the end, -1 refers to the last item,
-2 refers to the second last item etc.
Example
Print the last item of the list:
print(StuList[-1])
Output: Rekha
Range of Indexes: You can specify a range of indexes by specifying start and end position of the
range.
 When specifying a range, the return value will be a new list with the specified items.
Example
Return the third, fourth, and fifth item:
StuList = ["Prajwal", "Sunny", "Rekha", "Sam", "Adi", "Ram", "Shilu"]
print(StuList[2:5])

Output: ["Rekha", "Sam", "Adi"]
Note: The search will start at index 2 (included) and end at index 5 (not included).
Range of Negative Indexes: Specify negative indexes if you want to start the search from the end
of the list:
Example
This example returns the items from index -4 (included) to index -1 (excluded)
StuList = ["Prajwal", "Sunny", "Rekha", "Sam", "Adi", "Ram", "Shilu"]
print(StuList[-4:-1])
Output: ["Sam", "Adi", "Ram"]
Change Item Value: To change the value of a specific item, refer to the index number:
Example
Change the second item:
StuList[1] = "Adi"
print(StuList)
Output: ["Prajwal", "Adi", "Rekha"]
Loop through a List: You can loop through the list items by using a for loop:
Example
Print all items in the list, one by one:
for x in StuList:
print(x)
Output: Prajwal
Sunny
Rekha
Check if Item Exists: To determine if a specified item is present in a list use the in keyword:
Example
Check if “Sunny” is present in the list:
if "Prajwal" in StuList:
print("Yes, 'Sunny' is in the Student list")
Output: Yes, 'Sunny' is in the Student list
List Length
len() method: To determine how many items a list has use the:
Example
Print the number of items in the list:
print(len(StuList))
Output: 3

Add Items: To add an item to the end of the list, use the append), insert(), method:
 Using the append() method to append an item:
Example
StuList.append("Sam")
print(StuList)
Output: ['Prajwal', 'Sunny', 'Rekha', 'Sam']
 To add an item at the specified index, use the insert() method:
Example
Insert an item as the second position:
StuList.insert(1, "Sam")
print(StuList)
Output: ['Prajwal', 'Sam', 'Sunny', 'Rekha']
Remove Item: There are several methods to remove items from a list using: remove(), pop(), del,
clear()
 The remove() method removes the specified item:
Example
StuList.remove("Sunny")
print(StuList)
Output: ['Prajwal', 'Rekha']
 The pop() method removes the specified index, (or the last item if index is not specified):
Example
StuList.pop()
print(StuList)
Output: ['Prajwal', 'Sunny']
 The del keyword removes the specified index:
Example
del StuList[0]
print(StuList)
Output: ['Sunny', 'Rekha']
 The del keyword can also delete the list completely:
Example
del StuList

 The clear() method empties the list:
Example
StuList.clear()
print(StuList)
Copy a List: You cannot copy a list simply by typing list2 = list1, because: list2 will only be a
reference to list1, and changes made in list1 will automatically also be made in list2.
There are ways to make a copy, one-way is to use the built-in copy(), list() method.
 copy() method: Make a copy of a list:
Example
mylist = StuList.copy()
print(mylist)
 list() method: Make a copy of a list:
Example
mylist = list(StuList)
print(mylist)
Join Two Lists: There are several ways to join, or concatenate, two or more lists in Python.
 One of the easiest ways are by using the + operator.
Example
list1 = ["a", "b" , "c"]
list2 = [1, 2, 3]
list3 = list1 + list2
print(list3)
Output: ['a', 'b', 'c', 1, 2, 3]
Another way to join two lists are by appending all the items from list2 into list1, one by one:
Example
Append list2 into list1:
list1 = ["a", "b”, "c"]
list2 = [1, 2, 3]
for x in list2:
list1.append(x)
print(list1)
Output: ['a', 'b', 'c', 1, 2, 3]
 Use the extend() method, which purpose is to add elements from one list to another list:
Example

Use the extend() method to add list2 at the end of list1:
list1 = ["a", "b”, "c"]
list2 = [1, 2, 3]
list1.extend(list2)
print(list1)
Output: ['a', 'b', 'c', 1, 2, 3]
The list() Constructor: It is also possible to use the list() constructor to make a new list.
Example
Using the list() constructor to make a List:
StuList = list(("Prajwal", "Sunny", "Rekha")) # note the double round-brackets
print(StuList)
List Methods
Python has a set of built-in methods that you can use on lists.
Method Description
append() Adds an element at the end of the list
clear() Removes all the elements from the list
copy() Returns a copy of the list
count() Returns the number of elements with the specified value
extend() Add the elements of a list (or any iterable), to the end of the current list
index() Returns the index of the first element with the specified value
insert() Adds an element at the specified position
pop() Removes the element at the specified position
remove() Removes the item with the specified value
reverse() Reverses the order of the list
sort() Sorts the list

Python Tuples
A tuple is a collection, which is ordered and unchangeable. In Python, tuples are written with
round brackets.
Example
Create a Tuple:
Stu_tuple = (“Prajwal”, “Sunny”, “Rekha”)
print(Stu_tuple)
Output: ('Prajwal', 'Sunny', 'Rekha')
Access Tuple Items: Tuple items access by referring to the index number, inside square
brackets:
Example
Print the second item in the tuple:
print(Stu_tuple[1])
Output: Sunny
Negative Indexing: Negative indexing means beginning from the end, -1 refers to the last item,
and -2 refers to the second last item etc.
Example
Print the last item of the tuple:
print(Stu_tuple[-1])
Output: Rekha
Range of Indexes: You can specify a range of indexes by specifying start and end of the range.
When specifying a range, the return value will be a new tuple with the specified items.
Example
Return the third, fourth, and fifth item:
Stu_tuple = ("Prajwal", "Sunny", "Rekha", "Sam", "Adi", "Ram", "Shilu")
print(Stu_tuple[2:5])
Output: (“Rekha", "Sam", "Adi”)
Note: The search will start at index 2 (included) and end at index 5 (not included).
Range of Negative Indexes: Specify negative indexes if you want to start the search from the end
of the tuple:
Example
This example returns the items from index -4 (included) to index -1 (excluded)
Stu_tuple = ("Prajwal", "Sunny", "Rekha", "Sam", "Adi", "Ram", "Shilu")
print(Stu_tuple[-4:-1])
Output: ("Sam", "Adi", "Ram")

Change Tuple Values: Once a tuple is created, you cannot change its values. Tuples are
unchangeable or immutable as it also is called. However, by converting the tuple into a list, change
the list, and convert the list back into a tuple.
Example
Convert the tuple into a list to be able to change it:
x = (“Prajwal”, “Sunny”, “Rekha”)
y = list(x)
y[1] = "Adi"
x = tuple(y)
print(x)
Output: (“Prajwal”, “Adi”, “Rekha”)
Loop through a Tuple: You can loop through the tuple items by using a for loop.
Example
Iterate through the items and print the values:
for x in Stu_tuple:
print(x,”t”)
Output: Prajwal Sunny Rekha
Check if Item Exists: To determine if a specified item is present in a tuple use the in keyword:
Example
Check if "Prajwal" is present in the tuple:
if "Prajwal" in Stu_tuple:
print("Yes, 'Prajwal' is in the Student tuple")
Output: Yes, 'Prajwal' is in the Student tuple
Tuple Length: To determine how many items a tuple has, use the len() method:
Example
Print the number of items in the tuple:
print(len(Stu_tuple))
Output: 3
Adding Items: Once a tuple is created, you cannot add items to it. Tuples are unchangeable.
Example
You cannot add items to a tuple:
Stu_tuple[3] = "Adi" # This will raise an error
print(Stu_tuple) Output: TypeError: 'tuple' object does not support item assignment
Create Tuple With One Item: To create a tuple with only one item, you have add a comma
after the item, unless Python will not recognize the variable as a tuple.

Example
One item tuple, remember the comma:
Stu_tuple = ("Prajwal",)
print(type(Stu_tuple))
Stu_tuple = ("Prajwal") #NOT a tuple
print(type(Stu_tuple))
Output: <class 'tuple'>
<class 'str'>
Remove Items: Tuples are unchangeable, so you cannot remove items from it, but you can delete
the tuple completely:
Example
The del keyword can delete the tuple completely:
del Stu_tuple
print(Stu_tuple) #this will raise an error because the tuple no longer exists
Join Two Tuples: To join two or more tuples you can use the + operator:
Example
Join two tuples:
tuple1 = ("a", "b" , "c")
tuple2 = (1, 2, 3)
tuple3 = tuple1 + tuple2
print(tuple3)
Output: ('a', 'b', 'c', 1, 2, 3)
The tuple() Constructor: It is also possible to use the tuple() constructor to make a tuple.
Example
Using the tuple() method to make a tuple:
Stu_tuple = tuple((“Prajwal”, “Sunny”, “Rekha”)) # note the double round-brackets
print(Stu_tuple)
Output: ('Prajwal', 'Sunny', 'Rekha')
Tuple Methods
Python has two built-in methods that you can use on tuples.
Method Description
count() Returns the number of times a specified value occurs in a tuple
index() Searches the tuple for a specified value and returns the position of where it
was found

Dictionaries
Introduction
 Dictionaries are mutable unordered collections of data values in the form of key: value pairs.
 Dictionaries are also called associative arrays or mappings or hashes
 The keys of the dictionary must be of immutable type like strings and numbers. Tuples can also
be used as keys if they contain only immutable objects. Giving a mutable key like a list will
give you a TypeError:unhashable type" error
 Values in the dictionaries can be of any type.
 The curly brackets mark the beginning and end of the dictionary.
 Each entry (Key: Value) consists of a pair separated by a colon (:) between them.
 The key-value pairs are separated by commas (,)
Dictionary-name = {<key1> :< value1>, <key2> :< value2>...}
e.g Day 0f The Week "Sunday":1, "Monday" 2, "Tuesday":3, "Wednesday":4,
"Thursday":5, "Friday":6,"Saturday":7)
 As the elements (key, value pairs) in a dictionary are unordered, we cannot access elements as
per any specific order. Dictionaries are not a sequence like string, list and tuple.
 The key value pairs are held in a dictionary as references.
 Keys in a dictionary have to be unique.
Definitions:
Dictionary is listed in curly brackets { }, inside these curly brackets, keys and values are declared.
Each key is separated from its value by a colon (:) while commas separate each element.
To create a dictionary, you need to include the key: value pairs in a curly braces as per following
syntax:
my_dict = {'key1': 'value1','key2': 'value2','key3': 'value3'…'keyn': 'valuen'}
Example:
dictr = {'Name': Sunny', 'Age': 18, 'Place': 'Bangalore'}
Properties of Dictionary Keys?
 There are two important points while using dictionary keys
 More than one entry per key is not allowed ( no duplicate key is allowed)
 The values in the dictionary can be of any type while the keys must be immutable like
numbers, tuples or strings.
 Dictionary keys are case sensitive- Same key name but with the different case are treated
as different keys in Python dictionaries.

Creation, initializing and accessing the elements in a Dictionary
The function dict ( ) is used to create a new dictionary with no items. This function is
called built-in function. We can also create dictionary using {}.
Example:
>>> D=dict()
>>> print D
{}
{} represents empty string. To add an item to the dictionary (empty string), we can use
square brackets for accessing and initializing dictionary values.
Example
>>> H=dict()
>>> H["one"]="keyboard"
>>> H["two"]="Mouse"
>>> H["three"]="printer"
>>> H["Four"]="scanner"
>>> print H
{'Four': 'scanner', 'three': 'printer', 'two': 'Mouse', 'one': 'keyboard'}
Traversing a dictionary
Let us visit each element of the dictionary to display its values on screen. This can be done by
using ‘for-loop’.
Example
H={'Four': 'scanner', 'three': 'printer', 'two': 'Mouse', 'one': 'keyboard'}
for i in H:
print i,":", H[i]," ",
Output
>>> Four: scanner one: keyboard three: printer two: Mouse
Creating, initializing values during run time (Dynamic allocation)
We can create a dictionary during run time also by using dict () function. This way of creation is
called dynamic allocation. Because, during the run time, memory keys and values are added to
the dictionary.
Example
Appending values to the dictionary
We can add new elements to the existing dictionary, extend it with single pair of values or join
two dictionaries into one. If we want to add only one element to the dictionary, then we should
use the following method.
Syntax:
Dictionary name [key]=value

List few Dictionary Methods?
Python has a set of built-in methods that you can use on dictionaries.
Method Description
clear() Removes all the elements from the dictionary
copy() Returns a copy of the dictionary
fromkeys() Returns a dictionary with the specified keys and values
get() Returns the value of the specified key
items() Returns a list containing a tuple for each key value pair
keys() Returns a list containing the dictionary's keys
pop() Removes the element with the specified key
popitem() Removes the last inserted key-value pair
setdefault() Returns the value of the specified key. If the key does not
exist: insert the key, with the specified value
update() Updates the dictionary with the specified key-value pairs
values() Returns a list of all the values in the dictionary
clear ( ) Method: It removes all items from the particular dictionary.
Syntax:
d.clear( ) #d dictionary
Example
Day={'mon':'Monday','tue':'Tuesday','wed':'Wednesday'}
print Day
Day.clear( )
print Day
Output: {'wed': 'Wednesday', 'mon': 'Monday', 'tue': 'Tuesday'}
{}
cmp ( ) Method: This is used to check whether the given dictionaries are same or not. If both are
same, it will return ‘zero’, otherwise return 1 or -1. If the first dictionary having more number of
items, then it will return 1, otherwise return -1.
Syntax:
cmp(d1,d2) #d1and d2 are dictionary.

returns 0 or 1 or -1
Example
Day1={'1':'Sun','2':'Mon','3':'Tues',’4':'Wed','5':'Thu','6':'Fri,'7':'Sat'}
Day3={'1':'Sun','2':'Mon','3':'Tues','4':'Wed'}
cmp(D1,D3) #both are not equal
Output: 1
cmp(D1,D2) #both are equal
Output: 0
cmp(D3,D1)
Output: -1
del() Method: This is used to Removing an item from dictionary. We can remove item from the
existing dictionary by using del key word.
Syntax:
del dicname[key]
Example
del Day1["3"]
print Day
Output: {'1':'Sun','2':'Mon',’4':'Wed','5':'Thu','6':'Fri,'7':'Sat'}
len( ) Method: This method returns number of key-value pairs in the given dictionary.
Syntax:
len(d) #d dictionary returns number of items in the list.
Example
Day={'1':'Sun','2':'Mon','3':'Tues',’4':'Wed','5':'Thu','6':'Fri,'7':'Sat'}
len(Day)
Output: 4
Merging dictionaries: An update ( )
Two dictionaries can be merged in to one by using update ( ) method. It merges the
keys and values of one dictionary into another and overwrites values of the same key.
Syntax:
Dic_name1.update (dic_name2)
Using this dic_name2 is added with Dic_name1.
Example
>>> d1={1:10,2:20,3:30}
>>> d2={4:40,5:50}
>>> d1.update(d2)
>>> print d1
{1: 10, 2: 20, 3: 30, 4: 40, 5: 50}
get(k, x ) Method: There are two arguments (k, x) passed in ‘get( )’ method. The first argument
is key value, while the second argument is corresponding value. If a dictionary has a given key
(k), which is equal to given value (x), it returns the corresponding value (x) of given key (k). If
omitted and the dictionary has no key equal to the given key value, then it returns None.
Syntax:
D.get (k, x) #D dictionary, k key and x value

Example
 Day.get('4',"wed") # corresponding value 3 'Wed'
 Day.get("6","mon") # default value of 6 ‘fri’
 Day.get("2") # default value of 2 ‘mon’
 Day.get("8") # None
has_key( ) Method: This function returns ‘True’, if dictionary has a key, otherwise it returns
‘False’.
Syntax:
D.has_key(k) #D dictionary and k key
Example
D.has_key("2")
True
D.has_key("8")
False
items( ) Method: It returns the content of dictionary as a list of key and value. The key and
value pair will be in the form of a tuple, which is not in any particular order.
Syntax:
D.items() # D dictionary
Example
D.items()
['1':'Sun','2':'Mon','3':'Tues',’4':'Wed','5':'Thu','6':'Fri,'7':'Sat']
keys() Method: It returns a list of the key values in a dictionary, which is not in any particular
order.
Syntax:
D.keys( ) #D dictionary
Example
D.keys()
[1', '2', 3', 4', '5', '6, 7']
values() Method: It returns a list of values from key-value pairs in a dictionary, which is not in
any particular order. However, if we call both the items () and values() method without changing
the dictionary's contents between these two (items() and values()), Python guarantees that the
order of the two results will be the same.
Syntax:
D.values() #D values
Example

Day.values()
['Wed', 'Sun, 'Thu', 'Tue', 'Mon', 'Fri', 'Sat']
Day.items()
['1':'Sun','2':'Mon','3':'Tues',’4':'Wed','5':'Thu','6':'Fri,'7':'Sat']
Python Dictionaries
What is Dictionary?
Dictionary is listed in curly brackets { }, inside these curly brackets, keys and values are declared.
Each key is separated from its value by a colon (:) while commas separate each element.
How to creating a Dictionary?
To create a dictionary, you need to include the key: value pairs in a curly braces as per following
syntax:
<dictionary-name>={<key>:<Value>, <key>:<Value>,...}
Example:
dictr = {'Name': Sunny', 'Age': 18, 'Place': 'Bangalore'}
What are the properties of Dictionary Keys?
 There are two important points while using dictionary keys
 More than one entry per key is not allowed ( no duplicate key is allowed)
 The values in the dictionary can be of any type while the keys must be immutable like
numbers, tuples or strings.
 Dictionary keys are case sensitive- Same key name but with the different case are treated
as different keys in Python dictionaries.
How to access elements from a dictionary?
While indexing is used with other container types to access values, dictionary uses keys. Key can
be used either inside square brackets or with the get() method.
Example:
>>>dict = {'Name':Sunny', 'Age': 18,'Place':'Bangalore'}
>>>print(my_dict['name'])
# Output: Sunny
There is also a method called get() that will give you the same result:
>>>print(dict.get('age'))
# Output: 18

How to change or add elements in a dictionary?
Dictionary are mutable. We can add new items or change the value of existing items using
assignment operator.
If the key is already present, value gets updated, else a new key: value pair is added to the
dictionary.
dict = {'Name':Sunny', 'Age': 18,'Place':'Bangalore'}
update value to Dictionary
dict['age'] = 27
print(dict)
#Output: {'age': 27, 'name': 'Sunny'}
add item to Dictionary
dict['address'] = 'Downtown'
print(dict)
Output: {'address': 'Downtown', 'age': 27, 'name': 'Jack'}
How to delete or remove elements from a dictionary?
We can remove a particular item in a dictionary by using the method pop(). This method removes
as item with the provided key and returns the value.
The method, popitem() can be used to remove and return an arbitrary item (key, value) form the
dictionary. All the items can be removed at once using the clear() method.
We can also use the del keyword to remove individual items or the entire dictionary itself.
# create a dictionary
dirc = {1:1, 2:4, 3:9, 4:16, 5:25}
# remove a particular item
>>>print(dirc.pop(4))
# Output: 16
>>>print(dirc)
# Output: {1: 1, 2: 4, 3: 9, 5: 25}
# remove an arbitrary item
>>>print(dirc.popitem())
# Output: (1, 1)
# delete a particular item
>>>del dirc[5]
>>>print(squares)

Output: {2: 4, 3: 9}
# remove all items
dirc.clear()
>>>print(dirc)
# Output: {}
# delete the dictionary itself
>>>del squares
>>> print(dirc)
Output: Throws Error
Dictionary Membership Test
We can test if a key is in a dictionary or not using the keyword in. Notice that membership test is
for keys only, not for values.
Example:
squares = {1: 1, 3: 9, 5: 25, 7: 49, 9: 81}
Case1:
print(1 in squares)
# Output: True
Case2:
print(2 not in squares)
# Output: True
Loop Through a Dictionary
Loop through a dictionary is done through using a for loop.
When looping through a dictionary, the return value are the keys of the dictionary, but there are
methods to return the values as well.
Example
Print all key names in the dictionary, one by one:
for x in thisdict:
print(x)
Example
Print all values in the dictionary, one by one:
for x in thisdict:
print(thisdict[x])
Example
You can also use the values() function to return values of a dictionary:

for x in thisdict.values():
print(x)
Example
Loop through both keys and values, by using the items() function:
for x, y in thisdict.items():
print(x, y)
Dictionary Length
To determine how many items (key-value pairs) a dictionary has, use the len() method.
Example
Print the number of items in the dictionary:
print(len(thisdict))
Copy a Dictionary
You cannot copy a dictionary simply by typing dict2 = dict1, because: dict2 will only be
a reference to dict1, and changes made in dict1 will automatically also be made in dict2. There are
ways to make a copy, one way is to use the built-in Dictionary method copy().
Example
Make a copy of a dictionary with the copy() method:
thisdict = {
"brand": "Ford",
"model": "Mustang",
"year": 1964
}
mydict = thisdict.copy()
print(mydict)
Another way to make a copy is to use the built-in method dict().
Example
Make a copy of a dictionary with the dict() method:
Ruthisdict = {
"brand": "Ford",
"model": "Mustang",
"year": 1964
}
mydict = dict(thisdict)
print(mydict)Run example »
n example »
Write a python program to input ‘n’ names and phone numbers to store it in a dictionary and to
input any name and to print the phone number of that particular name.
Code

phonebook=dict()
n=input("Enter total number of friends: ")
i=1
while i<=n:
a=raw_input("enter name: ")
b=raw_input("enter phone number: ")
phonebook[a]=b
i=i+1
name=input("enter name :")
f=0
l=phonebook.keys()
for i in l:
if (cmp(i,name)==0):
print "Phone number= ",phonebook[i]
f=1
if (f==0):
print "Given name not exist"
Output
Enter total number of friends 2
enter name: Prajwal
enter phone number: 23456745
enter name: Sunny
enter phone number: 45678956
2. Write a program to input ‘n’ employee number and name and to display all employee’s
information in ascending order based upon their number.
Code
empinfo=dict()
n=input("Enter total number of employees")
i=1
while i<=n:
a=raw_input("enter number")
b=raw_input("enter name")
empinfo[a]=b
i=i+1
l=empinfo.keys()
l.sort()
print "Employee Information"
print "Employee Number",'t',"Employee Name"
for i in l:
print i,'t',empinfo[i]
3. Write the output for the following Python codes.
A={1:100,2:200,3:300,4:400,5:500}
print A.items()
print A.keys()
print A.values()

Output
[(1, 100), (2, 200), (3, 300), (4, 400), (5, 500)]
[1, 2, 3, 4, 5]
[100, 200, 300, 400, 500]
4. Write a program to create a phone book and delete particular phone number using name.
Code
phonebook=dict()
n=input("Enter total number of friends")
i=1
while i<=n:
a=raw_input("enter name")
b=raw_input("enter phone number")
phonebook[a]=b
i=i+1
name=raw_input("enter name")
del phonebook[name]
l=phonebook.keys()
print "Phonebook Information"
print "Name",'t',"Phone number"
for i in l:
print i,'t',phonebook[i]
Write a program to input total number of sections and class teachers’ name in 11th class
and display all information on the output screen.
Code
classxi=dict()
n=input("Enter total number of section in xi class")
i=1
while i<=n:
a=raw_input("enter section")
b=raw_input ("enter class teacher name")
classxi[a]=b
i=i+1
print "Class","t","Section","t","teacher name"
for i in classxi:
print "XI","t",i,"t",classxi[i]

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Python data handling notes