Chapter: Problem Solving and Python Programming : Lists, Tuples, Dictionaries

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Python dictionary

Python dictionary is an unordered collection of items.

PYTHON DICTIONARY

 

Python dictionary is an unordered collection of items. While other compound data types have only value as an element, a dictionary has a key: value pair.

 

How to create a dictionary?

 

Creating a dictionary is as simple as placing items inside curly braces {} separated by comma.

An item has a key and the corresponding value expressed as a pair, key: value. While values can be of any data type and can repeat, keys must be of immutable type (string, number or tuple with immutable elements) and must be unique.

 

# empty dictionary

# my_dict = {}

 

# dictionary with integer keys

my_dict = {1: 'apple', 2: 'ball'}

 

# dictionary with mixed keys

my_dict = {'name': 'John', 1: [2, 4, 3]}

 

# using dict()

my_dict = dict({1:'apple', 2:'ball'})

 

#from sequence having each item as a pair

my_dict = dict([(1,'apple'), (2,'ball')])

As you can see above, we can also create a dictionary using the built-in function dict().

 

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.

The difference while using get() is that it returns None instead of KeyError, if the key is not found.

 

my_dict = {'name':'Jack', 'age': 26}

 

# Output: Jack

# print(my_dict['name'])

 

# Output: 26

print(my_dict.get('age'))

 

# Trying to access keys which doesn't exist throws error

# my_dict.get('address')

# my_dict['address']

 

OUTPUT:

Jack

26

 

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.

my_dict = {'name':'Jack', 'age': 26}

 

# update value

my_dict['age'] = 27

 

#Output: {'age': 27, 'name': 'Jack'}

print(my_dict)

 

# add item

my_dict['address'] = 'Downtown'

 

# Output: {'address': 'Downtown', 'age': 27, 'name': 'Jack'}

print(my_dict)

 

When you run the program, the output will be:

{'name': 'Jack', 'age': 27}

{'name': 'Jack', 'age': 27, 'address': 'Downtown'}

 

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

squares = {1:1, 2:4, 3:9, 4:16, 5:25}

 

# remove a particular item

# Output: 16

print(squares.pop(4))

 

# Output: {1: 1, 2: 4, 3: 9, 5: 25}

print(squares)

 

# remove an arbitrary item

# Output: (1, 1)

print(squares.popitem())

 

# Output: {2: 4, 3: 9, 5: 25}

print(squares)

 

# delete a particular item

del squares[5]

 

# Output: {2: 4, 3: 9}

print(squares)

 

# remove all items

squares.clear()

 

# Output: {}

print(squares)

 

# delete the dictionary itself

del squares

 

# Throws Error

# print(squares)

 

When you run the program, the output will be:

16

{1: 1, 2: 4, 3: 9, 5: 25}

(1, 1)

{2: 4, 3: 9, 5: 25}

{2: 4, 3: 9}

{}

 

Python Dictionary Methods

 

Methods that are available with dictionary are tabulated below. Some of them have already been used in the above examples.

 

Python Dictionary Methods

 

Method and Description

clear() : Remove all items form the dictionary.

copy() : Return a shallow copy of the dictionary.

fromkeys(seq[, v]) : Return a new dictionary with keys from seq and value equal to v (defaults to None).

get(key[,d]) : Return the value of key. If key doesnot exit, return d (defaults to None).

items() : Return a new view of the dictionary's items (key, value).

keys() : Return a new view of the dictionary's keys.

pop(key[,d]) : Remove the item with key and return its value or d if key is not found. If d is not provided and key is not found, raises KeyError.

popitem() : Remove and return an arbitary item (key, value). Raises KeyError if the dictionary is empty.

setdefault(key[,d]) : If key is in the dictionary, return its value. If not, insert key with a value of d and return d (defaults to None).

update([other]) : Update the dictionary with the key/value pairs from other, overwriting existing keys.

values() : Return a new view of the dictionary's values

 

Here are a few example use of these methods.

 

marks = {}.fromkeys(['Math','English','Science'], 0)

 

# Output: {'English': 0, 'Math': 0, 'Science': 0}

print(marks)

 

for item in marks.items():

print(item)

 

# Output: ['English', 'Math', 'Science']

list(sorted(marks.keys()))

 

Output: {'English': 0, 'Math': 0, 'Science': 0}

('English', 0)

('Math', 0)

('Science', 0)

 

Out[1]: ['English', 'Math', 'Science']

 

Built-in Functions with Dictionary

 

Built-in functions like all(), any(), len(), cmp(), sorted() etc. are commonly used with dictionary to perform different tasks.

 

Built-in Functions with Dictionary

 

Function and Description


all() : Return True if all keys of the dictionary are true (or if the dictionary is empty).

any() : Return True if any key of the dictionary is true. If the dictionary is empty, return False.

len() : Return the length (the number of items) in the dictionary.

cmp() : Compares items of two dictionaries.

sorted() : Return a new sorted list of keys in the dictionary.

 

Here are some examples that uses built-in functions to work with dictionary. squares = {1: 1, 3: 9, 5: 25, 7: 49, 9: 81}

#  Output: 5

print(len(squares))

#  Output: [1, 3, 5, 7, 9]

print(sorted(squares))

Output: 5

[1, 3, 5, 7, 9]

 

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