Dictionary

A dictionary just like a list, tuple and a set which is a sequence. A dictionary is rather key-value pair. String, list, etc are number-indexed. A dictionary is key-indexed.

Structure of a dictionary

# dict_var = { key : value }

# empty dictionary
my_dict = {}

# profile dictionary
profile = {
    'name': 'John Doe',
    'age': 32,
    'job': 'Software engineer'
}

Casting

Casting is done with dict()

# passing a keyword and a value

# dict(key-word=value, ... )
my_dict = dict(
    name='John Doe',
    age=32,
    job='Software engineer'
)

# convert a list of tuple to a dictionary
my_tupled_list = [('name', 'John Doe'), ('age', 32, ),
                  ('job', 'Software engineer')]
my_dict = dict(my_tupled_list)

print(my_tupled_list)
# output-> 
# {'name': 'John Doe', 'age': 32, 'job': 'Software engineer'}

Indexing and updating a dictionary

Indexing and updating are done just as we would do to a list.

# consider this dictionary
profile = {
    'name': 'John Doe',
    'age': 32,
    'job': 'Software engineer'
}

# get the name and job
name = profile['name']
job = profile['job']

print(f"Candidates name is {name} and works, {job}")

# update the age
profile['age'] = 30

# add a new key
profile['lang'] = 'Python'

# delete a key to delete a value using - del
del profile['age']

dictionary functions

Functions	description
clear	deletes all the items in the dictionary, similar to reassigning it to `dict()`
copy	returns a copy of the dictionary
get( `key` , `default value` )	returns a value of that key just like dict_object[key] but returns default value when key doesn't exist. This does not update the dictionary
items	returns the items in the dictionary
values	returns the values of the dictionary
keys	returns the keys of the dictionary
pop( `key` )	deletes item with the key just like del
popitem	deletes the last item in the dictionary
setdefault( `key` , `value` )	adds key value to dictionary if key doesn't exist, unlike `get`

Examples

# empty dict
profile = dict()

# add an item - 3 ways use any
profile['name'] = 'John Doe'  # we use this more - simplier

profile.update(age=32)

profile.setdefault('job', 'Software engineer')

print(profile)

# get an item from the dict

# get the keys from a dictionary
profile_keys = profile.keys()
print(profile_keys)
# output-> dict_keys(['name', 'age', 'job'])

# get the values from a dictionary
profile_values = profile.values()
print(profile_values)
# output-> dict_values(['John Doe', 32, 'Software engineer'])

# get the key and value as items
profile_content = profile.items()
print(profile_content)
# do the printing

# get element by key
username = profile['name']
print(f"user name: {username}")

# what if the key doesn't exist
# use get with default value
# profile['height'] -> KeyError
height = profile.get('height', 130)
print(height)  # -> 130

# but height won't be added to the dict
# use set default, update or dict[key] = value
if not 'height' in profile.keys():
    # any of this would work
    profile['height'] = 120
    # profile.update(height = 120)
    # profile.setdefault('height', 120)
else:
    print('Profile updated, height added')

print(profile)

# copy
new_profile = profile.copy()
print(new_profile)

# pop - remove height
profile.pop('height')  # or
# del profile['height']

# delete all items in the dict
profile.popitem()

print(len(new_profile) == len(profile))

Looping through a dictionary

Looping is the same everywhere in a list, set and tuple, in even a string, but for a dictionary, we may loop using a key or and value.

# consider this sample dictionary
profile = {
    'name': 'John Doe',
    'age': 32,
    'job': 'Software engineer'
}

# looping through keys
for key in profile.keys():
    print(f"key: {key}")

# looping through values
for value in profile.values():
    print(f"value: {value}")

# looping through the items in the dictionary
# we loop through the key and value at the same time
for key, value in profile.items():
    print(f"{key} has a value of {value}")

Practicals

Write a function that removes items with duplicate values
Write a function that takes a dictionary as an argument, return another dictionary that has the frequency of the length of the value, if value is int or float, frequency is the number of digits. Keep the keys of the old as the new.

Summary

is a key-value pair sequence
is of structure, my_dict = {key:value}
dict(name='name') casts to a dict
dict_obj[ key ] returns the value at key
dict_obj[key] = value to create new item in dictionary or update
loop through dictionary by keys, values and items

Python3 Programming - Exercise 17 - Dictionary