A simple dictionary

ailen_0 = {'color':'green','point':5}

print(alien_0['color'])
print(alien_0['points'])

Dictionary alien_0 stores alien colors and scores

Use dictionary

A dictionary is a series of key value pairs. The values associated with a key can be numbers, strings, lists, or dictionaries
In python, dictionaries are represented by a series of key value pairs enclosed in curly braces ({})
Key value pairs are pairs of two associated values. When a key is specified, the value associated with it is returned, separated by a colon between the key and value, and separated by a comma between key value pairs

Accessing values in the dictionary

Add key value pair

alien_0['x_position'] = 0
alien_0['y_position'] = 0

Add two key value pairs

Modify values in the dictionary

alien_0 = {'color':'green'}
print(f"the alien is {alien_0['color']}.")

alien_0['color' = 'yellow']
print(f"the alien is now {alien_0['color']}.")

Change the value associated with 'color' to 'yellow'

Delete key value pair

Using the del statement, you must specify the dictionary name and the key to delete

alien_0 = {'color': 'green' , 'points':5}
del alien_0['points']

A dictionary of similar objects

fav_lan = { 
	'jan':'python',
	'sarah':'c',
	'edward':'ruby',
	'phil':'python',
	}

language = fav_lan['sarah'].title()
print(f"sarah favorite language is {language}.")

Use get() to access values

An error will be reported when the specified key accessed from the dictionary does not exist
Show traceback
So the method get() is used to return a default value when the specified key does not exist

alien_0 = {'color':'green','speed':'slow'}

point_value = alien_0.get('points','no point value assiged.')
print(point_value)

Output result: no point value assigned

Traversal dictionary

Traverse all key value pairs

Use for loop

user = {
	'username':'efermi',
	'first':'enrico',
	'last':'femi',
	}

for key,value in user.items():
	print(f"\nKey:{key}")
	print(f"Value:{value}")

Output results

Key:username
Value:efermi

Key:first
Value:enrico

Key:last
Value:femi

Traverse all values in the dictionary

The method keys() is useful when you do not need to use the values in the dictionary

fav_lan = { 
	'jan':'python',
	'sarah':'c',
	'edward':'ruby',
	'phil':'python',
	}

for name in fav_lan.keys():
	print(name.title())

The last penultimate line can also be written as
for name in fav_lan:
Output unchanged

Traverse all keys in the dictionary in a specific order

Sort the returned values in the for loop, and use the function sorted() to get a copy of the list of keys in a specific order

fav_lan = { 
	'jan':'python',
	'sarah':'c',
	'edward':'ruby',
	'phil':'python',
	}

for name in sorted(fav_lan.keys()):
	print(f"{name.title()},thank you for taking the poll.")

sorted() uses ASCII code for comparison. If the first character is the same, the next one is compared

Traverse all values in the dictionary

Use the method values() to return a list of values without any keys

fav_lan = { 
	'jan':'python',
	'sarah':'c',
	'edward':'ruby',
	'phil':'python',
	}

for language in fav_lan.values():
	print(language.title())

To eliminate duplicates, you can use the set. Each element in the set must be unique

fav_lan = { 
	'jan':'python',
	'sarah':'c',
	'edward':'ruby',
	'phil':'python',
	}

for language in set(fav_lan.values()):
	print(language.title())

Simpler usage

languages = {'python','ruby','c','python'}
languages
{'python','ruby','c'}

nesting

Storing a series of dictionaries in a list or a group of lists as values in a dictionary is called nesting

Dictionary list

Create a list of three alien s

alien_0 = {'color':'green','points':5}
alien_1 = {'color':'yellow','points':10}
alien_2 = {'color':'red','points':15}

aliens = [alien_0,alien_1,alien_2]
for alien in aliens:
	print(alien)

In reality, there are more than three aliens. For example, the following uses range() to generate 30 aliens with the same characteristics

# Create a storage empty list
aliens = []

# Create 30
for alien_number in range(30):
    new_alien = {'color':'green','points':5,'speed':'slow'}
    aliens.append(new_alien)

# Show top five
for alien in aliens[:5]:
    print(alien)

# Displays how many have been created
print(f"total number of aliens:{len(aliens)}")

Modify some data

for alien in aliens[:3]:
    if alien['color'] == 'green':
        alien['color'] == 'yellow'
        alien['speed'] == 'medium'
        alien['points'] = 10
    elif alien['color'] == 'yellow':
        alien['color'] == 'red'
        alien['speed'] == 'fast'
        alien['points'] = 15

Store list in dictionary

Sometimes you need to store a list in a dictionary

# Store information about pizza ordered
pizza = {
    'crust':'thick',
    'toppings':['mushrooms','extra cheese'],
}

# Overview of pizza ordered
print(f"you ordered a {pizza['crust']}-crust pizza "
    "with the following toppings:")

for topping in pizza['toppings']:
    print("\t" + topping)

To print ingredients, write a for loop. To access the list of ingredients, use the key 'toppings'

You can nest a list in the dictionary whenever you need to associate a key to multiple values in the dictionary

fav_lan = { 
	'jan':['python','ruby'],
	'sarah':['c'],
	'edward':['ruby','go'],
	'phil':['python','haskell'],
	}

for name,languages in fav_lan.items():
	print(f"\n{name.title()}'s favorite languages are:")
	for language in languages:
		print(f"\t{language.title()}")

Now the value associated with each name is a list. In the main loop of traversing the dictionary, another for loop is used to traverse the list of languages everyone likes

Store dictionary in dictionary

The code can be complicated

users = {
    'aeinstein':{
        'first':'albert',
        'last':'einstein',
        'location':'princetion',
    },

    'mcurie':{
        'first':'marie',
        'last':'curie',
        'location':'paris',
    },
}

for username,user_info in users.items():
    print(f"\nUsername:{username}")
    full_name = f"{user_info['first']}{user_info['last']}"
    location = user_info['location']

    print(f"\tFull name:{full_name.title()}")
    print(f"\tLocation:{location.title()}")

First, define a user dictionary, which contains two keys' aeinstein 'and' mcurie '
The value associated with each key is a dictionary, then traverse users, and then start accessing the internal dictionary, the variable user_info contains the user information dictionary