Not enough information!

instructor = ["Colt", True, 4, "Python", False]

We want to describe this data in more detail! 

A dictionary!

A data structure that consists of key value pairs.

We use the keys to describe our data and the values to represent the data

keys and values separated by a colon

instructor = {
    "name": "Colt",
    "owns_dog": True,
    "num_courses": 4,
    "favorite_language": "Python",
    "is_hilarious": False,
    44: "my favorite number!"
}

Our keys are almost always numbers or strings 

Our values can be anything!

Another approach is to use the dict function. You assign values to keys by passing in keys and values separated by an =

another_dictionary = dict(key = 'value')
another_dictionary # {'key': 'value'}

instructor = {
    "name": "Colt",
    "owns_dog": True,
    "num_courses": 4,
    "favorite_language": "Python",
    "is_hilarious": False,
    44: "my favorite number!"
}

There is a more forgiving dictionary method we'll learn about later! 

instructor["name"] # "Colt"

instructor["thing"] # KeyError

instructor = {
    "name": "Colt",
    "owns_dog": True,
    "num_courses": 4,
    "favorite_language": "Python",
    "is_hilarious": False,
    44: "my favorite number!"
}

print(instructor["name"])
print(instructor["owns_dog"])
print(instructor["num_courses"])
print(instructor["favorite_language"])
print(instructor["is_hilarious"])
print(instructor[44])

No, no, no......

Use .values()

instructor = {
    "name": "Colt",
    "owns_dog": True,
    "num_courses": 4,
    "favorite_language": "Python",
    "is_hilarious": False,
    44: "my favorite number!"
}

for value in instructor.values():
    print(value)

# "Colt"
# True
# 4
# "Python"
# False
# "my favorite number!"

Use .keys()

instructor = {
    "name": "Colt",
    "owns_dog": True,
    "num_courses": 4,
    "favorite_language": "Python",
    "is_hilarious": False,
    44: "my favorite number!"
}

for key in instructor.keys():
    print(key)

# name
# owns_dog
# num_courses
# favorite_language
# is_hilarious
# 44

Use .items()

for key,value in instructor.items():
    print(key,value)

# name "Colt"
# owns_dog True
# num_courses 4
# favorite_language "Python"
# is_hilarious False
# 44 "my favorite number!"

instructor = {
    "name": "Colt",
    "owns_dog": True,
    "num_courses": 4,
    "favorite_language": "Python",
    "is_hilarious": False,
    44: "my favorite number!"
}

Does a dictionary have a key?

"Colt" in instructor.values() # True
"Nope!" in instructor.values() # False

"name" in instructor # True
"awesome" in instructor # False

Does a dictionary have a value?

instructor = {
    "name": "Colt",
    "owns_dog": True,
    "num_courses": 4,
    "favorite_language": "Python",
    "is_hilarious": False,
    44: "my favorite number!"
}

Dictionaries are a fundamental data structure for organizing and describing data into key-value pairs

Just remember, there's no ordering!

Working with dictionaries is very common - there are quite a few things we can do!

d = dict(a=1,b=2,c=3)
d.clear()
d # {}

Clears all the keys and values in a dictionary:

d = dict(a=1,b=2,c=3)
c = d.copy()
c # {'a': 1, 'b': 2, 'c': 3}
c is d # False

Makes a copy of a dictionary

{}.fromkeys("a","b") # {'a': 'b'}
{}.fromkeys(["email"], 'unknown') # {'email': 'unknown'}
{}.fromkeys("a",[1,2,3,4,5]) # {'a': [1, 2, 3, 4, 5]}

Creates key-value pairs from comma separated values:

d = dict(a=1,b=2,c=3)
d['a'] # 1
d.get('a') # 1
d['b'] # 2
d.get('b') # 2
d['no_key'] # KeyError
d.get('no_key') # None

Retrieves a key in an object and return None instead of a KeyError if the key does not exist:

d = dict(a=1,b=2,c=3)
d.pop() # TypeError: pop expected at least 1 arguments, got 0
d.pop('a') # 1
d # {'c': 3, 'b': 2}
d.pop('e') # KeyError

Takes a single argument corresponding to a key and removes that key-value pair from the dictionary. Returns the value corresponding to the key that was removed. 

d = dict(a=1,b=2,c=3,d=4,e=5)
d.popitem() # ('d', 4)
d.popitem('a') # TypeError: popitem() takes no arguments (1 given)

Removes a random key in a dictionary:

first = dict(a=1,b=2,c=3,d=4,e=5)
second = {}

second.update(first)
second # {'a': 1, 'b': 2, 'c': 3, 'd': 4, 'e': 5}

# let's overwrite an existng key
second['a'] = "AMAZING"

# if we update again
second.update(first) # {'a': 1, 'b': 2, 'c': 3, 'd': 4, 'e': 5}

# the update overrides our values
second # {'a': 1, 'b': 2, 'c': 3, 'd': 4, 'e': 5}

Update keys and values in a dictionary with another set of key value pairs.

{ ____:____ for ___ in ____}

- iterates over keys by default

- to iterate over keys and values using .items()

numbers = dict(first=1, second=2, third=3)

squared_numbers = {key: value ** 2 for key,value in numbers.items()}

print(squared_numbers) # {'first': 1, 'second': 4, 'third': 9}

str1 = "ABC"
str2 = "123"
combo = {str1[i]: str2[i] for i in range(0,len(str1))} 
print(combo) # # {'A': '1', 'B': '2', 'C': '3'}

{num: num**2 for num in [1,2,3,4,5]}

num_list = [1,2,3,4]

{ num:("even" if num % 2 == 0 else "odd") for num in num_list }

# {1: 'odd', 2: 'even', 3: 'odd', 4: 'even'}