#!/usr/bin/env python
# coding: utf-8

# # Dictionaries
# 
# We've been learning about *sequences* in Python but now we're going to switch gears and learn about *mappings* in Python. If you're familiar with other languages you can think of these Dictionaries as hash tables. 
# 
# This section will serve as a brief introduction to dictionaries and consist of:
# 
#     1.) Constructing a Dictionary
#     2.) Accessing objects from a dictionary
#     3.) Nesting Dictionaries
#     4.) Basic Dictionary Methods
# 
# So what are mappings? Mappings are a collection of objects that are stored by a *key*, unlike a sequence that stored objects by their relative position. This is an important distinction, since mappings won't retain order since they have objects defined by a key.
# 
# A Python dictionary consists of a key and then an associated value. That value can be almost any Python object.
# 
# 
# ## Constructing a Dictionary
# Let's see how we can construct dictionaries to get a better understanding of how they work!

# In[1]:


# Make a dictionary with {} and : to signify a key and a value
my_dict = {'key1':'value1','key2':'value2'}


# In[2]:


# Call values by their key
my_dict['key2']


# Its important to note that dictionaries are very flexible in the data types they can hold. For example:

# In[13]:


my_dict = {'key1':123,'key2':[12,23,33],'key3':['item0','item1','item2']}


# In[4]:


#Lets call items from the dictionary
my_dict['key3']


# In[5]:


# Can call an index on that value
my_dict['key3'][0]


# In[7]:


#Can then even call methods on that value
my_dict['key3'][0].upper()


# We can effect the values of a key as well. For instance:

# In[14]:


my_dict['key1']


# In[15]:


# Subtract 123 from the value
my_dict['key1'] = my_dict['key1'] - 123


# In[16]:


#Check
my_dict['key1']


# A quick note, Python has a built-in method of doing a self subtraction or addition (or multiplication or division). We could have also used += or -= for the above statement. For example:

# In[17]:


# Set the object equal to itself minus 123 
my_dict['key1'] -= 123
my_dict['key1']


# We can also create keys by assignment. For instance if we started off with an empty dictionary, we could continually add to it:

# In[21]:


# Create a new dictionary
d = {}


# In[22]:


# Create a new key through assignment
d['animal'] = 'Dog'


# In[24]:


# Can do this with any object
d['answer'] = 42


# In[25]:


#Show
d


# ## Nesting with Dictionaries
# 
# Hopefully you're starting to see how powerful Python is with its flexibility of nesting objects and calling methods on them. Let's see a dictionary nested inside a dictionary:

# In[26]:


# Dictionary nested inside a dictionary nested in side a dictionary
d = {'key1':{'nestkey':{'subnestkey':'value'}}}


# Wow! That's a quite the inception of dictionaries! Let's see how we can grab that value:

# In[29]:


# Keep calling the keys
d['key1']['nestkey']['subnestkey']


# ## A few Dictionary Methods
# 
# There are a few methods we can call on a dictionary. Let's get a quick introduction to a few of them:

# In[30]:


# Create a typical dictionary
d = {'key1':1,'key2':2,'key3':3}


# In[35]:


# Method to return a list of all keys 
d.keys()


# In[36]:


# Method to grab all values
d.values()


# In[33]:


# Method to return tuples of all items  (we'll learn about tuples soon)
d.items()


# Hopefully you now have a good basic understanding how to construct dictionaries. There's a lot more to go into here, but we will revisit dictionaries at later time. After this section all you need to know is how to create a dictionary and how to retrieve values from it.