Dictionaries

A Python dictionary is a mutable data structure that can be used to associate keys with values. They are created using {} braces. You can think of dictionaries as lists, except that instead of extracting elements by their position you extract them using their keys. One way of creating them follows.



In [1]:

    
numbers = {1: "one", 2: "two", 3: "three"}
numbers









    Out[1]:





{1: 'one', 2: 'two', 3: 'three'}

In the numbers dictionary the keys are 1, 2 and 3 and their corresponding values are "one", "two" and "three". Extracting a value from a dictionary is very similar to extracting an element from a list.



In [2]:

    
numbers[2]









    Out[2]:





'two'

Creating Dictionaries and Extracting Elements

The power of dictionary comes from the fact that keys don't have to be numbers. They can be any immutable data structure.



In [3]:

    
elements = {
    "H": "Hydrogen",
    "He": "Helium",
    "Li": "Lithium",
}
elements









    Out[3]:





{'H': 'Hydrogen', 'He': 'Helium', 'Li': 'Lithium'}



In [4]:

    
elements["H"]









    Out[4]:





'Hydrogen'



In [5]:

    
houses = {
    ("Harry", "Potter"): "Gryffindor",
    ("Hermione", "Granger"): "Gryffindor",
    ("Draco", "Malfoy"): "Slytherin",
    ("Cho", "Chang"): "Ravenclaw"
}
houses









    Out[5]:





{('Cho', 'Chang'): 'Ravenclaw',
 ('Draco', 'Malfoy'): 'Slytherin',
 ('Harry', 'Potter'): 'Gryffindor',
 ('Hermione', 'Granger'): 'Gryffindor'}



In [6]:

    
houses[("Harry", "Potter")]









    Out[6]:





'Gryffindor'

Adding and Removing Elements

Dictionaries are mutable so just like lists you can add and remove new mappings.



In [7]:

    
# Adding a mapping.
elements["Au"] = "Gold"
elements









    Out[7]:





{'Au': 'Gold', 'H': 'Hydrogen', 'He': 'Helium', 'Li': 'Lithium'}



In [8]:

    
# Removing a mapping.
elements.pop("H")









    Out[8]:





'Hydrogen'



In [9]:

    
elements









    Out[9]:





{'Au': 'Gold', 'He': 'Helium', 'Li': 'Lithium'}

Much like with lists, attempting to access an element that doesn't exist will result in an error.



In [10]:

    
elements["Ag"]









    



---------------------------------------------------------------------------
KeyError                                  Traceback (most recent call last)
<ipython-input-10-c5571bb0bb35> in <module>()
----> 1 elements["Ag"]

KeyError: 'Ag'

It is possible to check if a key is in a dictionary.



In [11]:

    
("Harry", "Potter") in houses









    Out[11]:





True



In [12]:

    
("Ronald", "Weasley") in houses









    Out[12]:





False