Dictionaries

Dictionaries in python are Mappings (or associations). Means they store values in key and value associations (pair).

Example: 'color' : 'Blue'

Here color is the key and Blue is the value. notice that key and value are seperated by :. These key value pairs are enclosed in { and }.

You can consider them as associative arrays in some languages.



In [1]:

    
myDict = {'color':'blue', 'taste':'sweet'}



In [2]:

    
myDict









    Out[2]:





{'color': 'blue', 'taste': 'sweet'}

We can access the elements of a dictionary using index, but here we do not use number indexes. Instead we use the key as index to access the value.



In [3]:

    
myDict['taste']









    Out[3]:





'sweet'

The values in a dictionary can be any python object. It can be a integer, floating point, string, List or even another dictionary.



In [4]:

    
myDict = {'first':123,'second':3.1415,'third':'hello world!'}



In [5]:

    
myDict









    Out[5]:





{'first': 123, 'second': 3.1415, 'third': 'hello world!'}



In [6]:

    
myDict['second']









    Out[6]:





3.1415



In [7]:

    
myDict['third']









    Out[7]:





'hello world!'

You can use any function on the value in a dictionary.



In [8]:

    
myDict['third'].upper()









    Out[8]:





'HELLO WORLD!'



In [9]:

    
# Accessing a single character
myDict['third'][0]









    Out[9]:





'h'



In [10]:

    
# Slicing the string
myDict['third'][0:5]









    Out[10]:





'hello'



In [11]:

    
# Reversing the string
myDict['third'][::-1]









    Out[11]:





'!dlrow olleh'



In [12]:

    
myDict['second']*2









    Out[12]:





6.283

All these are temporary operations, they dont change the dictionary values permanently.



In [13]:

    
myDict









    Out[13]:





{'first': 123, 'second': 3.1415, 'third': 'hello world!'}

Creating Dictionary through assignment.

We can create a dictionary by assignment also. As with lists, dictionary are also not of fixed size. You can add any number of values to a dictionary.



In [14]:

    
d = {}

Now we have created a dictionary which doesn't has any values. An empty dictionary.



In [15]:

    
d









    Out[15]:





{}

We can insert a value directly into the dictionary by using its key.



In [16]:

    
d['element'] = 'Oxygen'



In [17]:

    
d['weight'] = 15.999



In [18]:

    
d









    Out[18]:





{'element': 'Oxygen', 'weight': 15.999}

We can put a list as a value in a dictionary.



In [19]:

    
d['list'] = [1,2,3]



In [20]:

    
d









    Out[20]:





{'element': 'Oxygen', 'list': [1, 2, 3], 'weight': 15.999}



In [21]:

    
d['list'][1]









    Out[21]:





2

Nested Dictionaries

Like lists and any other objects, dictionaries can also be nested.



In [22]:

    
nestDict = {'d1': {'element':'Oxygen', 'weight':15.999}, 'd2':{'element': 'Hydrogen', 'weight':1}}



In [23]:

    
nestDict









    Out[23]:





{'d1': {'element': 'Oxygen', 'weight': 15.999},
 'd2': {'element': 'Hydrogen', 'weight': 1}}



In [24]:

    
nestDict['d1']['element']









    Out[24]:





'Oxygen'



In [25]:

    
nestDict['d2']['weight']









    Out[25]:





1

Basic functions for Dictionaries



In [28]:

    
# Keys in a dictionary.
myDict.keys()









    Out[28]:





dict_keys(['first', 'second', 'third'])



In [29]:

    
# Values in a dictionary.
myDict.values()









    Out[29]:





dict_values([123, 3.1415, 'hello world!'])



In [36]:

    
myDict.items()









    Out[36]:





dict_items([('first', 123), ('second', 3.1415), ('third', 'hello world!')])

Python offers many functions on dictionaries like clear(), copy(), fromkeys(), get(), pop(), popitem(), setdefault(), update().

You can also use len() function on a dictionary.



In [38]:

    
len(myDict)









    Out[38]:





3