Python dictionary (Avoiding Mistakes)

What is dict in python ? 
Python dictionary is similar to hash table in languages like C++. Dictionary are used to create a key value pair in python. In place of key there can be used String Number and Tuple etc. In place of values there can be anything. Python Dictionary is represented by curly braces. An empty dictionary is represented by {}. In Python dictionary key and values are separated by ‘:’ and key values pair are separated by ‘, ‘ . Below example explains it clearly 
Example 

Python3

# program to understand dictionary in python

# creating an empty dictionary
mydictionary ={}

# inserting values in dictionary
mydictionary ={'name':'Ankit',
               'college':'MNNIT',
               'address':'Allahabad'};

# print the dictionary
print(mydictionary)

Output : 

  {'address': 'Allahabad', 'name': 'Ankit', 'college': 'MNNIT'}

where is used ? 
Dictionaries gives us power to model a variety of real world applications. We can create a dictionary of person where we can store all information related to that person as name age contact location etc. In dictionaries can be store any type of information such as words and their meaning. Python dictionaries are used very much in machine learning where machine talks with human in that situation some predefined words are stored as keys and their meaning as values and when user want anything that thing is searched in keys if it is found then its value is returned otherwise it shows some error. In fact python dictionary can be used anywhere where hashing is used in normal old language. 
Example to understand use of dict 

Python3

# program to understand dictionary in python

# creating an empty dictionary
mydictionary ={}

# inserting values in dictionary
mydictionary ={'greeting':'Hello',
               'status':'how are you',
               'thanks':'thanks visit again'};

# print values according to choice
print(mydictionary['greeting'])
print(mydictionary['status'])
print(mydictionary['thanks'])

Output : 

Hello
how are you
thanks visit again

Common mistakes while using dicts and overcomes
Following are some mistake while using dict in python. 
1 ) To access element of dictionary in python never direct access element using key name always try to use .get method. If key is not present then .get method would print none while [key] will terminate the whole program. 

Python3

# program to understand dictionary in python

# creating an empty dictionary
mydictionary ={}

# inserting values in dictionary
mydictionary ={'greeting':'Hello',
               'status':'how are you',
               'thanks':'thanks visit again'};

# print values according to choice
print(mydictionary['greeting'])
print(mydictionary['status'])
print(mydictionary['thanks'])

# this will print none
print(mydictionary.get('college'))

# this will throw an error
print(mydictionary['college'])

Output : 

Hello
how are you
thanks visit again
None

Runtime Error 

 Traceback (most recent call last):
  File "/home/ce65dd34285f0cb0781de2a068e658fa.py", line 14, in 
    print(mydictionary['college'])
KeyError: 'college'

2 ) When we want to copy a dictionary to another dictionary then there should be proper knowledge of copying method 

• new_dictionary = old_dictionary : This line means that old_dictionary and new_dictionary will refer to same object means that change in one dictionary will reflect to other dictionary.
• new_dictionary = dict(old_dictionary) and new_dictionary = old_dictionary.copy() : will copy old dictionary to new dictionary means that update in old will not reflect update in d but values in e will be copied by using references . This will perform shallow copy
• new_dictionary = copy.deepcopy(old_dictionary) : This will generate an deep copy .

Python3

# program to understand dictionary in python

# creating an empty dictionary
mydictionary ={}

# inserting values in dictionary
mydictionary ={'greeting':'Hello',
               'status':'how are you',
               'thanks':'thanks visit again'};

# print values according to choice
print(mydictionary['greeting'])
print(mydictionary['status'])
print(mydictionary['thanks'])

# copying dictionary
m = mydictionary
print(m)

Output : 

Hello
how are you
thanks visit again
{'greeting': 'Hello', 'status': 'how are you', 'thanks': 'thanks visit again'}

When not to use dicts ? 
Python dict are useful in many situation but in some situation their use must be avoided .In python never think that only dictionary are associative array. In dict we should try to store values of same type. 

• If we want to search values whether values is present into dictionary or not always use in that situation python set because in python set is an associative array with bool values whether element is present or not.
• For fixed number of attribute always use Class or named tuple in python .
• When you want a predefined message when a particular key then use collections.defaultdict in python