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DICTIONARIES IN PYTHON

This mark down file helps understand DICTIONARIES in PYTHON.

📋 DICTIONARIES

  • Dictionary in Python is an unordered collection of data values, used to store data values like a map, which unlike other Data Types that hold only single value as an element, Dictionary holds key:value pair. Key value is provided in the dictionary to make it more optimized.

  • Each key is separated from its value by a colon (:), the items are separated by commas, and the whole thing is enclosed in curly braces. An empty dictionary without any items is written with just two curly braces, like this: {}.

  • Keys are unique within a dictionary while values may not be. The values of a dictionary can be of any type, but the keys must be of an immutable data type such as strings, numbers, or tuples.

# Example
thisdict = {
"Company": "Maruti",
"model": "Swift",
"year": 2020
}
print(thisdict)
# Output:
{'Company': 'Maruti', 'model': 'Swift', 'year': 2020}

# Example 2:
dict = {'Name': 'Zara', 'Age': 7, 'Class': 'First'}
print("dict['Name']: ", dict['Name'])
print("dict['Age']: ", dict['Age'])
# Output:
dict['Name']:  Zara
dict['Age']:  7

📋 PROPERTIES OF DICTIONARY

Dictionary values have no restrictions. They can be any arbitrary Python object, either standard objects or user-defined objects. However, same is not true for the keys.

There are two important points to remember about dictionary keys:

  1. More than one entry per key not allowed. Which means no duplicate key is allowed. When duplicate keys encountered during assignment, the last assignment wins. For example:
thisdict = {
"Company": "Maruti",
"model": "Swift",
"year": 2020,
 "model": "WagonR"   #then the value which is last assigned is considered as the value of the key.
}
print(thisdict)
# Output:
{'Company': 'Maruti', 'model': 'WagonR', 'year': 2020}
  1. Keys must be immutable. Which means you can use strings, numbers or tuples as dictionary keys but something like ['key'] is not allowed. Following is a simple example:
dict = {['Name']: 'Zara', 'Age': 7}
print("dict['Name']: ", dict['Name'])
# When the above code is executed, it produces the following result which will be an error
---------------------------------------------------------------------------

TypeError                                 Traceback (most recent call last)

<ipython-input-10-bd5391ecc03c> in <module>()
----> 1 dict = {['Name']: 'Zara', 'Age': 7}
      2 print("dict['Name']: ", dict['Name'])
      3 # When the above code is executed, it produces the following result which will be an error


TypeError: unhashable type: 'list'

📋 ACCESSING ITEMS

  • You can access the items of a dictionary by referring to its key name, inside square brackets:
#Example1:
thisdict = {
"Company": "Maruti",
"model": "Swift",
"year": 2020
}
x = thisdict["model"]
print(x)
# Output:
Swift

#Example2:
dict = {'Name': 'Zara', 'Age': 7, 'Class': 'First'}
print("dict['Name']: ", dict['Name'])
print("dict['Age']: ", dict['Age'])
# Output:
dict['Name']:  Zara
dict['Age']:  7

#📋 UPDATING DICTIONARY

  • You can update a dictionary by adding a new entry or a key-value pair, modifying an existing entry, or deleting an existing entry as shown below in the simple example:
# Example:
dict = {'Name': 'Zara', 'Age': 7, 'Class': 'First'}
dict['Age'] = 8; # update existing entry
dict['School'] = "DPS School"; # Add new entry

print("dict['Age']: ", dict['Age'])
print("dict['School']: ", dict['School'])
# Output:
dict['Age']:  8
dict['School']:  DPS School

#📋 LOOP THROUGH A DICTIONARY

  • You can loop through a dictionary by using a for loop.
  • When looping through a dictionary, the return value are the keys of the dictionary, but there are methods to return the values as well.
#Example1
thisdict =	{
  "brand": "Ford",
  "model": "Mustang",
  "year": 1964
}
for x in thisdict.values():
  print(x)

#Output  
Ford
Mustang
1964

#Example2
## You can also use the values() method to return values of a dictionary:
statesAndCapitals = {
                     'Gujarat' : 'Gandhinagar',
                     'Maharashtra' : 'Mumbai',
                     'Rajasthan' : 'Jaipur',
                     'Bihar' : 'Patna'
                    }
                      
print('List Of given capitals:\n')
  
# Iterating over values
for capital in statesAndCapitals.values():
    print(capital)
#Output   
List Of given capitals:

Gandhinagar
Mumbai
Jaipur
Patna

#Example3
#Loop through both keys and values, by using the items() method:
statesAndCapitals = {
                     'Gujarat' : 'Gandhinagar',
                     'Maharashtra' : 'Mumbai',
                     'Rajasthan' : 'Jaipur',
                     'Bihar' : 'Patna'
                    }
                      
print('List Of given states and their capitals:\n')
  
for state, capital in statesAndCapitals.items():
    print(state, ":", capital)
 #Output   
List Of given states and their capitals:

Gujarat : Gandhinagar
Maharashtra : Mumbai
Rajasthan : Jaipur
Bihar : Patna

#📋 DICTIONARY LENGTH

  • To determine how many items (key-value pairs) a dictionary has, use the len() function.
#Example1
dict1 ={'Name':'Steve', 'Age':30, 'Designation':'Programmer'}
  
# using len() counts the
# number of entries
print("len() method :", len(dict1))
#OutPut
len() method : 3

#Example2
dict1 ={'Name':'Steve', 'Age':30, 'Designation':'Programmer'}
# using dict.keys() counts
# the no.of.keys in dictionary
print("len() method with keys() :", len(dict1.keys()))
#Output
len() method with keys() : 3

#Example3
dict1 ={'Name':'Steve', 'Age':30, 'Designation':'Programmer'}
# using dict.values() counts
# the no.of.valuess in dictionary
print("len() method with values():", len(dict1.values()))
#OutPut
len() method with values(): 3

#📋 ADD NEW KEYS TO A DICTIONARY

#Method1
dict = {'FirstName':'Rohit', 'LastName':'Sharma'}
print("Current Dict is: ", dict)
# using the subscript notation
# Dictionary_Name[New_Key_Name] = New_Key_Value
dict['FirstName'] = 'Mahendra'
dict['LastName'] = 'Singh Dhoni'
print("Updated Dict is: ", dict)
#Output
Current Dict is:  {'FirstName': 'Rohit', 'LastName': 'Sharma'}
Updated Dict is:  {'FirstName': 'Mahendra', 'LastName': 'Singh Dhoni'}

#Method2
#Using update() method
dict = {'FirstName':'Rohit', 'LastName':'Sharma'}
dict.update({'Gender':'Male'})
print("Updated Dict ",dict)
#Output
Updated Dict  {'FirstName': 'Rohit', 'LastName': 'Sharma', 'Gender': 'Male'}

#Method3
#Using __setitem__
dict = {'FirstName':'Rohit', 'LastName':'Sharma'}
dict.__setitem__('Height', "5'9")
print("Updated Dict",dict)
#Output
Updated Dict {'FirstName': 'Rohit', 'LastName': 'Sharma', 'Height': "5'9"}

#📋 REMOVING ITEMS

  • There are several methods to remove items from a dictionary:
#Method1
test_dict = {"Arushi" : 22, "Anuradha" : 21, "Mani" : 21, "Haritha" : 21}
  
# Using del to remove a dict
# removes Mani
del test_dict['Mani']
print(test_dict)
#Output
{'Arushi': 22, 'Anuradha': 21, 'Haritha': 21}

#Method2
test_dict = {"Arushi" : 22, "Anuradha" : 21, "Mani" : 21, "Haritha" : 21}
  
# Using pop() to remove a dict. pair
# removes Anuradha
removed_value = test_dict.pop('Anuradha')
print(test_dict)
#Output
{'Arushi': 22, 'Mani': 21, 'Haritha': 21}

#Method3
test_dict = {"Arushi" : 22, "Anuradha" : 21, "Mani" : 21, "Haritha" : 21}
  
# Using items() + dict comprehension to remove a dict. pair
# removes Arushi
new_dict = {key:val for key, val in test_dict.items() if key != 'Arushi'}
print(new_dict)
#Output
{'Anuradha': 21, 'Mani': 21, 'Haritha': 21}

#📋 COPY A DICTIONARY

  • They copy() method returns a shallow copy of the dictionary
#Example1
# of dictionary copy
original = {1:'Webdev', 2:'Machine Learning'}
  
# copying using copy() function
new = original.copy()
  
# removing all elements from the list
# Only new list becomes empty as copy()
# does shallow copy.
new.clear() 
  
print('new: ', new)
print('original: ', original)
#Output
new:  {1: 'Webdev', 2: 'Machine Learning'}
original:  {1: 'Webdev', 2: 'Machine Learning'}

#Example2
#Unlike copy(), the assignment operator does deep copy.
original = {1:'Webdev', 2:'Machine Learning'}
  
# copying using copy() function
new = original.copy()
  
# removing all elements from new list
# and printing both
new.clear()
print('new: ', new)
print('original: ', original)
  
original = {1:'andriodDev', 2:'Data Science'}
  
# copying using =
new = original
  
# removing all elements from new list
# and printing both
new.clear()
print('new: ', new)
print('original: ', original)
#Output
new:  {}
original:  {1: 'Webdev', 2: 'Machine Learning'}
new:  {}
original:  {}

##📋 BUILT-IN METHODS OF DICTIONARY

Methods Description
clear() Removes all items from the dictionary.
copy() Returns a shallow copy of the dictionary.
fromkeys(seq[, v]) Returns a new dictionary with keys from seq and value equal to v (defaults to None).
get(key[,d]) Returns the value of the key. If the key does not exist, returns d (defaults to None).
items() Return a new object of the dictionary's items in (key, value) format.
keys() Returns a new.
all() Return True if all keys of the dictionary are True (or if the dictionary is empty).
any() Return True if any key of the dictionary is true. If the dictionary is empty, return False.
len() Return the length (the number of items) in the dictionary.
cmp() Compares items of two dictionaries. (Not available in Python 3)
sorted() Return a new sorted list of keys in the dictionary.