Python Dictionary

Introduction

• It is one to one relation with the key to value but not value to key.
• Dictionaries are Mutable and unordered data structures
• Values of a dictionary can be any type (list, function, strings etc),
  but the keys must be of an immutable (numbers, strings, tuples etc) data type
• If the keys will be mutable as there will be a chance of key duplication that will creating the confusion

Define a Dictionary

Dictionaries are written within curly brackets {}

 > mydict = {}   Empty Dictionary
 > mydict = {1:"one", 2:"two", 3:"three"}  Dictionary with items
 > mydict = dict() We can also create a dictionary object by calling its python built in dict() keyword

We can check the type of a dictionary using

print(type(mydict)) --> <class 'dict'>

Access values from a dictionary

We can access dictionary values using square brackets [ ]
The below code uses the key python to access the value codementor

mydict = {'python':"codementor", 'good':"bad", 'hi':"hello"}
print(mydict['python'])

Output: codementor

If the key doesnot exist it will give KeyError. This error tells that there is no key called "python1" in the dictionary

print(mydict['python1'])

Output: 
Traceback (most recent call last):
  File "C:/dictionary.py", line 2, in <module>
    print(mydict['python1'])
KeyError: 'python1'

If we want to avoid this error we can use get() method. It will check the dictionary, if it doesn't contain the key it will give None as output

mydict = {0:"codementor", 1:"bad", 'hi':"hello"}
print(mydict.get(2))

Output:   None

Update keys

Updating the keys means adding keys to a dictionary

mydict = {0:"One", 1:"Two", 2:"Three"}
mydict.update({3:"Three", 4:"Four"})
print(mydict)

Output:
{0: 'One', 1: 'Two', 2: 'Three', 3: 'Three', 4: 'Four'}

Removing Keys

we can remove keys using two methods pop() & del()

• The pop() will check if the key is in the dictionary or not
• If the key is there it will remove the key
• If the key is not there it will give KeyError

mydict = {0:"One", 1:"Two", 2:"Three"}
print(mydict.pop(5))

Output: 
Traceback (most recent call last):
  File "C:/dictionary.py", line 2, in <module>
    print(mydict.pop(5))
KeyError: 5

We can suppress the key error by passing a second argument

mydict = {0:"One", 1:"Two", 2:"Three"}
print(mydict.pop(5,None))

Output:  None

mydict = {0:"One", 1:"Two", 2:"Three"}
print(mydict.pop(0))
print(mydict)

Output:
One
{1: 'Two', 2: 'Three'}

• The other way to remove items from a dictionary using pythons built-in del() method.
• This will delete the specific key,value from a dictionary.
• If the key isn't there it will give a KeyError.
• That's why we actually use pop() method.

mydict = {0:"One", 1:"Two", 2:"Three"}
del mydict[1]
print(mydict)

Output: {0: 'One', 2: 'Three'}

Using keys() method we can access all the keys of a dictionary

mydict = {1:"one", 2:"two", 3:"three", 4:"four"}
print(mydict.keys())

Output: dict_keys([1, 2, 3, 4])

Using values() method we can access all the values of a dictionary

mydict = {1:"one", 2:"two", 3:"three", 4:"four"}
print(mydict.values())

Output: dict_values(['one', 'two', 'three', 'four'])

Using items() method we can access all the items(key,value) of a dictionary

mydict = {1:"one", 2:"two", 3:"three", 4:"four"}
print(mydict.items())

Output: dict_items([(1, 'one'), (2, 'two'), (3, 'three'), (4, 'four')])

Iterate through a dictionary

Python dictionary allows the programmer to iterate over it's keys, values using forloop. Let's look into this
We can iterate through the keys of a dictionary

mydict = {1:"one", 2:"two", 3:"three", 4:"four"}
for mykey in mydict.keys():
    print(mykey)
    
 Output:
 1
 2
 3
 4

iterate through the values of a dictionary

mydict = {1:"one", 2:"two", 3:"three", 4:"four"}
for myval in mydict.values():
    print(myval)
    
Output:
one
two
three
four

Iterate both keys&values of a dictionary

mydict = {1:"one", 2:"two", 3:"three", 4:"four"}
for key,val in mydict.items():
    print(key,":",val)

Output:
1 : one
2 : two
3 : three
4 : four

Methods of a dictionary

For Practice Dictionary Exercises click here

This is all we have to know about Dictionaries. Practice more and more till you get this handy.