Python - Dictionary

Python’s dictionary is also a collection type object. However, unlike list or tuple, it is not a sequence. It is a mapping type object. Dictionary is an unordered collection of comma separated key-value pairs. Key is mapped to its value by special hashing mechanism of Python interpreter. Association of key to value is marked by : symbol between them.

Some examples of dictionary objects are shown below:

In [1]:
capitals={"Maharashtra":"Mumbai", "Telangana":"Hyderabad", "Tamilnadu":"Chennai", "Karnataka":"Bengaluru", "Bihar":"Patna"}
rankings={1:"India", 2:"England", 3:"South Africa", 4:"Australia", 5:"New Zealand"}
prices={"pen":25, "bike":50000, "mobile": 5000,"book": 250 }
In [2]:
print ("state capitals:", capitals)
print ("Team rankings:", rankings)
print ("prices:",prices)
Out[2]:
state capitals: {'Maharashtra': 'Mumbai', 'Telangana': 'Hyderabad', 'Tamilnadu': 'Chennai', 'Karnataka': 'Bengaluru', 'Bihar': 'Patna'}
Team rankings: {1: 'India', 2: 'England', 3: 'South Africa', 4: 'Australia', 5: 'New Zealand'}
prices: {'pen': 25, 'bike': 50000, 'mobile': 5000, 'book': 250}

Key should be an immutable object. Number, string or tuple can be used as key in dictionary. Use of mutable object as key will raise TypeError. Any object is valid for use as value.

In [3]:
manufacturers={("Nokia", "Samsung","MicroMax"):"Mobiles", ("Nike", "Adidas", "Puma"):"Sports Goods",
              ("Honda","Bajaj", "TVS"):"Bikes"}
sports={"indoors":["chess", "Badminton", "Table Tennis"], "outdoor":["Cricket", "Football", "Rugby"]}
In [4]:
animals={["snake", "Lizard"]:"reptiles", ["Parrot", "Crow"]:"birds"}
---------------------------------------------------------------------------
TypeError                                 Traceback (most recent call last)
<ipython-input-4-1904d3d80b08> in <module>()
----> 1 animals={["snake", "Lizard"]:"reptiles", ["Parrot", "Crow"]:"birds"}

TypeError: unhashable type: 'list'

Same Key doesn’t appear more than once in one collection. One value can be assigned to more than one keys. If key is used more than once, only the last will be retained.

In [5]:
marks={"Laxman":56, "Kavita":65, "Amit":51, "Julie":68, "Kavita":66, "Amar":48}
marks
Out[5]:
{'Laxman': 56, 'Kavita': 66, 'Amit': 51, 'Julie': 68, 'Amar': 48}

To access value:

To obtain value associated with certain key use get() method. It can also be fetched by using [] operator.

In [6]:
capitals={"Maharashtra":"Mumbai", "Telangana":"Hyderabad", "Tamilnadu":"Chennai", "Karnataka":"Bengaluru", "Bihar":"Patna"}
capitals.get("Karnataka")
Out[6]:
'Bengaluru'
In [7]:
prices={"pen":25, "bike":50000, "mobile": 5000,"book": 250 }
prices['bike']
Out[7]:
50000

To update value:

To update the dictionary, just assign a new value to existing key. If the key is not currently used, a new key-value pair is added to dictionary.

In [8]:
ChiefMinisters={"Maharashtra":"Fadanvis","UP":"Yogi", "Karnataka":"KumarSwamy", "MP":"chauhan"}
ChiefMinisters["MP"]="KamalNath"
ChiefMinisters["Maharashtra"]="Thakre"
ChiefMinisters
Out[8]:
{'Maharashtra': 'Thakre',
'UP': 'Yogi',
'Karnataka': 'KumarSwamy',
'MP': 'KamalNath'}
In [9]:
ChiefMinisters["Delhi"]="Kejriwal"
ChiefMinisters
Out[9]:
{'Maharashtra': 'Thakre',
'UP': 'Yogi',
'Karnataka': 'KumarSwamy',
'MP': 'KamalNath',
'Delhi': 'Kejriwal'}

Dictionary Built-in functions

In [10]:
players={"A":"Anderson", "B":"Bravo", "C":"Currran","D":"Dhawan"}
len(players)
Out[10]:
4
In [11]:
print ('max :',max(players))
print ('min : ', min(players))       
max : D
min :  A
In [12]:
digwords={2:"two",9:"nine", 6:"six", 1:"one"}
print ('max:', max(digwords))
print ('min:', min(digwords))
max: 9
min: 1

Dictionary class methods

dict() is a built-in function. If used without parameter, it creates an empty dictionary. The function uses a list of two item tuples as parameter.The new dictionary object is constructed with each tuple treated as key-value pair.

In [13]:
d1=dict()
d1
Out[13]:
{}
In [14]:
dict([(1,100),(2,200),(3,300)])
Out[14]:
{1: 100, 2: 200, 3: 300}
In [15]:
dict(a=1, b=2, c=3)
Out[15]:
{'a': 1, 'b': 2, 'c': 3}

items(), keys() and values() methods return view objects that are lists of k:v pairs, keys and values respectively. Any change in composition of dictionary will automatically reflect in these view objects.

In [16]:
CMs={'Maharashtra': 'Thakre', 'UP': 'Yogi', 'Karnataka': 'KumarSwamy', 'MP': 'KamalNath', 'Delhi': 'Kejriwal'}
CMs.items()
Out[16]:
dict_items([('Maharashtra', 'Thakre'), ('UP', 'Yogi'), ('Karnataka', 'KumarSwamy'), ('MP', 'KamalNath'), ('Delhi', 'Kejriwal')])
In [17]:
CMs.keys()
Out[17]:
dict_keys(['Maharashtra', 'UP', 'Karnataka', 'MP', 'Delhi'])
In [18]:
CMs.values()
Out[18]:
dict_values(['Thakre', 'Yogi', 'KumarSwamy', 'KamalNath', 'Kejriwal'])
In [19]:
CMs.pop("UP")
Out[19]:
'Yogi'

update() method merges two dictionaries by updating values with common keys in both and adding items with new keys from another dictionary.

In [20]:
CMs
Out[20]:
{'Maharashtra': 'Thakre',
'Karnataka': 'KumarSwamy',
'MP': 'KamalNath',
'Delhi': 'Kejriwal'}
In [21]:
d2={"Punjab":"Amrindersingh","Bengal":"Mamata", "Odisha":"Patnaik"}
CMs.update(d2)
In [22]:
CMs
Out[22]:
{'Maharashtra': 'Thakre',
'Karnataka': 'KumarSwamy',
'MP': 'KamalNath',
'Delhi': 'Kejriwal',
'Punjab': 'Amrindersingh',
'Bengal': 'Mamata',
'Odisha': 'Patnaik'}

clear() method removes all items from dictionary, though the object remains in the memory. The del keyword in Python removes the object from memory altogether.

In [23]:
CMs.clear()
CMs
Out[23]:
{}
In [24]:
del CMs
CMs
---------------------------------------------------------------------------
NameError                                 Traceback (most recent call last)
<ipython-input-24-66687de12b16> in <module>()
      1 del CMs
----> 2 CMs

NameError: name 'CMs' is not defined