Python - Dictionaries

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Dictionary is one of the built-in data types in Python. Python's dictionary is example of mapping type. A mapping object 'maps' value of one object with another.

In a language dictionary we have pairs of word and corresponding meaning. Two parts of pair are key (word) and value (meaning). Similarly, Python dictionary is also a collection of key:value pairs. The pairs are separated by comma and put inside curly brackets {}.

To establish mapping between key and value, the colon ':' symbol is put between the two.

Given below are some examples of Python dictionary objects −

capitals = {"Maharashtra":"Mumbai", "Gujarat":"Gandhinagar", "Telangana":"Hyderabad", "Karnataka":"Bengaluru"}
numbers = {10:"Ten", 20:"Twenty", 30:"Thirty",40:"Forty"}
marks = {"Savita":67, "Imtiaz":88, "Laxman":91, "David":49}

Example 1

Only a number, string or tuple can be used as key. All of them are immutable. You can use an object of any type as the value. Hence following definitions of dictionary are also valid −

d1 = {"Fruit":["Mango","Banana"], "Flower":["Rose", "Lotus"]}
d2 = {('India, USA'):'Countries', ('New Delhi', 'New York'):'Capitals'}
print (d1)
print (d2)

It will produce the following output −

{'Fruit': ['Mango', 'Banana'], 'Flower': ['Rose', 'Lotus']}
{'India, USA': 'Countries', ('New Delhi', 'New York'): 'Capitals'}

Example 2

Python doesn't accept mutable objects such as list as key, and raises TypeError.

d1 = {["Mango","Banana"]:"Fruit", "Flower":["Rose", "Lotus"]}
print (d1)

It will raise a TypeError −

Traceback (most recent call last):
   File "C:\Users\Sairam\PycharmProjects\pythonProject\main.py", line 8, in <module>
d1 = {["Mango","Banana"]:"Fruit", "Flower":["Rose", "Lotus"]}
     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
TypeError: unhashable type: 'list'

Example 3

You can assign a value to more than one keys in a dictionary, but a key cannot appear more than once in a dictionary.

d1 = {"Banana":"Fruit", "Rose":"Flower", "Lotus":"Flower", "Mango":"Fruit"}
d2 = {"Fruit":"Banana","Flower":"Rose", "Fruit":"Mango", "Flower":"Lotus"}
print (d1)
print (d2)

It will produce the following output −

{'Banana': 'Fruit', 'Rose': 'Flower', 'Lotus': 'Flower', 'Mango': 'Fruit'}
{'Fruit': 'Mango', 'Flower': 'Lotus'}

Python Dictionary Operators

In Python, following operators are defined to be used with dictionary operands. In the example, the following dictionary objects are used.

d1 = {'a': 2, 'b': 4, 'c': 30}
d2 = {'a1': 20, 'b1': 40, 'c1': 60}

Operator	Description	Example
dict[key]	Extract/assign the value mapped with key	print (d1['b']) retrieves 4 d1['b'] = 'Z' assigns new value to key 'b'
dict1|dict2	Union of two dictionary objects, retuning new object	d3=d1|d2 ; print (d3) {'a': 2, 'b': 4, 'c': 30, 'a1': 20, 'b1': 40, 'c1': 60}
dict1|=dict2	Augmented dictionary union operator	d1|=d2; print (d1) {'a': 2, 'b': 4, 'c': 30, 'a1': 20, 'b1': 40, 'c1': 60}