Python Data Structures – Dictionary

Dictionary is a collection of Key value pairs. key must be unique, value can be duplicated. Value part can be retrieved using the key part.

Python Dictionary is a collection of Key value pairs. i.e., An element is a key & a value pair. The key is a unique value for which a value will be associated. The element’s value part can be retrieved using the key part.

Key must be a unique value but values can be duplicated.

Mutability:

  • A dictionary is a mutable objects and its elements also is mutable by using the key part.
  • Access an item using the key.
  • Add items directly by dict_name[new_key_name] = new_value
  • Remove any values using pop or popitem
  • pop – takes in a key value as argument and removes that item. So you can remove any item regardless of its position.
  • popitem – no arguments. Simply removes the last item.

Syntax:

variable_name = {key1: value1, key2: value2, ......, keyN: valueN}

Example:

Syntax to access an element: dictionary_name[key]

dict1={'a':'test', 1:'second', 3: 'last'}
print(dict1)
print("Access an element of key 'a':", dict1["a"])
{'a': 'new', 1: 'second', 3: 'last'}
Access an element of key 'a': new

What happens if a key is duplicated?

dict1={'a':'test', 1:'second', 'a':'new', 3: 'last'}
print(dict1)
{'a': 'new', 1: 'second', 3: 'last'}

In the above example, the key ‘a’ is duplicated. When a key is duplicated, the first occurrence value will be overridden by the last occurrence’s value.


Pop function:

Pop function takes an mandatory argument as a key value. It removes the pair from dictionary and returns the value part.

dict1={1:'First', 2:'Second', 3: 'Last'}
print("Original Dictionary: ",dict1)
print("Value of key '1'", dict1.pop(1))
print("After pop:", dict1)
Original Dictionary:  {1: 'First', 2: 'Second', 3: 'Last'}
Value of key '1' First
After pop: {2: 'Second', 3: 'Last'}

We have seen how to change the value part. But how to change the key part of a pair?

There is no direct method. However there is a workaround.

The pop function removes the pair and returns the value. We can use this return value and add a new element with a new name. As there is no way to access using the index of an element in Dictionary, the order does not matter.

dict1={1:'First', 2:'Second', 3: 'Last'}
print("Before changes: ",dict1)
dict1["changed"] = dict1.pop(1)
print("After changes:", dict1)
Before changes:  {1: 'First', 2: 'Second', 3: 'Last'}
After changes: {2: 'Second', 3: 'Last', 'changed': 'First'}

Popitem function:

Popitem function removes and returns an element in random choice. The returned value is a size 2 Tuple containing the key and value parts.

dict1={1:'First', 2:'Second', 3: 'Last'}
print("Before changes: ",dict1)
removed = dict1.popitem()
print("Popitem result:", removed)
print("Type of return value:", type(removed))
print("After changes:", dict1)
Before changes:  {1: 'First', 2: 'Second', 3: 'Last'}
Popitem result: (3, 'Last')
Type of return value: <class 'tuple'>
After changes: {1: 'First', 2: 'Second'}

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Asha Ponraj

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